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_folio_noalloc() 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.
v3 changes: * rename ->get_folio() to ->get_folio_noalloc() * fix build errors reported by kbuild * pull handling of UFFD_MINOR out of hotpath in __do_fault() * update guest_memfs changes so its ->fault() and ->get_folio_noalloc() follow the same semantics as shmem and hugetlb. * s/MISSING/MINOR/g in changelogs * added review tags
v2: https://lore.kernel.org/all/20251125183840.2368510-1-rppt@kernel.org * 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 | 5 +- mm/shmem.c | 20 +++- mm/userfaultfd.c | 80 ++++++++++++--- .../testing/selftests/kvm/guest_memfd_test.c | 97 +++++++++++++++++++ virt/kvm/guest_memfd.c | 33 ++++++- 8 files changed, 236 insertions(+), 54 deletions(-)
base-commit: ac3fd01e4c1efce8f2c054cdeb2ddd2fc0fb150d
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 Reviewed-by: Liam R. Howlett Liam.Howlett@oracle.com 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_noalloc() 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_noalloc() method for shmem and slightly refactor userfaultfd's mfill_atomic() and mfill_atomic_pte_continue() to support this new API.
Acked-by: David Hildenbrand (Red Hat) david@kernel.org Reviewed-by: Liam R. Howlett Liam.Howlett@oracle.com 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..6351a9cde360 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_noalloc)(struct inode *inode, pgoff_t pgoff); +#endif };
#ifdef CONFIG_NUMA_BALANCING diff --git a/mm/shmem.c b/mm/shmem.c index 5a3f0f754dc0..9f8c54ad0e32 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -3262,6 +3262,18 @@ 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 @@ -5294,6 +5306,9 @@ 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_noalloc = shmem_get_folio_noalloc, +#endif };
static const struct vm_operations_struct shmem_anon_vm_ops = { @@ -5303,6 +5318,9 @@ 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_noalloc = 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..5610f29dac73 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_noalloc(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_noalloc; + + 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_noalloc) + 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_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_MINOR) to complete a page fault.
Replace a call to handle_userfault(VM_UFFD_MINOR) 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 | 5 ++++- mm/shmem.c | 2 +- 3 files changed, 14 insertions(+), 3 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 90e5790c318f..c92a52c572c3 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..8d2180ec6933 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -5280,8 +5280,11 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
ret = vma->vm_ops->fault(vmf); if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY | - VM_FAULT_DONE_COW))) + VM_FAULT_DONE_COW | VM_FAULT_UFFD_MINOR))) { + if (ret & VM_FAULT_UFFD_MINOR) + return handle_userfault(vmf, VM_UFFD_MINOR); return ret; + }
folio = page_folio(vmf->page); if (unlikely(PageHWPoison(vmf->page))) { diff --git a/mm/shmem.c b/mm/shmem.c index 9f8c54ad0e32..2c32e2398e99 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2460,7 +2460,7 @@ 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_folio_noalloc() to guest_memfd vm_ops.
Reviewed-by: Liam R. Howlett Liam.Howlett@oracle.com Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org --- virt/kvm/guest_memfd.c | 33 ++++++++++++++++++++++++++++++++- 1 file changed, 32 insertions(+), 1 deletion(-)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index ffadc5ee8e04..dca6e373937b 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"
@@ -359,7 +360,15 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) if (!((u64)inode->i_private & GUEST_MEMFD_FLAG_INIT_SHARED)) return VM_FAULT_SIGBUS;
- folio = kvm_gmem_get_folio(inode, vmf->pgoff); + folio = filemap_lock_folio(inode->i_mapping, vmf->pgoff); + if (!IS_ERR_OR_NULL(folio) && userfaultfd_minor(vmf->vma)) { + ret = VM_FAULT_UFFD_MINOR; + goto out_folio; + } + + if (PTR_ERR(folio) == -ENOENT) + folio = kvm_gmem_get_folio(inode, vmf->pgoff); + if (IS_ERR(folio)) { int err = PTR_ERR(folio);
@@ -390,8 +399,30 @@ 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_noalloc(struct inode *inode, + pgoff_t pgoff) +{ + struct folio *folio; + + folio = filemap_lock_folio(inode->i_mapping, 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_noalloc = kvm_gmem_get_folio_noalloc, +#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.
Acked-by: Liam R. Howlett Liam.Howlett@oracle.com 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 | 97 +++++++++++++++++++ 1 file changed, 97 insertions(+)
diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c index e7d9aeb418d3..d0cf57d41cc9 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 +258,98 @@ 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, + }; + 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; + int uffd, ret; + off_t offset = page_size; + void *fault_addr; + + 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"); + + memset(mem_nofault + offset, 0xaa, page_size); + + 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"); + + + 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"); + close(uffd); +} + #define gmem_test(__test, __vm, __flags) \ do { \ int fd = vm_create_guest_memfd(__vm, page_size * 4, __flags); \ @@ -273,6 +369,7 @@ 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); }
On 11/30/25 12:18, 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_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_MINOR) to complete a page fault.
Replace a call to handle_userfault(VM_UFFD_MINOR) 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
LGTM and does not look too invasive for me.
Acked-by: David Hildenbrand (Red Hat) david@kernel.org
On 11/30/25 12:18, 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_folio_noalloc() to guest_memfd vm_ops.
Reviewed-by: Liam R. Howlett Liam.Howlett@oracle.com Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
virt/kvm/guest_memfd.c | 33 ++++++++++++++++++++++++++++++++- 1 file changed, 32 insertions(+), 1 deletion(-)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index ffadc5ee8e04..dca6e373937b 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" @@ -359,7 +360,15 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) if (!((u64)inode->i_private & GUEST_MEMFD_FLAG_INIT_SHARED)) return VM_FAULT_SIGBUS;
- folio = kvm_gmem_get_folio(inode, vmf->pgoff);
- folio = filemap_lock_folio(inode->i_mapping, vmf->pgoff);
- if (!IS_ERR_OR_NULL(folio) && userfaultfd_minor(vmf->vma)) {
Can we ever get NULL here?
ret = VM_FAULT_UFFD_MINOR;goto out_folio;- }
- if (PTR_ERR(folio) == -ENOENT)
folio = kvm_gmem_get_folio(inode, vmf->pgoff);
Was briefly wondering what the performance impact of that two-step approach is (two lookups in case we have to create it IIUC)
Wouldn't it be better to limit it to the userfaultfd_minor(vmf->vma) case?
if (userfaultfd_minor(vmf->vma)) { folio = filemap_lock_folio(inode->i_mapping, vmf->pgoff); if (!IS_ERR(folio)) { ret = VM_FAULT_UFFD_MINOR; goto out_folio; } } else { folio = kvm_gmem_get_folio(inode, vmf->pgoff); }
if (IS_ERR(folio)) { ...
On 30/11/2025 11:18, 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_folio_noalloc() to guest_memfd vm_ops.
Reviewed-by: Liam R. Howlett Liam.Howlett@oracle.com Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
virt/kvm/guest_memfd.c | 33 ++++++++++++++++++++++++++++++++- 1 file changed, 32 insertions(+), 1 deletion(-)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index ffadc5ee8e04..dca6e373937b 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"
@@ -359,7 +360,15 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) if (!((u64)inode->i_private & GUEST_MEMFD_FLAG_INIT_SHARED)) return VM_FAULT_SIGBUS;
folio = kvm_gmem_get_folio(inode, vmf->pgoff);
folio = filemap_lock_folio(inode->i_mapping, vmf->pgoff);if (!IS_ERR_OR_NULL(folio) && userfaultfd_minor(vmf->vma)) {ret = VM_FAULT_UFFD_MINOR;goto out_folio;}
I realised that I might have been wrong in [1] saying that the noalloc get folio was ok for our use case. Unfortunately we rely on a minor fault to get generated even when the page is being allocated. Peter and I discussed it originally in [2]. Since we want to populate guest memory with the content supplied by userspace on demand, we have to be able to intercept the very first access, meaning we either need a minor or major UFFD event for that. We decided to make use of the minor at the time. If we have to preserve the shmem semantics, it forces us to implement support for major/UFFDIO_COPY.
[1] https://lore.kernel.org/all/4405c306-9d7c-4fd6-9ea6-2ed1b73f5c2e@amazon.com [2] https://lore.kernel.org/kvm/Z9HhTjEWtM58Zfxf@x1.local
if (PTR_ERR(folio) == -ENOENT)folio = kvm_gmem_get_folio(inode, vmf->pgoff);if (IS_ERR(folio)) { int err = PTR_ERR(folio);@@ -390,8 +399,30 @@ 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_noalloc(struct inode *inode,
pgoff_t pgoff)+{
struct folio *folio;folio = filemap_lock_folio(inode->i_mapping, 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_noalloc = kvm_gmem_get_folio_noalloc,+#endif };
static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
2.51.0
On 12/1/25 14:39, Nikita Kalyazin wrote:
On 30/11/2025 11:18, 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_folio_noalloc() to guest_memfd vm_ops.
Reviewed-by: Liam R. Howlett Liam.Howlett@oracle.com Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
virt/kvm/guest_memfd.c | 33 ++++++++++++++++++++++++++++++++- 1 file changed, 32 insertions(+), 1 deletion(-)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index ffadc5ee8e04..dca6e373937b 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"
@@ -359,7 +360,15 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) if (!((u64)inode->i_private & GUEST_MEMFD_FLAG_INIT_SHARED)) return VM_FAULT_SIGBUS;
folio = kvm_gmem_get_folio(inode, vmf->pgoff);
folio = filemap_lock_folio(inode->i_mapping, vmf->pgoff);if (!IS_ERR_OR_NULL(folio) && userfaultfd_minor(vmf->vma)) {ret = VM_FAULT_UFFD_MINOR;goto out_folio;}I realised that I might have been wrong in [1] saying that the noalloc get folio was ok for our use case. Unfortunately we rely on a minor fault to get generated even when the page is being allocated. Peter and I discussed it originally in [2]. Since we want to populate guest memory with the content supplied by userspace on demand, we have to be able to intercept the very first access, meaning we either need a minor or major UFFD event for that. We decided to make use of the minor at the time. If we have to preserve the shmem semantics, it forces us to implement support for major/UFFDIO_COPY.
If we want missing semantics then likely we should be adding ... missing support? :)
On 01/12/2025 15:54, David Hildenbrand (Red Hat) wrote:
On 12/1/25 14:39, Nikita Kalyazin wrote:
On 30/11/2025 11:18, 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_folio_noalloc() to guest_memfd vm_ops.
Reviewed-by: Liam R. Howlett Liam.Howlett@oracle.com Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
virt/kvm/guest_memfd.c | 33 ++++++++++++++++++++++++++++++++- 1 file changed, 32 insertions(+), 1 deletion(-)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index ffadc5ee8e04..dca6e373937b 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"
@@ -359,7 +360,15 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) if (!((u64)inode->i_private & GUEST_MEMFD_FLAG_INIT_SHARED)) return VM_FAULT_SIGBUS;
- folio = kvm_gmem_get_folio(inode, vmf->pgoff); + folio = filemap_lock_folio(inode->i_mapping, vmf->pgoff); + if (!IS_ERR_OR_NULL(folio) && userfaultfd_minor(vmf->vma)) { + ret = VM_FAULT_UFFD_MINOR; + goto out_folio; + }
I realised that I might have been wrong in [1] saying that the noalloc get folio was ok for our use case. Unfortunately we rely on a minor fault to get generated even when the page is being allocated. Peter and I discussed it originally in [2]. Since we want to populate guest memory with the content supplied by userspace on demand, we have to be able to intercept the very first access, meaning we either need a minor or major UFFD event for that. We decided to make use of the minor at the time. If we have to preserve the shmem semantics, it forces us to implement support for major/UFFDIO_COPY.
If we want missing semantics then likely we should be adding ... missing support? :)
I believe I found the precise point where we convinced ourselves that minor support was sufficient: [1]. If at this moment we don't find that reasoning valid anymore, then indeed implementing missing is the only option.
[1] https://lore.kernel.org/kvm/Z9GsIDVYWoV8d8-C@x1.local
-- Cheers
David
On Mon, Dec 01, 2025 at 04:48:22PM +0000, Nikita Kalyazin wrote:
I believe I found the precise point where we convinced ourselves that minor support was sufficient: [1]. If at this moment we don't find that reasoning valid anymore, then indeed implementing missing is the only option.
Now after I re-read the discussion, I may have made a wrong statement there, sorry. I could have got slightly confused on when the write() syscall can be involved.
I agree if you want to get an event when cache missed with the current uffd definitions and when pre-population is forbidden, then MISSING trap is required. That is, with/without the need of UFFDIO_COPY being available.
Do I understand it right that UFFDIO_COPY is not allowed in your case, but only write()?
One way that might work this around, is introducing a new UFFD_FEATURE bit allowing the MINOR registration to trap all pgtable faults, which will change the MINOR fault semantics.
That'll need some further thoughts, meanwhile we may also want to make sure the old shmem/hugetlbfs semantics are kept (e.g. they should fail MINOR registers if the new feature bit is enabled in the ctx somehow; or support them properly in the codebase).
Thanks,
On 01/12/2025 18:35, Peter Xu wrote:
On Mon, Dec 01, 2025 at 04:48:22PM +0000, Nikita Kalyazin wrote:
I believe I found the precise point where we convinced ourselves that minor support was sufficient: [1]. If at this moment we don't find that reasoning valid anymore, then indeed implementing missing is the only option.
Now after I re-read the discussion, I may have made a wrong statement there, sorry. I could have got slightly confused on when the write() syscall can be involved.
I agree if you want to get an event when cache missed with the current uffd definitions and when pre-population is forbidden, then MISSING trap is required. That is, with/without the need of UFFDIO_COPY being available.
Do I understand it right that UFFDIO_COPY is not allowed in your case, but only write()?
No, UFFDIO_COPY would work perfectly fine. We will still use write() whenever we resolve stage-2 faults as they aren't visible to UFFD. When a userfault occurs at an offset that already has a page in the cache, we will have to keep using UFFDIO_CONTINUE so it looks like both will be required:
- user mapping major fault -> UFFDIO_COPY (fills the cache and sets up userspace PT) - user mapping minor fault -> UFFDIO_CONTINUE (only sets up userspace PT) - stage-2 fault -> write() (only fills the cache)
One way that might work this around, is introducing a new UFFD_FEATURE bit allowing the MINOR registration to trap all pgtable faults, which will change the MINOR fault semantics.
This would equally work for us. I suppose this MINOR+MAJOR semantics would be more intrusive from the API point of view though.
That'll need some further thoughts, meanwhile we may also want to make sure the old shmem/hugetlbfs semantics are kept (e.g. they should fail MINOR registers if the new feature bit is enabled in the ctx somehow; or support them properly in the codebase).
Thanks,
-- Peter Xu
On Mon, Dec 01, 2025 at 08:12:38PM +0000, Nikita Kalyazin wrote:
On 01/12/2025 18:35, Peter Xu wrote:
On Mon, Dec 01, 2025 at 04:48:22PM +0000, Nikita Kalyazin wrote:
I believe I found the precise point where we convinced ourselves that minor support was sufficient: [1]. If at this moment we don't find that reasoning valid anymore, then indeed implementing missing is the only option.
Now after I re-read the discussion, I may have made a wrong statement there, sorry. I could have got slightly confused on when the write() syscall can be involved.
I agree if you want to get an event when cache missed with the current uffd definitions and when pre-population is forbidden, then MISSING trap is required. That is, with/without the need of UFFDIO_COPY being available.
Do I understand it right that UFFDIO_COPY is not allowed in your case, but only write()?
No, UFFDIO_COPY would work perfectly fine. We will still use write() whenever we resolve stage-2 faults as they aren't visible to UFFD. When a userfault occurs at an offset that already has a page in the cache, we will have to keep using UFFDIO_CONTINUE so it looks like both will be required:
- user mapping major fault -> UFFDIO_COPY (fills the cache and sets up
userspace PT)
- user mapping minor fault -> UFFDIO_CONTINUE (only sets up userspace PT)
- stage-2 fault -> write() (only fills the cache)
Is stage-2 fault about KVM_MEMORY_EXIT_FLAG_USERFAULT, per James's series?
It looks fine indeed, but it looks slightly weird then, as you'll have two ways to populate the page cache. Logically here atomicity is indeed not needed when you trap both MISSING + MINOR.
One way that might work this around, is introducing a new UFFD_FEATURE bit allowing the MINOR registration to trap all pgtable faults, which will change the MINOR fault semantics.
This would equally work for us. I suppose this MINOR+MAJOR semantics would be more intrusive from the API point of view though.
Yes it is, it's just that I don't know whether it'll be harder when you want to completely support UFFDIO_COPY here, per previous discussions.
After a 2nd thought, such UFFD_FEATURE is probably not a good design, because it essentially means that feature bit will functionally overlap with what MISSING trap was trying to do, however duplicating that concept in a VMA that was registered as MINOR only.
Maybe it's possible instead if we allow a module to support MISSING trap, but without supporting UFFDIO_COPY ioctl.
That is, the MISSING events will be properly generated if MISSING traps are supported, however the module needs to provide its own way to resolve it if UFFDIO_COPY ioctl isn't available. Gmem is fine in this case as long as it'll always be registered with both MISSING+MINOR traps, then resolving using write()s would work.
Such would be possible when with something like my v3 previously:
https://lore.kernel.org/all/20250926211650.525109-1-peterx@redhat.com/#t
Then gmem needs to declare VM_UFFD_MISSING + VM_UFFD_MINOR in uffd_features, but _UFFDIO_CONTINUE only (without _UFFDIO_COPY) in uffd_ioctls.
Since Mike already took this series over, I'll leave that to you all to decide.
On 01/12/2025 20:57, Peter Xu wrote:
On Mon, Dec 01, 2025 at 08:12:38PM +0000, Nikita Kalyazin wrote:
On 01/12/2025 18:35, Peter Xu wrote:
On Mon, Dec 01, 2025 at 04:48:22PM +0000, Nikita Kalyazin wrote:
I believe I found the precise point where we convinced ourselves that minor support was sufficient: [1]. If at this moment we don't find that reasoning valid anymore, then indeed implementing missing is the only option.
Now after I re-read the discussion, I may have made a wrong statement there, sorry. I could have got slightly confused on when the write() syscall can be involved.
I agree if you want to get an event when cache missed with the current uffd definitions and when pre-population is forbidden, then MISSING trap is required. That is, with/without the need of UFFDIO_COPY being available.
Do I understand it right that UFFDIO_COPY is not allowed in your case, but only write()?
No, UFFDIO_COPY would work perfectly fine. We will still use write() whenever we resolve stage-2 faults as they aren't visible to UFFD. When a userfault occurs at an offset that already has a page in the cache, we will have to keep using UFFDIO_CONTINUE so it looks like both will be required:
- user mapping major fault -> UFFDIO_COPY (fills the cache and sets up
userspace PT)
- user mapping minor fault -> UFFDIO_CONTINUE (only sets up userspace PT)
- stage-2 fault -> write() (only fills the cache)
Is stage-2 fault about KVM_MEMORY_EXIT_FLAG_USERFAULT, per James's series?
Yes, that's the one ([1]).
[1] https://lore.kernel.org/kvm/20250618042424.330664-1-jthoughton@google.com
It looks fine indeed, but it looks slightly weird then, as you'll have two ways to populate the page cache. Logically here atomicity is indeed not needed when you trap both MISSING + MINOR.
I reran the test based on the UFFDIO_COPY prototype I had using your series [2], and UFFDIO_COPY is slower than write() to populate 512 MiB: 237 vs 202 ms (+17%). Even though UFFDIO_COPY alone is functionally sufficient, I would prefer to have an option to use write() where possible and only falling back to UFFDIO_COPY for userspace faults to have better performance.
[2] https://lore.kernel.org/all/7666ee96-6f09-4dc1-8cb2-002a2d2a29cf@amazon.com
One way that might work this around, is introducing a new UFFD_FEATURE bit allowing the MINOR registration to trap all pgtable faults, which will change the MINOR fault semantics.
This would equally work for us. I suppose this MINOR+MAJOR semantics would be more intrusive from the API point of view though.
Yes it is, it's just that I don't know whether it'll be harder when you want to completely support UFFDIO_COPY here, per previous discussions.
After a 2nd thought, such UFFD_FEATURE is probably not a good design, because it essentially means that feature bit will functionally overlap with what MISSING trap was trying to do, however duplicating that concept in a VMA that was registered as MINOR only.
Maybe it's possible instead if we allow a module to support MISSING trap, but without supporting UFFDIO_COPY ioctl.
That is, the MISSING events will be properly generated if MISSING traps are supported, however the module needs to provide its own way to resolve it if UFFDIO_COPY ioctl isn't available. Gmem is fine in this case as long as it'll always be registered with both MISSING+MINOR traps, then resolving using write()s would work.
Yes, this would also work for me. This is almost how it was (accidentally) working until this version of the patches. If this is a lighter undertaking compared to implementing UFFDIO_COPY, I'd be happy with it too.
Such would be possible when with something like my v3 previously:
https://lore.kernel.org/all/20250926211650.525109-1-peterx@redhat.com/#t
Then gmem needs to declare VM_UFFD_MISSING + VM_UFFD_MINOR in uffd_features, but _UFFDIO_CONTINUE only (without _UFFDIO_COPY) in uffd_ioctls.
Since Mike already took this series over, I'll leave that to you all to decide.
Thanks for you input, Peter.
-- Peter Xu
linux-kselftest-mirror@lists.linaro.org
-
David Hildenbrand (Red Hat) -
Mike Rapoport -
Nikita Kalyazin -
Peter Xu