From: "Mike Rapoport (Microsoft)" rppt@kernel.org
Hi,
These patches allow guest_memfd to notify userspace about minor page faults using userfaultfd and let userspace to resolve these page faults using UFFDIO_CONTINUE.
To allow UFFDIO_CONTINUE outside of the core mm I added a get_shmem_folio() callback to vm_ops that allows an address space backing a VMA to return a folio that exists in it's page cache (patch 2)
In order for guest_memfd to notify userspace about page faults, there is a new VM_FAULT_UFFD_MINOR that a ->fault() handler can return to inform the page fault handler that it needs to call handle_userfault() to complete the fault (patch 3).
Patch 4 plumbs these new goodies into guest_memfd.
This series is the minimal change I've been able to come up with to allow integration of guest_memfd with uffd and while refactoring uffd and making mfill_atomic() flow more linear would have been a nice improvement, it's way out of the scope of enabling uffd with guest_memfd.
v2 changes: * rename ->get_shared_folio() to ->get_folio() * hardwire VM_FAULF_UFFD_MINOR to 0 when CONFIG_USERFAULTFD=n
v1: https://patch.msgid.link/20251123102707.559422-1-rppt@kernel.org * Introduce VM_FAULF_UFFD_MINOR to avoid exporting handle_userfault() * Simplify vma_can_mfill_atomic() * Rename get_pagecache_folio() to get_shared_folio() and use inode instead of vma as its argument
rfc: https://patch.msgid.link/20251117114631.2029447-1-rppt@kernel.org
Mike Rapoport (Microsoft) (4): userfaultfd: move vma_can_userfault out of line userfaultfd, shmem: use a VMA callback to handle UFFDIO_CONTINUE mm: introduce VM_FAULT_UFFD_MINOR fault reason guest_memfd: add support for userfaultfd minor mode
Nikita Kalyazin (1): KVM: selftests: test userfaultfd minor for guest_memfd
include/linux/mm.h | 9 ++ include/linux/mm_types.h | 10 +- include/linux/userfaultfd_k.h | 36 +----- mm/memory.c | 2 + mm/shmem.c | 20 +++- mm/userfaultfd.c | 80 +++++++++++--- .../testing/selftests/kvm/guest_memfd_test.c | 103 ++++++++++++++++++ virt/kvm/guest_memfd.c | 28 +++++ 8 files changed, 236 insertions(+), 52 deletions(-)
base-commit: 6a23ae0a96a600d1d12557add110e0bb6e32730c
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
vma_can_userfault() has grown pretty big and it's not called on performance critical path.
Move it out of line.
No functional changes.
Reviewed-by: David Hildenbrand (Red Hat) david@kernel.org Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org --- include/linux/userfaultfd_k.h | 36 ++--------------------------------- mm/userfaultfd.c | 34 +++++++++++++++++++++++++++++++++ 2 files changed, 36 insertions(+), 34 deletions(-)
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h index c0e716aec26a..e4f43e7b063f 100644 --- a/include/linux/userfaultfd_k.h +++ b/include/linux/userfaultfd_k.h @@ -208,40 +208,8 @@ static inline bool userfaultfd_armed(struct vm_area_struct *vma) return vma->vm_flags & __VM_UFFD_FLAGS; }
-static inline bool vma_can_userfault(struct vm_area_struct *vma, - vm_flags_t vm_flags, - bool wp_async) -{ - vm_flags &= __VM_UFFD_FLAGS; - - if (vma->vm_flags & VM_DROPPABLE) - return false; - - if ((vm_flags & VM_UFFD_MINOR) && - (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma))) - return false; - - /* - * If wp async enabled, and WP is the only mode enabled, allow any - * memory type. - */ - if (wp_async && (vm_flags == VM_UFFD_WP)) - return true; - -#ifndef CONFIG_PTE_MARKER_UFFD_WP - /* - * If user requested uffd-wp but not enabled pte markers for - * uffd-wp, then shmem & hugetlbfs are not supported but only - * anonymous. - */ - if ((vm_flags & VM_UFFD_WP) && !vma_is_anonymous(vma)) - return false; -#endif - - /* By default, allow any of anon|shmem|hugetlb */ - return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) || - vma_is_shmem(vma); -} +bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags, + bool wp_async);
static inline bool vma_has_uffd_without_event_remap(struct vm_area_struct *vma) { diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index af61b95c89e4..8dc964389b0d 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1977,6 +1977,40 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, return moved ? moved : err; }
+bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags, + bool wp_async) +{ + vm_flags &= __VM_UFFD_FLAGS; + + if (vma->vm_flags & VM_DROPPABLE) + return false; + + if ((vm_flags & VM_UFFD_MINOR) && + (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma))) + return false; + + /* + * If wp async enabled, and WP is the only mode enabled, allow any + * memory type. + */ + if (wp_async && (vm_flags == VM_UFFD_WP)) + return true; + +#ifndef CONFIG_PTE_MARKER_UFFD_WP + /* + * If user requested uffd-wp but not enabled pte markers for + * uffd-wp, then shmem & hugetlbfs are not supported but only + * anonymous. + */ + if ((vm_flags & VM_UFFD_WP) && !vma_is_anonymous(vma)) + return false; +#endif + + /* By default, allow any of anon|shmem|hugetlb */ + return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) || + vma_is_shmem(vma); +} + static void userfaultfd_set_vm_flags(struct vm_area_struct *vma, vm_flags_t vm_flags) {
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
When userspace resolves a page fault in a shmem VMA with UFFDIO_CONTINUE it needs to get a folio that already exists in the pagecache backing that VMA.
Instead of using shmem_get_folio() for that, add a get_folio() method to 'struct vm_operations_struct' that will return a folio if it exists in the VMA's pagecache at given pgoff.
Implement get_folio() method for shmem and slightly refactor userfaultfd's mfill_atomic() and mfill_atomic_pte_continue() to support this new API.
Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org --- include/linux/mm.h | 9 ++++++++ mm/shmem.c | 18 ++++++++++++++++ mm/userfaultfd.c | 52 +++++++++++++++++++++++++++++----------------- 3 files changed, 60 insertions(+), 19 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h index 7c79b3369b82..c8647707d75b 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -690,6 +690,15 @@ struct vm_operations_struct { struct page *(*find_normal_page)(struct vm_area_struct *vma, unsigned long addr); #endif /* CONFIG_FIND_NORMAL_PAGE */ +#ifdef CONFIG_USERFAULTFD + /* + * Called by userfault to resolve UFFDIO_CONTINUE request. + * Should return the folio found at pgoff in the VMA's pagecache if it + * exists or ERR_PTR otherwise. + * The returned folio is locked and with reference held. + */ + struct folio *(*get_folio)(struct inode *inode, pgoff_t pgoff); +#endif };
#ifdef CONFIG_NUMA_BALANCING diff --git a/mm/shmem.c b/mm/shmem.c index 58701d14dd96..e16c7c8c3e1e 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -3263,6 +3263,18 @@ int shmem_mfill_atomic_pte(pmd_t *dst_pmd, shmem_inode_unacct_blocks(inode, 1); return ret; } + +static struct folio *shmem_get_folio_noalloc(struct inode *inode, pgoff_t pgoff) +{ + struct folio *folio; + int err; + + err = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC); + if (err) + return ERR_PTR(err); + + return folio; +} #endif /* CONFIG_USERFAULTFD */
#ifdef CONFIG_TMPFS @@ -5295,6 +5307,9 @@ static const struct vm_operations_struct shmem_vm_ops = { .set_policy = shmem_set_policy, .get_policy = shmem_get_policy, #endif +#ifdef CONFIG_USERFAULTFD + .get_folio = shmem_get_folio_noalloc, +#endif };
static const struct vm_operations_struct shmem_anon_vm_ops = { @@ -5304,6 +5319,9 @@ static const struct vm_operations_struct shmem_anon_vm_ops = { .set_policy = shmem_set_policy, .get_policy = shmem_get_policy, #endif +#ifdef CONFIG_USERFAULTFD + .get_folio = shmem_get_folio_noalloc, +#endif };
int shmem_init_fs_context(struct fs_context *fc) diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 8dc964389b0d..9f0f879b603a 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -388,15 +388,12 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd, struct page *page; int ret;
- ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC); + folio = dst_vma->vm_ops->get_folio(inode, pgoff); /* Our caller expects us to return -EFAULT if we failed to find folio */ - if (ret == -ENOENT) - ret = -EFAULT; - if (ret) - goto out; - if (!folio) { - ret = -EFAULT; - goto out; + if (IS_ERR_OR_NULL(folio)) { + if (PTR_ERR(folio) == -ENOENT || !folio) + return -EFAULT; + return PTR_ERR(folio); }
page = folio_file_page(folio, pgoff); @@ -411,13 +408,12 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd, goto out_release;
folio_unlock(folio); - ret = 0; -out: - return ret; + return 0; + out_release: folio_unlock(folio); folio_put(folio); - goto out; + return ret; }
/* Handles UFFDIO_POISON for all non-hugetlb VMAs. */ @@ -694,6 +690,15 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd, return err; }
+static __always_inline bool vma_can_mfill_atomic(struct vm_area_struct *vma, + uffd_flags_t flags) +{ + if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) + return vma->vm_ops && vma->vm_ops->get_folio; + + return vma_is_anonymous(vma) || vma_is_shmem(vma); +} + static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx, unsigned long dst_start, unsigned long src_start, @@ -766,10 +771,7 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx, return mfill_atomic_hugetlb(ctx, dst_vma, dst_start, src_start, len, flags);
- if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma)) - goto out_unlock; - if (!vma_is_shmem(dst_vma) && - uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) + if (!vma_can_mfill_atomic(dst_vma, flags)) goto out_unlock;
while (src_addr < src_start + len) { @@ -1985,9 +1987,21 @@ bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags, if (vma->vm_flags & VM_DROPPABLE) return false;
- if ((vm_flags & VM_UFFD_MINOR) && - (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma))) - return false; + if (vm_flags & VM_UFFD_MINOR) { + /* + * If only MINOR mode is requested and we can request an + * existing folio from VMA's page cache, allow it + */ + if (vm_flags == VM_UFFD_MINOR && vma->vm_ops && + vma->vm_ops->get_folio) + return true; + /* + * Only hugetlb and shmem can support MINOR mode in combination + * with other modes + */ + if (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)) + return false; + }
/* * If wp async enabled, and WP is the only mode enabled, allow any
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
When a VMA is registered with userfaulfd in minor mode, its ->fault() method should check if a folio exists in the page cache and if yes ->fault() should call handle_userfault(VM_UFFD_MISSING).
Instead of calling handle_userfault() directly from a specific ->fault() implementation introduce new fault reason VM_FAULT_UFFD_MINOR that will notify the core page fault handler that it should call handle_userfaultfd(VM_UFFD_MISSING) to complete a page fault.
Replace a call to handle_userfault(VM_UFFD_MISSING) in shmem and use the new VM_FAULT_UFFD_MINOR there instead.
For configurations that don't enable CONFIG_USERFAULTFD, VM_FAULT_UFFD_MINOR is set to 0.
Suggested-by: David Hildenbrand (Red Hat) david@kernel.org Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org --- include/linux/mm_types.h | 10 +++++++++- mm/memory.c | 2 ++ mm/shmem.c | 2 +- 3 files changed, 12 insertions(+), 2 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 90e5790c318f..df71b057111b 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -1523,6 +1523,8 @@ typedef __bitwise unsigned int vm_fault_t; * fsync() to complete (for synchronous page faults * in DAX) * @VM_FAULT_COMPLETED: ->fault completed, meanwhile mmap lock released + * @VM_FAULT_UFFD_MINOR: ->fault did not modify page tables and needs + * handle_userfault(VM_UFFD_MINOR) to complete * @VM_FAULT_HINDEX_MASK: mask HINDEX value * */ @@ -1540,6 +1542,11 @@ enum vm_fault_reason { VM_FAULT_DONE_COW = (__force vm_fault_t)0x001000, VM_FAULT_NEEDDSYNC = (__force vm_fault_t)0x002000, VM_FAULT_COMPLETED = (__force vm_fault_t)0x004000, +#ifdef CONFIG_USERFAULTFD + VM_FAULT_UFFD_MINOR = (__force vm_fault_t)0x008000, +#else + VM_FAULT_UFFD_MINOR = (__force vm_fault_t)0x000000, +#endif VM_FAULT_HINDEX_MASK = (__force vm_fault_t)0x0f0000, };
@@ -1564,7 +1571,8 @@ enum vm_fault_reason { { VM_FAULT_FALLBACK, "FALLBACK" }, \ { VM_FAULT_DONE_COW, "DONE_COW" }, \ { VM_FAULT_NEEDDSYNC, "NEEDDSYNC" }, \ - { VM_FAULT_COMPLETED, "COMPLETED" } + { VM_FAULT_COMPLETED, "COMPLETED" }, \ + { VM_FAULT_UFFD_MINOR, "UFFD_MINOR" }, \
struct vm_special_mapping { const char *name; /* The name, e.g. "[vdso]". */ diff --git a/mm/memory.c b/mm/memory.c index b59ae7ce42eb..94acbac8cefb 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -5279,6 +5279,8 @@ static vm_fault_t __do_fault(struct vm_fault *vmf) }
ret = vma->vm_ops->fault(vmf); + if (unlikely(ret & VM_FAULT_UFFD_MINOR)) + return handle_userfault(vmf, VM_UFFD_MINOR); if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY | VM_FAULT_DONE_COW))) return ret; diff --git a/mm/shmem.c b/mm/shmem.c index e16c7c8c3e1e..a9a31c0b5979 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2461,7 +2461,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, if (folio && vma && userfaultfd_minor(vma)) { if (!xa_is_value(folio)) folio_put(folio); - *fault_type = handle_userfault(vmf, VM_UFFD_MINOR); + *fault_type = VM_FAULT_UFFD_MINOR; return 0; }
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
userfaultfd notifications about minor page faults used for live migration and snapshotting of VMs with memory backed by shared hugetlbfs or tmpfs mappings as described in detail in commit 7677f7fd8be7 ("userfaultfd: add minor fault registration mode").
To use the same mechanism for VMs that use guest_memfd to map their memory, guest_memfd should support userfaultfd minor mode.
Extend ->fault() method of guest_memfd with ability to notify core page fault handler that a page fault requires handle_userfault(VM_UFFD_MINOR) to complete and add implementation of ->get_shared_folio() to guest_memfd vm_ops.
Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org --- virt/kvm/guest_memfd.c | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index ffadc5ee8e04..2a2b076293f9 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -4,6 +4,7 @@ #include <linux/kvm_host.h> #include <linux/pagemap.h> #include <linux/anon_inodes.h> +#include <linux/userfaultfd_k.h>
#include "kvm_mm.h"
@@ -369,6 +370,12 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) return vmf_error(err); }
+ if (userfaultfd_minor(vmf->vma)) { + folio_unlock(folio); + folio_put(folio); + return VM_FAULT_UFFD_MINOR; + } + if (WARN_ON_ONCE(folio_test_large(folio))) { ret = VM_FAULT_SIGBUS; goto out_folio; @@ -390,8 +397,29 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) return ret; }
+#ifdef CONFIG_USERFAULTFD +static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t pgoff) +{ + struct folio *folio; + + folio = kvm_gmem_get_folio(inode, pgoff); + if (IS_ERR_OR_NULL(folio)) + return folio; + + if (!folio_test_uptodate(folio)) { + clear_highpage(folio_page(folio, 0)); + kvm_gmem_mark_prepared(folio); + } + + return folio; +} +#endif + static const struct vm_operations_struct kvm_gmem_vm_ops = { .fault = kvm_gmem_fault_user_mapping, +#ifdef CONFIG_USERFAULTFD + .get_folio = kvm_gmem_get_folio, +#endif };
static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
From: Nikita Kalyazin kalyazin@amazon.com
The test demonstrates that a minor userfaultfd event in guest_memfd can be resolved via a memcpy followed by a UFFDIO_CONTINUE ioctl.
Signed-off-by: Nikita Kalyazin kalyazin@amazon.com Co-developed-by: Mike Rapoport (Microsoft) rppt@kernel.org Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org --- .../testing/selftests/kvm/guest_memfd_test.c | 103 ++++++++++++++++++ 1 file changed, 103 insertions(+)
diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c index e7d9aeb418d3..a5d3ed21d7bb 100644 --- a/tools/testing/selftests/kvm/guest_memfd_test.c +++ b/tools/testing/selftests/kvm/guest_memfd_test.c @@ -10,13 +10,17 @@ #include <errno.h> #include <stdio.h> #include <fcntl.h> +#include <pthread.h>
#include <linux/bitmap.h> #include <linux/falloc.h> #include <linux/sizes.h> +#include <linux/userfaultfd.h> #include <sys/mman.h> #include <sys/types.h> #include <sys/stat.h> +#include <sys/syscall.h> +#include <sys/ioctl.h>
#include "kvm_util.h" #include "test_util.h" @@ -254,6 +258,104 @@ static void test_guest_memfd_flags(struct kvm_vm *vm) } }
+struct fault_args { + char *addr; + volatile char value; +}; + +static void *fault_thread_fn(void *arg) +{ + struct fault_args *args = arg; + + /* Trigger page fault */ + args->value = *args->addr; + return NULL; +} + +static void test_uffd_minor(int fd, size_t total_size) +{ + struct uffdio_api uffdio_api = { + .api = UFFD_API, + .features = UFFD_FEATURE_MINOR_GENERIC, + }; + struct uffdio_register uffd_reg; + struct uffdio_continue uffd_cont; + struct uffd_msg msg; + struct fault_args args; + pthread_t fault_thread; + void *mem, *mem_nofault, *buf = NULL; + int uffd, ret; + off_t offset = page_size; + void *fault_addr; + + ret = posix_memalign(&buf, page_size, total_size); + TEST_ASSERT_EQ(ret, 0); + + memset(buf, 0xaa, total_size); + + uffd = syscall(__NR_userfaultfd, O_CLOEXEC); + TEST_ASSERT(uffd != -1, "userfaultfd creation should succeed"); + + ret = ioctl(uffd, UFFDIO_API, &uffdio_api); + TEST_ASSERT(ret != -1, "ioctl(UFFDIO_API) should succeed"); + + mem = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); + TEST_ASSERT(mem != MAP_FAILED, "mmap should succeed"); + + mem_nofault = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); + TEST_ASSERT(mem_nofault != MAP_FAILED, "mmap should succeed"); + + uffd_reg.range.start = (unsigned long)mem; + uffd_reg.range.len = total_size; + uffd_reg.mode = UFFDIO_REGISTER_MODE_MINOR; + ret = ioctl(uffd, UFFDIO_REGISTER, &uffd_reg); + TEST_ASSERT(ret != -1, "ioctl(UFFDIO_REGISTER) should succeed"); + + ret = fallocate(fd, FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE, + offset, page_size); + TEST_ASSERT(!ret, "fallocate(PUNCH_HOLE) should succeed"); + + fault_addr = mem + offset; + args.addr = fault_addr; + + ret = pthread_create(&fault_thread, NULL, fault_thread_fn, &args); + TEST_ASSERT(ret == 0, "pthread_create should succeed"); + + ret = read(uffd, &msg, sizeof(msg)); + TEST_ASSERT(ret != -1, "read from userfaultfd should succeed"); + TEST_ASSERT(msg.event == UFFD_EVENT_PAGEFAULT, "event type should be pagefault"); + TEST_ASSERT((void *)(msg.arg.pagefault.address & ~(page_size - 1)) == fault_addr, + "pagefault should occur at expected address"); + + memcpy(mem_nofault + offset, buf + offset, page_size); + + uffd_cont.range.start = (unsigned long)fault_addr; + uffd_cont.range.len = page_size; + uffd_cont.mode = 0; + ret = ioctl(uffd, UFFDIO_CONTINUE, &uffd_cont); + TEST_ASSERT(ret != -1, "ioctl(UFFDIO_CONTINUE) should succeed"); + + /* + * wait for fault_thread to finish to make sure fault happened and was + * resolved before we verify the values + */ + ret = pthread_join(fault_thread, NULL); + TEST_ASSERT(ret == 0, "pthread_join should succeed"); + + TEST_ASSERT(args.value == *(char *)(mem_nofault + offset), + "memory should contain the value that was copied"); + TEST_ASSERT(args.value == *(char *)(mem + offset), + "no further fault is expected"); + + ret = munmap(mem_nofault, total_size); + TEST_ASSERT(!ret, "munmap should succeed"); + + ret = munmap(mem, total_size); + TEST_ASSERT(!ret, "munmap should succeed"); + free(buf); + close(uffd); +} + #define gmem_test(__test, __vm, __flags) \ do { \ int fd = vm_create_guest_memfd(__vm, page_size * 4, __flags); \ @@ -273,6 +375,7 @@ static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags) if (flags & GUEST_MEMFD_FLAG_INIT_SHARED) { gmem_test(mmap_supported, vm, flags); gmem_test(fault_overflow, vm, flags); + gmem_test(uffd_minor, vm, flags); } else { gmem_test(fault_private, vm, flags); }
Hi, Mike,
On Tue, Nov 25, 2025 at 08:38:38PM +0200, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
When a VMA is registered with userfaulfd in minor mode, its ->fault() method should check if a folio exists in the page cache and if yes ->fault() should call handle_userfault(VM_UFFD_MISSING).
s/MISSING/MINOR/
Instead of calling handle_userfault() directly from a specific ->fault() implementation introduce new fault reason VM_FAULT_UFFD_MINOR that will notify the core page fault handler that it should call handle_userfaultfd(VM_UFFD_MISSING) to complete a page fault.
Same.
Replace a call to handle_userfault(VM_UFFD_MISSING) in shmem and use the
Same.
new VM_FAULT_UFFD_MINOR there instead.
Personally I'd keep the fault path as simple as possible, because that's the more frequently used path (rather than when userfaultfd is armed). I also see it slightly a pity that even with flags introduced, it only solves the MINOR problem, not MISSING.
If it's me, I'd simply export handle_userfault().. I confess I still don't know why exporting it is a problem, but maybe I missed something.
Only my two cents. Feel free to go with whatever way you prefer.
Thanks,
@@ -1564,7 +1571,8 @@ enum vm_fault_reason { { VM_FAULT_FALLBACK, "FALLBACK" }, \ { VM_FAULT_DONE_COW, "DONE_COW" }, \ { VM_FAULT_NEEDDSYNC, "NEEDDSYNC" }, \
- { VM_FAULT_COMPLETED, "COMPLETED" }
- { VM_FAULT_COMPLETED, "COMPLETED" }, \
- { VM_FAULT_UFFD_MINOR, "UFFD_MINOR" }, \
struct vm_special_mapping { const char *name; /* The name, e.g. "[vdso]". */ diff --git a/mm/memory.c b/mm/memory.c index b59ae7ce42eb..94acbac8cefb 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -5279,6 +5279,8 @@ static vm_fault_t __do_fault(struct vm_fault *vmf) } ret = vma->vm_ops->fault(vmf);
- if (unlikely(ret & VM_FAULT_UFFD_MINOR))
return handle_userfault(vmf, VM_UFFD_MINOR);
If we want to reduce the overhead on the fast path, we can simply do
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY | VM_FAULT_DONE_COW | VM_FAULT_UFFD_MINOR))) { if (unlikely(ret & VM_FAULT_UFFD_MINOR)) return handle_userfault(vmf, VM_UFFD_MINOR); return ret; }
Maybe the compiler already does that to improve the likely case.
LGTM
On 11/25/25 19:38, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
When userspace resolves a page fault in a shmem VMA with UFFDIO_CONTINUE it needs to get a folio that already exists in the pagecache backing that VMA.
Instead of using shmem_get_folio() for that, add a get_folio() method to 'struct vm_operations_struct' that will return a folio if it exists in the VMA's pagecache at given pgoff.
Implement get_folio() method for shmem and slightly refactor userfaultfd's mfill_atomic() and mfill_atomic_pte_continue() to support this new API.
Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
Acked-by: David Hildenbrand (Red Hat) david@kernel.org
On 11/25/25 19:38, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
userfaultfd notifications about minor page faults used for live migration and snapshotting of VMs with memory backed by shared hugetlbfs or tmpfs mappings as described in detail in commit 7677f7fd8be7 ("userfaultfd: add minor fault registration mode").
To use the same mechanism for VMs that use guest_memfd to map their memory, guest_memfd should support userfaultfd minor mode.
Extend ->fault() method of guest_memfd with ability to notify core page fault handler that a page fault requires handle_userfault(VM_UFFD_MINOR) to complete and add implementation of ->get_shared_folio() to guest_memfd vm_ops.
Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
No exports and still looks clean to me, nice. :)
* Mike Rapoport rppt@kernel.org [251125 13:39]:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
vma_can_userfault() has grown pretty big and it's not called on performance critical path.
Move it out of line.
No functional changes.
Reviewed-by: David Hildenbrand (Red Hat) david@kernel.org Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
Reviewed-by: Liam R. Howlett Liam.Howlett@oracle.com
include/linux/userfaultfd_k.h | 36 ++--------------------------------- mm/userfaultfd.c | 34 +++++++++++++++++++++++++++++++++ 2 files changed, 36 insertions(+), 34 deletions(-)
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h index c0e716aec26a..e4f43e7b063f 100644 --- a/include/linux/userfaultfd_k.h +++ b/include/linux/userfaultfd_k.h @@ -208,40 +208,8 @@ static inline bool userfaultfd_armed(struct vm_area_struct *vma) return vma->vm_flags & __VM_UFFD_FLAGS; } -static inline bool vma_can_userfault(struct vm_area_struct *vma,
vm_flags_t vm_flags,bool wp_async)-{
- vm_flags &= __VM_UFFD_FLAGS;
- if (vma->vm_flags & VM_DROPPABLE)
return false;- if ((vm_flags & VM_UFFD_MINOR) &&
(!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))return false;- /*
* If wp async enabled, and WP is the only mode enabled, allow any* memory type.*/- if (wp_async && (vm_flags == VM_UFFD_WP))
return true;-#ifndef CONFIG_PTE_MARKER_UFFD_WP
- /*
* If user requested uffd-wp but not enabled pte markers for* uffd-wp, then shmem & hugetlbfs are not supported but only* anonymous.*/- if ((vm_flags & VM_UFFD_WP) && !vma_is_anonymous(vma))
return false;-#endif
- /* By default, allow any of anon|shmem|hugetlb */
- return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
vma_is_shmem(vma);-} +bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
bool wp_async);static inline bool vma_has_uffd_without_event_remap(struct vm_area_struct *vma) { diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index af61b95c89e4..8dc964389b0d 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1977,6 +1977,40 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, return moved ? moved : err; } +bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
bool wp_async)+{
- vm_flags &= __VM_UFFD_FLAGS;
- if (vma->vm_flags & VM_DROPPABLE)
return false;- if ((vm_flags & VM_UFFD_MINOR) &&
(!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))return false;- /*
* If wp async enabled, and WP is the only mode enabled, allow any* memory type.*/- if (wp_async && (vm_flags == VM_UFFD_WP))
return true;+#ifndef CONFIG_PTE_MARKER_UFFD_WP
- /*
* If user requested uffd-wp but not enabled pte markers for* uffd-wp, then shmem & hugetlbfs are not supported but only* anonymous.*/- if ((vm_flags & VM_UFFD_WP) && !vma_is_anonymous(vma))
return false;+#endif
- /* By default, allow any of anon|shmem|hugetlb */
- return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
vma_is_shmem(vma);+}
static void userfaultfd_set_vm_flags(struct vm_area_struct *vma, vm_flags_t vm_flags) { -- 2.50.1
* Mike Rapoport rppt@kernel.org [251125 13:39]:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
When userspace resolves a page fault in a shmem VMA with UFFDIO_CONTINUE it needs to get a folio that already exists in the pagecache backing that VMA.
Instead of using shmem_get_folio() for that, add a get_folio() method to 'struct vm_operations_struct' that will return a folio if it exists in the VMA's pagecache at given pgoff.
Implement get_folio() method for shmem and slightly refactor userfaultfd's mfill_atomic() and mfill_atomic_pte_continue() to support this new API.
Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
Reviewed-by: Liam R. Howlett Liam.Howlett@oracle.com
include/linux/mm.h | 9 ++++++++ mm/shmem.c | 18 ++++++++++++++++ mm/userfaultfd.c | 52 +++++++++++++++++++++++++++++----------------- 3 files changed, 60 insertions(+), 19 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h index 7c79b3369b82..c8647707d75b 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -690,6 +690,15 @@ struct vm_operations_struct { struct page *(*find_normal_page)(struct vm_area_struct *vma, unsigned long addr); #endif /* CONFIG_FIND_NORMAL_PAGE */ +#ifdef CONFIG_USERFAULTFD
- /*
* Called by userfault to resolve UFFDIO_CONTINUE request.* Should return the folio found at pgoff in the VMA's pagecache if it* exists or ERR_PTR otherwise.* The returned folio is locked and with reference held.*/- struct folio *(*get_folio)(struct inode *inode, pgoff_t pgoff);
+#endif }; #ifdef CONFIG_NUMA_BALANCING diff --git a/mm/shmem.c b/mm/shmem.c index 58701d14dd96..e16c7c8c3e1e 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -3263,6 +3263,18 @@ int shmem_mfill_atomic_pte(pmd_t *dst_pmd, shmem_inode_unacct_blocks(inode, 1); return ret; }
+static struct folio *shmem_get_folio_noalloc(struct inode *inode, pgoff_t pgoff) +{
- struct folio *folio;
- int err;
- err = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
- if (err)
return ERR_PTR(err);- return folio;
+} #endif /* CONFIG_USERFAULTFD */ #ifdef CONFIG_TMPFS @@ -5295,6 +5307,9 @@ static const struct vm_operations_struct shmem_vm_ops = { .set_policy = shmem_set_policy, .get_policy = shmem_get_policy, #endif +#ifdef CONFIG_USERFAULTFD
- .get_folio = shmem_get_folio_noalloc,
+#endif }; static const struct vm_operations_struct shmem_anon_vm_ops = { @@ -5304,6 +5319,9 @@ static const struct vm_operations_struct shmem_anon_vm_ops = { .set_policy = shmem_set_policy, .get_policy = shmem_get_policy, #endif +#ifdef CONFIG_USERFAULTFD
- .get_folio = shmem_get_folio_noalloc,
+#endif }; int shmem_init_fs_context(struct fs_context *fc) diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 8dc964389b0d..9f0f879b603a 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -388,15 +388,12 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd, struct page *page; int ret;
- ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
- folio = dst_vma->vm_ops->get_folio(inode, pgoff); /* Our caller expects us to return -EFAULT if we failed to find folio */
- if (ret == -ENOENT)
ret = -EFAULT;- if (ret)
goto out;- if (!folio) {
ret = -EFAULT;goto out;
- if (IS_ERR_OR_NULL(folio)) {
if (PTR_ERR(folio) == -ENOENT || !folio)return -EFAULT;return PTR_ERR(folio);page = folio_file_page(folio, pgoff); @@ -411,13 +408,12 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd, goto out_release; folio_unlock(folio);
- ret = 0;
-out:
- return ret;
- return 0;
out_release: folio_unlock(folio); folio_put(folio);
- goto out;
- return ret;
I really like this part.
} /* Handles UFFDIO_POISON for all non-hugetlb VMAs. */ @@ -694,6 +690,15 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t *dst_pmd, return err; } +static __always_inline bool vma_can_mfill_atomic(struct vm_area_struct *vma,
uffd_flags_t flags)+{
- if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
return vma->vm_ops && vma->vm_ops->get_folio;- return vma_is_anonymous(vma) || vma_is_shmem(vma);
+}
static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx, unsigned long dst_start, unsigned long src_start, @@ -766,10 +771,7 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx, return mfill_atomic_hugetlb(ctx, dst_vma, dst_start, src_start, len, flags);
- if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
goto out_unlock;- if (!vma_is_shmem(dst_vma) &&
uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
- if (!vma_can_mfill_atomic(dst_vma, flags)) goto out_unlock;
while (src_addr < src_start + len) { @@ -1985,9 +1987,21 @@ bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags, if (vma->vm_flags & VM_DROPPABLE) return false;
- if ((vm_flags & VM_UFFD_MINOR) &&
(!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))return false;
- if (vm_flags & VM_UFFD_MINOR) {
/** If only MINOR mode is requested and we can request an* existing folio from VMA's page cache, allow it*/if (vm_flags == VM_UFFD_MINOR && vma->vm_ops &&vma->vm_ops->get_folio)return true;/** Only hugetlb and shmem can support MINOR mode in combination* with other modes*/if (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma))return false;- }
/* * If wp async enabled, and WP is the only mode enabled, allow any -- 2.50.1
* Mike Rapoport rppt@kernel.org [251125 13:39]:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
When a VMA is registered with userfaulfd in minor mode, its ->fault() method should check if a folio exists in the page cache and if yes ->fault() should call handle_userfault(VM_UFFD_MISSING).
Instead of calling handle_userfault() directly from a specific ->fault() implementation introduce new fault reason VM_FAULT_UFFD_MINOR that will notify the core page fault handler that it should call handle_userfaultfd(VM_UFFD_MISSING) to complete a page fault.
Replace a call to handle_userfault(VM_UFFD_MISSING) in shmem and use the new VM_FAULT_UFFD_MINOR there instead.
For configurations that don't enable CONFIG_USERFAULTFD, VM_FAULT_UFFD_MINOR is set to 0.
Suggested-by: David Hildenbrand (Red Hat) david@kernel.org Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
Same nit as David, but the rest looks good.
include/linux/mm_types.h | 10 +++++++++- mm/memory.c | 2 ++ mm/shmem.c | 2 +- 3 files changed, 12 insertions(+), 2 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 90e5790c318f..df71b057111b 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -1523,6 +1523,8 @@ typedef __bitwise unsigned int vm_fault_t;
fsync() to complete (for synchronous page faultsin DAX)- @VM_FAULT_COMPLETED: ->fault completed, meanwhile mmap lock released
- @VM_FAULT_UFFD_MINOR: ->fault did not modify page tables and needs
*/
handle_userfault(VM_UFFD_MINOR) to complete- @VM_FAULT_HINDEX_MASK: mask HINDEX value
@@ -1540,6 +1542,11 @@ enum vm_fault_reason { VM_FAULT_DONE_COW = (__force vm_fault_t)0x001000, VM_FAULT_NEEDDSYNC = (__force vm_fault_t)0x002000, VM_FAULT_COMPLETED = (__force vm_fault_t)0x004000, +#ifdef CONFIG_USERFAULTFD
- VM_FAULT_UFFD_MINOR = (__force vm_fault_t)0x008000,
+#else
- VM_FAULT_UFFD_MINOR = (__force vm_fault_t)0x000000,
+#endif VM_FAULT_HINDEX_MASK = (__force vm_fault_t)0x0f0000, }; @@ -1564,7 +1571,8 @@ enum vm_fault_reason { { VM_FAULT_FALLBACK, "FALLBACK" }, \ { VM_FAULT_DONE_COW, "DONE_COW" }, \ { VM_FAULT_NEEDDSYNC, "NEEDDSYNC" }, \
- { VM_FAULT_COMPLETED, "COMPLETED" }
- { VM_FAULT_COMPLETED, "COMPLETED" }, \
- { VM_FAULT_UFFD_MINOR, "UFFD_MINOR" }, \
struct vm_special_mapping { const char *name; /* The name, e.g. "[vdso]". */ diff --git a/mm/memory.c b/mm/memory.c index b59ae7ce42eb..94acbac8cefb 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -5279,6 +5279,8 @@ static vm_fault_t __do_fault(struct vm_fault *vmf) } ret = vma->vm_ops->fault(vmf);
- if (unlikely(ret & VM_FAULT_UFFD_MINOR))
return handle_userfault(vmf, VM_UFFD_MINOR);
I have the same concern as David here with adding instructions to the faults that are not UFFD_MINOR.. I suspect the compiler will remove the statement completely when UFFD is disabled (and thus ret & 0 in the check), but it might be worth looking at this closer in the case where uffd is enabled? It won't be as clean looking but might make the assembly better.
return ret;diff --git a/mm/shmem.c b/mm/shmem.c index e16c7c8c3e1e..a9a31c0b5979 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2461,7 +2461,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, if (folio && vma && userfaultfd_minor(vma)) { if (!xa_is_value(folio)) folio_put(folio);
*fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
*fault_type = VM_FAULT_UFFD_MINOR;2.50.1
* Mike Rapoport rppt@kernel.org [251125 13:39]:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
userfaultfd notifications about minor page faults used for live migration and snapshotting of VMs with memory backed by shared hugetlbfs or tmpfs mappings as described in detail in commit 7677f7fd8be7 ("userfaultfd: add minor fault registration mode").
To use the same mechanism for VMs that use guest_memfd to map their memory, guest_memfd should support userfaultfd minor mode.
Extend ->fault() method of guest_memfd with ability to notify core page fault handler that a page fault requires handle_userfault(VM_UFFD_MINOR) to complete and add implementation of ->get_shared_folio() to guest_memfd vm_ops.
Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
Reviewed-by: Liam R. Howlett Liam.Howlett@oracle.com
virt/kvm/guest_memfd.c | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index ffadc5ee8e04..2a2b076293f9 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -4,6 +4,7 @@ #include <linux/kvm_host.h> #include <linux/pagemap.h> #include <linux/anon_inodes.h> +#include <linux/userfaultfd_k.h> #include "kvm_mm.h" @@ -369,6 +370,12 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) return vmf_error(err); }
- if (userfaultfd_minor(vmf->vma)) {
folio_unlock(folio);folio_put(folio);return VM_FAULT_UFFD_MINOR;- }
- if (WARN_ON_ONCE(folio_test_large(folio))) { ret = VM_FAULT_SIGBUS; goto out_folio;
@@ -390,8 +397,29 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) return ret; } +#ifdef CONFIG_USERFAULTFD +static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t pgoff) +{
- struct folio *folio;
- folio = kvm_gmem_get_folio(inode, pgoff);
- if (IS_ERR_OR_NULL(folio))
return folio;- if (!folio_test_uptodate(folio)) {
clear_highpage(folio_page(folio, 0));kvm_gmem_mark_prepared(folio);- }
- return folio;
+} +#endif
static const struct vm_operations_struct kvm_gmem_vm_ops = { .fault = kvm_gmem_fault_user_mapping, +#ifdef CONFIG_USERFAULTFD
- .get_folio = kvm_gmem_get_folio,
+#endif }; static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma) -- 2.50.1
* Mike Rapoport rppt@kernel.org [251125 13:39]:
From: Nikita Kalyazin kalyazin@amazon.com
The test demonstrates that a minor userfaultfd event in guest_memfd can be resolved via a memcpy followed by a UFFDIO_CONTINUE ioctl.
Signed-off-by: Nikita Kalyazin kalyazin@amazon.com Co-developed-by: Mike Rapoport (Microsoft) rppt@kernel.org Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
Acked-by: Liam R. Howlett Liam.Howlett@oracle.com
.../testing/selftests/kvm/guest_memfd_test.c | 103 ++++++++++++++++++ 1 file changed, 103 insertions(+)
diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c index e7d9aeb418d3..a5d3ed21d7bb 100644 --- a/tools/testing/selftests/kvm/guest_memfd_test.c +++ b/tools/testing/selftests/kvm/guest_memfd_test.c @@ -10,13 +10,17 @@ #include <errno.h> #include <stdio.h> #include <fcntl.h> +#include <pthread.h> #include <linux/bitmap.h> #include <linux/falloc.h> #include <linux/sizes.h> +#include <linux/userfaultfd.h> #include <sys/mman.h> #include <sys/types.h> #include <sys/stat.h> +#include <sys/syscall.h> +#include <sys/ioctl.h> #include "kvm_util.h" #include "test_util.h" @@ -254,6 +258,104 @@ static void test_guest_memfd_flags(struct kvm_vm *vm) } } +struct fault_args {
- char *addr;
- volatile char value;
+};
+static void *fault_thread_fn(void *arg) +{
- struct fault_args *args = arg;
- /* Trigger page fault */
- args->value = *args->addr;
- return NULL;
+}
+static void test_uffd_minor(int fd, size_t total_size) +{
- struct uffdio_api uffdio_api = {
.api = UFFD_API,.features = UFFD_FEATURE_MINOR_GENERIC,- };
- struct uffdio_register uffd_reg;
- struct uffdio_continue uffd_cont;
- struct uffd_msg msg;
- struct fault_args args;
- pthread_t fault_thread;
- void *mem, *mem_nofault, *buf = NULL;
- int uffd, ret;
- off_t offset = page_size;
- void *fault_addr;
- ret = posix_memalign(&buf, page_size, total_size);
- TEST_ASSERT_EQ(ret, 0);
- memset(buf, 0xaa, total_size);
- uffd = syscall(__NR_userfaultfd, O_CLOEXEC);
- TEST_ASSERT(uffd != -1, "userfaultfd creation should succeed");
- ret = ioctl(uffd, UFFDIO_API, &uffdio_api);
- TEST_ASSERT(ret != -1, "ioctl(UFFDIO_API) should succeed");
- mem = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
- TEST_ASSERT(mem != MAP_FAILED, "mmap should succeed");
- mem_nofault = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
- TEST_ASSERT(mem_nofault != MAP_FAILED, "mmap should succeed");
- uffd_reg.range.start = (unsigned long)mem;
- uffd_reg.range.len = total_size;
- uffd_reg.mode = UFFDIO_REGISTER_MODE_MINOR;
- ret = ioctl(uffd, UFFDIO_REGISTER, &uffd_reg);
- TEST_ASSERT(ret != -1, "ioctl(UFFDIO_REGISTER) should succeed");
- ret = fallocate(fd, FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE,
offset, page_size);- TEST_ASSERT(!ret, "fallocate(PUNCH_HOLE) should succeed");
- fault_addr = mem + offset;
- args.addr = fault_addr;
- ret = pthread_create(&fault_thread, NULL, fault_thread_fn, &args);
- TEST_ASSERT(ret == 0, "pthread_create should succeed");
- ret = read(uffd, &msg, sizeof(msg));
- TEST_ASSERT(ret != -1, "read from userfaultfd should succeed");
- TEST_ASSERT(msg.event == UFFD_EVENT_PAGEFAULT, "event type should be pagefault");
- TEST_ASSERT((void *)(msg.arg.pagefault.address & ~(page_size - 1)) == fault_addr,
"pagefault should occur at expected address");- memcpy(mem_nofault + offset, buf + offset, page_size);
- uffd_cont.range.start = (unsigned long)fault_addr;
- uffd_cont.range.len = page_size;
- uffd_cont.mode = 0;
- ret = ioctl(uffd, UFFDIO_CONTINUE, &uffd_cont);
- TEST_ASSERT(ret != -1, "ioctl(UFFDIO_CONTINUE) should succeed");
- /*
* wait for fault_thread to finish to make sure fault happened and was* resolved before we verify the values*/- ret = pthread_join(fault_thread, NULL);
- TEST_ASSERT(ret == 0, "pthread_join should succeed");
- TEST_ASSERT(args.value == *(char *)(mem_nofault + offset),
"memory should contain the value that was copied");- TEST_ASSERT(args.value == *(char *)(mem + offset),
"no further fault is expected");- ret = munmap(mem_nofault, total_size);
- TEST_ASSERT(!ret, "munmap should succeed");
- ret = munmap(mem, total_size);
- TEST_ASSERT(!ret, "munmap should succeed");
- free(buf);
- close(uffd);
+}
#define gmem_test(__test, __vm, __flags) \ do { \ int fd = vm_create_guest_memfd(__vm, page_size * 4, __flags); \ @@ -273,6 +375,7 @@ static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags) if (flags & GUEST_MEMFD_FLAG_INIT_SHARED) { gmem_test(mmap_supported, vm, flags); gmem_test(fault_overflow, vm, flags);
gmem_test(uffd_minor, vm, flags);-- 2.50.1
On 25/11/2025 18:38, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
When a VMA is registered with userfaulfd in minor mode, its ->fault() method should check if a folio exists in the page cache and if yes ->fault() should call handle_userfault(VM_UFFD_MISSING).
Instead of calling handle_userfault() directly from a specific ->fault() implementation introduce new fault reason VM_FAULT_UFFD_MINOR that will notify the core page fault handler that it should call handle_userfaultfd(VM_UFFD_MISSING) to complete a page fault.
Replace a call to handle_userfault(VM_UFFD_MISSING) in shmem and use the new VM_FAULT_UFFD_MINOR there instead.
For configurations that don't enable CONFIG_USERFAULTFD, VM_FAULT_UFFD_MINOR is set to 0.
Suggested-by: David Hildenbrand (Red Hat) david@kernel.org Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
include/linux/mm_types.h | 10 +++++++++- mm/memory.c | 2 ++ mm/shmem.c | 2 +- 3 files changed, 12 insertions(+), 2 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 90e5790c318f..df71b057111b 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -1523,6 +1523,8 @@ typedef __bitwise unsigned int vm_fault_t;
fsync() to complete (for synchronous page faultsin DAX)- @VM_FAULT_COMPLETED: ->fault completed, meanwhile mmap lock released
- @VM_FAULT_UFFD_MINOR: ->fault did not modify page tables and needs
*/
handle_userfault(VM_UFFD_MINOR) to complete- @VM_FAULT_HINDEX_MASK: mask HINDEX value
@@ -1540,6 +1542,11 @@ enum vm_fault_reason { VM_FAULT_DONE_COW = (__force vm_fault_t)0x001000, VM_FAULT_NEEDDSYNC = (__force vm_fault_t)0x002000, VM_FAULT_COMPLETED = (__force vm_fault_t)0x004000, +#ifdef CONFIG_USERFAULTFD
VM_FAULT_UFFD_MINOR = (__force vm_fault_t)0x008000,+#else
VM_FAULT_UFFD_MINOR = (__force vm_fault_t)0x000000,+#endif VM_FAULT_HINDEX_MASK = (__force vm_fault_t)0x0f0000, };
@@ -1564,7 +1571,8 @@ enum vm_fault_reason { { VM_FAULT_FALLBACK, "FALLBACK" }, \ { VM_FAULT_DONE_COW, "DONE_COW" }, \ { VM_FAULT_NEEDDSYNC, "NEEDDSYNC" }, \
{ VM_FAULT_COMPLETED, "COMPLETED" }
{ VM_FAULT_COMPLETED, "COMPLETED" }, \{ VM_FAULT_UFFD_MINOR, "UFFD_MINOR" }, \
It looks like we have to keep the last element comma-less, otherwise I'm seeing compile errors somewhere in fs/dax.c.
struct vm_special_mapping { const char *name; /* The name, e.g. "[vdso]". */ diff --git a/mm/memory.c b/mm/memory.c index b59ae7ce42eb..94acbac8cefb 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -5279,6 +5279,8 @@ static vm_fault_t __do_fault(struct vm_fault *vmf) }
ret = vma->vm_ops->fault(vmf);
if (unlikely(ret & VM_FAULT_UFFD_MINOR))return handle_userfault(vmf, VM_UFFD_MINOR); if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY | VM_FAULT_DONE_COW))) return ret;diff --git a/mm/shmem.c b/mm/shmem.c index e16c7c8c3e1e..a9a31c0b5979 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2461,7 +2461,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, if (folio && vma && userfaultfd_minor(vma)) { if (!xa_is_value(folio)) folio_put(folio);
*fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
*fault_type = VM_FAULT_UFFD_MINOR; return 0; }-- 2.50.1
On 25/11/2025 18:38, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
userfaultfd notifications about minor page faults used for live migration and snapshotting of VMs with memory backed by shared hugetlbfs or tmpfs mappings as described in detail in commit 7677f7fd8be7 ("userfaultfd: add minor fault registration mode").
To use the same mechanism for VMs that use guest_memfd to map their memory, guest_memfd should support userfaultfd minor mode.
Extend ->fault() method of guest_memfd with ability to notify core page fault handler that a page fault requires handle_userfault(VM_UFFD_MINOR) to complete and add implementation of ->get_shared_folio() to guest_memfd vm_ops.
Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
virt/kvm/guest_memfd.c | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index ffadc5ee8e04..2a2b076293f9 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -4,6 +4,7 @@ #include <linux/kvm_host.h> #include <linux/pagemap.h> #include <linux/anon_inodes.h> +#include <linux/userfaultfd_k.h>
#include "kvm_mm.h"
@@ -369,6 +370,12 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) return vmf_error(err); }
if (userfaultfd_minor(vmf->vma)) {folio_unlock(folio);folio_put(folio);return VM_FAULT_UFFD_MINOR;}if (WARN_ON_ONCE(folio_test_large(folio))) { ret = VM_FAULT_SIGBUS; goto out_folio;@@ -390,8 +397,29 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) return ret; }
+#ifdef CONFIG_USERFAULTFD +static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t pgoff)
We have to name it differently, otherwise it clashes with the existing one in this file.
+{
struct folio *folio;folio = kvm_gmem_get_folio(inode, pgoff);
^^
if (IS_ERR_OR_NULL(folio))return folio;if (!folio_test_uptodate(folio)) {clear_highpage(folio_page(folio, 0));kvm_gmem_mark_prepared(folio);}return folio;+} +#endif
- static const struct vm_operations_struct kvm_gmem_vm_ops = { .fault = kvm_gmem_fault_user_mapping,
+#ifdef CONFIG_USERFAULTFD
.get_folio = kvm_gmem_get_folio,+#endif };
static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
2.50.1
On 25/11/2025 18:38, Mike Rapoport wrote:
From: Nikita Kalyazin kalyazin@amazon.com
The test demonstrates that a minor userfaultfd event in guest_memfd can be resolved via a memcpy followed by a UFFDIO_CONTINUE ioctl.
Signed-off-by: Nikita Kalyazin kalyazin@amazon.com Co-developed-by: Mike Rapoport (Microsoft) rppt@kernel.org Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
.../testing/selftests/kvm/guest_memfd_test.c | 103 ++++++++++++++++++ 1 file changed, 103 insertions(+)
diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c index e7d9aeb418d3..a5d3ed21d7bb 100644 --- a/tools/testing/selftests/kvm/guest_memfd_test.c +++ b/tools/testing/selftests/kvm/guest_memfd_test.c @@ -10,13 +10,17 @@ #include <errno.h> #include <stdio.h> #include <fcntl.h> +#include <pthread.h>
#include <linux/bitmap.h> #include <linux/falloc.h> #include <linux/sizes.h> +#include <linux/userfaultfd.h> #include <sys/mman.h> #include <sys/types.h> #include <sys/stat.h> +#include <sys/syscall.h> +#include <sys/ioctl.h>
#include "kvm_util.h" #include "test_util.h" @@ -254,6 +258,104 @@ static void test_guest_memfd_flags(struct kvm_vm *vm) } }
+struct fault_args {
char *addr;volatile char value;+};
+static void *fault_thread_fn(void *arg) +{
struct fault_args *args = arg;/* Trigger page fault */args->value = *args->addr;return NULL;+}
+static void test_uffd_minor(int fd, size_t total_size) +{
struct uffdio_api uffdio_api = {.api = UFFD_API,.features = UFFD_FEATURE_MINOR_GENERIC,
Should it be UFFD_FEATURE_MINOR_SHMEM instead? UFFD_FEATURE_MINOR_GENERIC was removed in the v1.
};struct uffdio_register uffd_reg;struct uffdio_continue uffd_cont;struct uffd_msg msg;struct fault_args args;pthread_t fault_thread;void *mem, *mem_nofault, *buf = NULL;int uffd, ret;off_t offset = page_size;void *fault_addr;ret = posix_memalign(&buf, page_size, total_size);TEST_ASSERT_EQ(ret, 0);memset(buf, 0xaa, total_size);uffd = syscall(__NR_userfaultfd, O_CLOEXEC);TEST_ASSERT(uffd != -1, "userfaultfd creation should succeed");ret = ioctl(uffd, UFFDIO_API, &uffdio_api);TEST_ASSERT(ret != -1, "ioctl(UFFDIO_API) should succeed");mem = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);TEST_ASSERT(mem != MAP_FAILED, "mmap should succeed");mem_nofault = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);TEST_ASSERT(mem_nofault != MAP_FAILED, "mmap should succeed");uffd_reg.range.start = (unsigned long)mem;uffd_reg.range.len = total_size;uffd_reg.mode = UFFDIO_REGISTER_MODE_MINOR;ret = ioctl(uffd, UFFDIO_REGISTER, &uffd_reg);TEST_ASSERT(ret != -1, "ioctl(UFFDIO_REGISTER) should succeed");ret = fallocate(fd, FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE,offset, page_size);TEST_ASSERT(!ret, "fallocate(PUNCH_HOLE) should succeed");fault_addr = mem + offset;args.addr = fault_addr;ret = pthread_create(&fault_thread, NULL, fault_thread_fn, &args);TEST_ASSERT(ret == 0, "pthread_create should succeed");ret = read(uffd, &msg, sizeof(msg));TEST_ASSERT(ret != -1, "read from userfaultfd should succeed");TEST_ASSERT(msg.event == UFFD_EVENT_PAGEFAULT, "event type should be pagefault");TEST_ASSERT((void *)(msg.arg.pagefault.address & ~(page_size - 1)) == fault_addr,"pagefault should occur at expected address");memcpy(mem_nofault + offset, buf + offset, page_size);uffd_cont.range.start = (unsigned long)fault_addr;uffd_cont.range.len = page_size;uffd_cont.mode = 0;ret = ioctl(uffd, UFFDIO_CONTINUE, &uffd_cont);TEST_ASSERT(ret != -1, "ioctl(UFFDIO_CONTINUE) should succeed");/** wait for fault_thread to finish to make sure fault happened and was* resolved before we verify the values*/ret = pthread_join(fault_thread, NULL);TEST_ASSERT(ret == 0, "pthread_join should succeed");TEST_ASSERT(args.value == *(char *)(mem_nofault + offset),"memory should contain the value that was copied");TEST_ASSERT(args.value == *(char *)(mem + offset),"no further fault is expected");ret = munmap(mem_nofault, total_size);TEST_ASSERT(!ret, "munmap should succeed");ret = munmap(mem, total_size);TEST_ASSERT(!ret, "munmap should succeed");free(buf);close(uffd);+}
- #define gmem_test(__test, __vm, __flags) \ do { \ int fd = vm_create_guest_memfd(__vm, page_size * 4, __flags); \
@@ -273,6 +375,7 @@ static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags) if (flags & GUEST_MEMFD_FLAG_INIT_SHARED) { gmem_test(mmap_supported, vm, flags); gmem_test(fault_overflow, vm, flags);
gmem_test(uffd_minor, vm, flags); } else { gmem_test(fault_private, vm, flags); }-- 2.50.1
On Wed, Nov 26, 2025 at 04:49:31PM +0000, Nikita Kalyazin wrote:
On 25/11/2025 18:38, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
+#ifdef CONFIG_USERFAULTFD +static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t pgoff)
We have to name it differently, otherwise it clashes with the existing one in this file.
It's all David's fault! ;-P How about kvm_gmem_get_prepared_folio() ?
On Wed, Nov 26, 2025 at 04:49:46PM +0000, Nikita Kalyazin wrote:
On 25/11/2025 18:38, Mike Rapoport wrote:
From: Nikita Kalyazin kalyazin@amazon.com
+static void test_uffd_minor(int fd, size_t total_size) +{
struct uffdio_api uffdio_api = {.api = UFFD_API,.features = UFFD_FEATURE_MINOR_GENERIC,Should it be UFFD_FEATURE_MINOR_SHMEM instead? UFFD_FEATURE_MINOR_GENERIC was removed in the v1.
I'll drop .features completely, the checks in UFFDIO_REGISTER are sufficient.
On Tue, Nov 25, 2025 at 02:21:16PM -0500, Peter Xu wrote:
Hi, Mike,
On Tue, Nov 25, 2025 at 08:38:38PM +0200, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
When a VMA is registered with userfaulfd in minor mode, its ->fault() method should check if a folio exists in the page cache and if yes ->fault() should call handle_userfault(VM_UFFD_MISSING).
s/MISSING/MINOR/
Thanks, fixed.
new VM_FAULT_UFFD_MINOR there instead.
Personally I'd keep the fault path as simple as possible, because that's the more frequently used path (rather than when userfaultfd is armed). I also see it slightly a pity that even with flags introduced, it only solves the MINOR problem, not MISSING.
With David's suggestion the likely path remains unchanged.
As for MISSING, let's take it baby steps. We have enough space in vm_fault_reason for UFFD_MISSING if we'd want to pull handle_userfault() from shmem and hugetlb.
If it's me, I'd simply export handle_userfault().. I confess I still don't know why exporting it is a problem, but maybe I missed something.
It's not only about export, it's also about not requiring ->fault() methods for pte-mapped memory call handle_userfault().
Only my two cents. Feel free to go with whatever way you prefer.
Thanks,
-- Peter Xu
On 27/11/2025 10:36, Mike Rapoport wrote:
On Wed, Nov 26, 2025 at 04:49:31PM +0000, Nikita Kalyazin wrote:
On 25/11/2025 18:38, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
+#ifdef CONFIG_USERFAULTFD +static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t pgoff)
We have to name it differently, otherwise it clashes with the existing one in this file.
It's all David's fault! ;-P How about kvm_gmem_get_prepared_folio() ?
I'm afraid it may not be ideal due to preparedness tracking being removed from guest_memfd at some point [1]. Would it be too bad to add an indication to userfaultfd in the name somehow given that it's already guarded by the config?
[1] https://lore.kernel.org/linux-coco/20251113230759.1562024-1-michael.roth@amd...
-- Sincerely yours, Mike.
On 11/27/25 11:36, Mike Rapoport wrote:
On Wed, Nov 26, 2025 at 04:49:31PM +0000, Nikita Kalyazin wrote:
On 25/11/2025 18:38, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
+#ifdef CONFIG_USERFAULTFD +static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t pgoff)
We have to name it differently, otherwise it clashes with the existing one in this file.
It's all David's fault! ;-P
As usual :)
How about kvm_gmem_get_prepared_folio() ?
Or maybe just spell out that it is for vm_ops
kvm_gmem_vm_ops_get_folio()
On Thu, Nov 27, 2025 at 01:18:10PM +0200, Mike Rapoport wrote:
On Tue, Nov 25, 2025 at 02:21:16PM -0500, Peter Xu wrote:
Hi, Mike,
On Tue, Nov 25, 2025 at 08:38:38PM +0200, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
When a VMA is registered with userfaulfd in minor mode, its ->fault() method should check if a folio exists in the page cache and if yes ->fault() should call handle_userfault(VM_UFFD_MISSING).
s/MISSING/MINOR/
Thanks, fixed.
new VM_FAULT_UFFD_MINOR there instead.
Personally I'd keep the fault path as simple as possible, because that's the more frequently used path (rather than when userfaultfd is armed). I also see it slightly a pity that even with flags introduced, it only solves the MINOR problem, not MISSING.
With David's suggestion the likely path remains unchanged.
It is not about the likely, it's about introducing flags into core path that makes the core path harder to follow, when it's not strictly required.
Meanwhile, personally I'm also not sure if we should have "unlikely" here.. My gut feeling is in reality we will only have two major use cases:
(a) when userfaultfd minor isn't in the picture
(b) when userfaultfd minor registered and actively being used (e.g. in a postcopy process)
Then without likely, IIUC the hardware should optimize path selected hence both a+b performs almost equally well.
My guessing is after adding unlikely, (a) works well, but (b) works badly. We may need to measure it, IIUC it's part of the reason why we sometimes do not encourage "likely/unlikely". But that's only my guess, some numbers would be more helpful.
One thing we can try is if we add "unlikely" then compare a sequential MINOR fault trapping on shmem and measure the time it takes, we need to better make sure we don't regress perf there. I wonder if James / Axel would care about it - QEMU doesn't yet support minor, but will soon, and we will also prefer better perf since the start.
As for MISSING, let's take it baby steps. We have enough space in vm_fault_reason for UFFD_MISSING if we'd want to pull handle_userfault() from shmem and hugetlb.
Yep.
If it's me, I'd simply export handle_userfault().. I confess I still don't know why exporting it is a problem, but maybe I missed something.
It's not only about export, it's also about not requiring ->fault() methods for pte-mapped memory call handle_userfault().
I also don't see it a problem.. as what shmem used to do. Maybe it's a personal preference? If so, I don't have a strong opinion.
Just to mention, if we want, I think we have at least one more option to do the same thing, but without even introducing a new flag to ->fault() retval.
That is, when we have get_folio() around, we can essentially do two faults in sequence, one lighter then the real one, only for minor vmas, something like (I didn't think deeper, so only a rough idea shown):
__do_fault(): if (uffd_minor(vma)) { ... folio = vma->get_folio(...); if (folio) return handle_userfault(vmf, VM_UFFD_MINOR); // fallthrough, which imply a cache miss } ret = vma->vm_ops->fault(vmf); ...
The risk of above is also perf-wise, but it's another angle where it might slow down page cache miss case where MINOR is registered only (hence, when cache missing we'll need to call both get_folio() and fault() now). However that's likely a less critical case than the unlikely, and I'm also guessing due to the shared code of get_folio() / fault(), codes will be preheated and it may not be measureable even if we write it like that.
Then maybe we can avoid this new flag completely but also achieve the same goal.
Thanks,
On Thu, Nov 27, 2025 at 11:19:35AM +0000, Nikita Kalyazin wrote:
On 27/11/2025 10:36, Mike Rapoport wrote:
On Wed, Nov 26, 2025 at 04:49:31PM +0000, Nikita Kalyazin wrote:
On 25/11/2025 18:38, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
+#ifdef CONFIG_USERFAULTFD +static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t pgoff)
We have to name it differently, otherwise it clashes with the existing one in this file.
It's all David's fault! ;-P How about kvm_gmem_get_prepared_folio() ?
I'm afraid it may not be ideal due to preparedness tracking being removed from guest_memfd at some point [1]. Would it be too bad to add an indication to userfaultfd in the name somehow given that it's already guarded by the config?
Hmm, shmem also has this clash. There I picked shmem_get_folio_noalloc() because that describes well what it does: lookup folio in the page cache, grab it if it's there or return -ENOENT if it's missing. That's also what hugetlb does for uffd minor fault.
The guest_memfd implementation I copied from one of the older postings allocates the folio if it's not in the page cache and it seems to me that it also should only look up existing folios to keep uffd minor semantics uniform.
Then it makes sense also to name the vm_ops method get_folio_noalloc().
linux-kselftest-mirror@lists.linaro.org
-
David Hildenbrand (Red Hat) -
Liam R. Howlett -
Mike Rapoport -
Nikita Kalyazin -
Peter Xu