From: Andrea Arcangeli aarcange@redhat.com
This implements the uABI of UFFDIO_REMAP.
Notably one mode bitflag is also forwarded (and in turn known) by the lowlevel remap_pages method.
Signed-off-by: Andrea Arcangeli aarcange@redhat.com Signed-off-by: Suren Baghdasaryan surenb@google.com --- Changes since v1: - add mmget_not_zero in userfaultfd_remap, per Jann Horn - removed extern from function definitions, per Matthew Wilcox - converted to folios in remap_pages_huge_pmd, per Matthew Wilcox - use PageAnonExclusive in remap_pages_huge_pmd, per David Hildenbrand - handle pgtable transfers between MMs, per Jann Horn - ignore concurrent A/D pte bit changes, per Jann Horn - split functions into smaller units, per David Hildenbrand - test for folio_test_large in remap_anon_pte, per Matthew Wilcox - use pte_swp_exclusive for swapcount check, per David Hildenbrand - eliminated use of mmu_notifier_invalidate_range_start_nonblock, per Jann Horn - simplified THP alignment checks, per Jann Horn - refactored the loop inside remap_pages, per Jann Horn - additional clarifying comments, per Jann Horn
fs/userfaultfd.c | 63 ++++ include/linux/rmap.h | 5 + include/linux/userfaultfd_k.h | 12 + include/uapi/linux/userfaultfd.h | 22 ++ mm/huge_memory.c | 130 +++++++ mm/khugepaged.c | 3 + mm/userfaultfd.c | 590 +++++++++++++++++++++++++++++++ 7 files changed, 825 insertions(+)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 56eaae9dac1a..5b6bb20f4518 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -2027,6 +2027,66 @@ static inline unsigned int uffd_ctx_features(__u64 user_features) return (unsigned int)user_features | UFFD_FEATURE_INITIALIZED; }
+static int userfaultfd_remap(struct userfaultfd_ctx *ctx, + unsigned long arg) +{ + __s64 ret; + struct uffdio_remap uffdio_remap; + struct uffdio_remap __user *user_uffdio_remap; + struct userfaultfd_wake_range range; + + user_uffdio_remap = (struct uffdio_remap __user *) arg; + + ret = -EAGAIN; + if (atomic_read(&ctx->mmap_changing)) + goto out; + + ret = -EFAULT; + if (copy_from_user(&uffdio_remap, user_uffdio_remap, + /* don't copy "remap" last field */ + sizeof(uffdio_remap)-sizeof(__s64))) + goto out; + + ret = validate_range(ctx->mm, uffdio_remap.dst, uffdio_remap.len); + if (ret) + goto out; + + ret = validate_range(current->mm, uffdio_remap.src, uffdio_remap.len); + if (ret) + goto out; + + ret = -EINVAL; + if (uffdio_remap.mode & ~(UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES| + UFFDIO_REMAP_MODE_DONTWAKE)) + goto out; + + if (mmget_not_zero(ctx->mm)) { + ret = remap_pages(ctx->mm, current->mm, + uffdio_remap.dst, uffdio_remap.src, + uffdio_remap.len, uffdio_remap.mode); + mmput(ctx->mm); + } else { + return -ESRCH; + } + + if (unlikely(put_user(ret, &user_uffdio_remap->remap))) + return -EFAULT; + if (ret < 0) + goto out; + + /* len == 0 would wake all */ + BUG_ON(!ret); + range.len = ret; + if (!(uffdio_remap.mode & UFFDIO_REMAP_MODE_DONTWAKE)) { + range.start = uffdio_remap.dst; + wake_userfault(ctx, &range); + } + ret = range.len == uffdio_remap.len ? 0 : -EAGAIN; + +out: + return ret; +} + /* * userland asks for a certain API version and we return which bits * and ioctl commands are implemented in this kernel for such API @@ -2113,6 +2173,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd, case UFFDIO_ZEROPAGE: ret = userfaultfd_zeropage(ctx, arg); break; + case UFFDIO_REMAP: + ret = userfaultfd_remap(ctx, arg); + break; case UFFDIO_WRITEPROTECT: ret = userfaultfd_writeprotect(ctx, arg); break; diff --git a/include/linux/rmap.h b/include/linux/rmap.h index 51cc21ebb568..614c4b439907 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -121,6 +121,11 @@ static inline void anon_vma_lock_write(struct anon_vma *anon_vma) down_write(&anon_vma->root->rwsem); }
+static inline int anon_vma_trylock_write(struct anon_vma *anon_vma) +{ + return down_write_trylock(&anon_vma->root->rwsem); +} + static inline void anon_vma_unlock_write(struct anon_vma *anon_vma) { up_write(&anon_vma->root->rwsem); diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h index ac8c6854097c..9ea2c43ad4b7 100644 --- a/include/linux/userfaultfd_k.h +++ b/include/linux/userfaultfd_k.h @@ -93,6 +93,18 @@ extern int mwriteprotect_range(struct mm_struct *dst_mm, extern long uffd_wp_range(struct vm_area_struct *vma, unsigned long start, unsigned long len, bool enable_wp);
+/* remap_pages */ +void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2); +void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2); +ssize_t remap_pages(struct mm_struct *dst_mm, struct mm_struct *src_mm, + unsigned long dst_start, unsigned long src_start, + unsigned long len, __u64 flags); +int remap_pages_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, + pmd_t *dst_pmd, pmd_t *src_pmd, pmd_t dst_pmdval, + struct vm_area_struct *dst_vma, + struct vm_area_struct *src_vma, + unsigned long dst_addr, unsigned long src_addr); + /* mm helpers */ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma, struct vm_userfaultfd_ctx vm_ctx) diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h index 62151706c5a3..22d1c43e39f9 100644 --- a/include/uapi/linux/userfaultfd.h +++ b/include/uapi/linux/userfaultfd.h @@ -49,6 +49,7 @@ ((__u64)1 << _UFFDIO_WAKE | \ (__u64)1 << _UFFDIO_COPY | \ (__u64)1 << _UFFDIO_ZEROPAGE | \ + (__u64)1 << _UFFDIO_REMAP | \ (__u64)1 << _UFFDIO_WRITEPROTECT | \ (__u64)1 << _UFFDIO_CONTINUE | \ (__u64)1 << _UFFDIO_POISON) @@ -72,6 +73,7 @@ #define _UFFDIO_WAKE (0x02) #define _UFFDIO_COPY (0x03) #define _UFFDIO_ZEROPAGE (0x04) +#define _UFFDIO_REMAP (0x05) #define _UFFDIO_WRITEPROTECT (0x06) #define _UFFDIO_CONTINUE (0x07) #define _UFFDIO_POISON (0x08) @@ -91,6 +93,8 @@ struct uffdio_copy) #define UFFDIO_ZEROPAGE _IOWR(UFFDIO, _UFFDIO_ZEROPAGE, \ struct uffdio_zeropage) +#define UFFDIO_REMAP _IOWR(UFFDIO, _UFFDIO_REMAP, \ + struct uffdio_remap) #define UFFDIO_WRITEPROTECT _IOWR(UFFDIO, _UFFDIO_WRITEPROTECT, \ struct uffdio_writeprotect) #define UFFDIO_CONTINUE _IOWR(UFFDIO, _UFFDIO_CONTINUE, \ @@ -340,6 +344,24 @@ struct uffdio_poison { __s64 updated; };
+struct uffdio_remap { + __u64 dst; + __u64 src; + __u64 len; + /* + * Especially if used to atomically remove memory from the + * address space the wake on the dst range is not needed. + */ +#define UFFDIO_REMAP_MODE_DONTWAKE ((__u64)1<<0) +#define UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES ((__u64)1<<1) + __u64 mode; + /* + * "remap" is written by the ioctl and must be at the end: the + * copy_from_user will not read the last 8 bytes. + */ + __s64 remap; +}; + /* * Flags for the userfaultfd(2) system call itself. */ diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 064fbd90822b..a8c898df36db 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1932,6 +1932,136 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, return ret; }
+#ifdef CONFIG_USERFAULTFD +/* + * The PT lock for src_pmd and the mmap_lock for reading are held by + * the caller, but it must return after releasing the + * page_table_lock. We're guaranteed the src_pmd is a pmd_trans_huge + * until the PT lock of the src_pmd is released. Just move the page + * from src_pmd to dst_pmd if possible. Return zero if succeeded in + * moving the page, -EAGAIN if it needs to be repeated by the caller, + * or other errors in case of failure. + */ +int remap_pages_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, + pmd_t *dst_pmd, pmd_t *src_pmd, pmd_t dst_pmdval, + struct vm_area_struct *dst_vma, + struct vm_area_struct *src_vma, + unsigned long dst_addr, unsigned long src_addr) +{ + pmd_t _dst_pmd, src_pmdval; + struct page *src_page; + struct folio *src_folio; + struct anon_vma *src_anon_vma, *dst_anon_vma; + spinlock_t *src_ptl, *dst_ptl; + pgtable_t src_pgtable, dst_pgtable; + struct mmu_notifier_range range; + int err = 0; + + src_pmdval = *src_pmd; + src_ptl = pmd_lockptr(src_mm, src_pmd); + + BUG_ON(!spin_is_locked(src_ptl)); + mmap_assert_locked(src_mm); + mmap_assert_locked(dst_mm); + + BUG_ON(!pmd_trans_huge(src_pmdval)); + BUG_ON(!pmd_none(dst_pmdval)); + BUG_ON(src_addr & ~HPAGE_PMD_MASK); + BUG_ON(dst_addr & ~HPAGE_PMD_MASK); + + src_page = pmd_page(src_pmdval); + if (unlikely(!PageAnonExclusive(src_page))) { + spin_unlock(src_ptl); + return -EBUSY; + } + + src_folio = page_folio(src_page); + folio_get(src_folio); + spin_unlock(src_ptl); + + /* preallocate dst_pgtable if needed */ + if (dst_mm != src_mm) { + dst_pgtable = pte_alloc_one(dst_mm); + if (unlikely(!dst_pgtable)) { + err = -ENOMEM; + goto put_folio; + } + } else { + dst_pgtable = NULL; + } + + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, src_mm, src_addr, + src_addr + HPAGE_PMD_SIZE); + mmu_notifier_invalidate_range_start(&range); + + /* block all concurrent rmap walks */ + folio_lock(src_folio); + + /* + * split_huge_page walks the anon_vma chain without the page + * lock. Serialize against it with the anon_vma lock, the page + * lock is not enough. + */ + src_anon_vma = folio_get_anon_vma(src_folio); + if (!src_anon_vma) { + err = -EAGAIN; + goto unlock_folio; + } + anon_vma_lock_write(src_anon_vma); + + dst_ptl = pmd_lockptr(dst_mm, dst_pmd); + double_pt_lock(src_ptl, dst_ptl); + if (unlikely(!pmd_same(*src_pmd, src_pmdval) || + !pmd_same(*dst_pmd, dst_pmdval) || + folio_mapcount(src_folio) != 1)) { + double_pt_unlock(src_ptl, dst_ptl); + err = -EAGAIN; + goto put_anon_vma; + } + + BUG_ON(!folio_test_head(src_folio)); + BUG_ON(!folio_test_anon(src_folio)); + + dst_anon_vma = (void *)dst_vma->anon_vma + PAGE_MAPPING_ANON; + WRITE_ONCE(src_folio->mapping, (struct address_space *) dst_anon_vma); + WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr)); + + src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd); + _dst_pmd = mk_huge_pmd(&src_folio->page, dst_vma->vm_page_prot); + _dst_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma); + set_pmd_at(dst_mm, dst_addr, dst_pmd, _dst_pmd); + + src_pgtable = pgtable_trans_huge_withdraw(src_mm, src_pmd); + if (dst_pgtable) { + pgtable_trans_huge_deposit(dst_mm, dst_pmd, dst_pgtable); + pte_free(src_mm, src_pgtable); + dst_pgtable = NULL; + + mm_inc_nr_ptes(dst_mm); + mm_dec_nr_ptes(src_mm); + add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR); + add_mm_counter(src_mm, MM_ANONPAGES, -HPAGE_PMD_NR); + } else { + pgtable_trans_huge_deposit(dst_mm, dst_pmd, src_pgtable); + } + double_pt_unlock(src_ptl, dst_ptl); + +put_anon_vma: + anon_vma_unlock_write(src_anon_vma); + put_anon_vma(src_anon_vma); +unlock_folio: + /* unblock rmap walks */ + folio_unlock(src_folio); + mmu_notifier_invalidate_range_end(&range); + if (dst_pgtable) + pte_free(dst_mm, dst_pgtable); +put_folio: + folio_put(src_folio); + + return err; +} +#endif /* CONFIG_USERFAULTFD */ + /* * Returns page table lock pointer if a given pmd maps a thp, NULL otherwise. * diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 88433cc25d8a..af23248b3551 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1135,6 +1135,9 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address, * Prevent all access to pagetables with the exception of * gup_fast later handled by the ptep_clear_flush and the VM * handled by the anon_vma lock + PG_lock. + * + * UFFDIO_REMAP is prevented to race as well thanks to the + * mmap_lock. */ mmap_write_lock(mm); result = hugepage_vma_revalidate(mm, address, true, &vma, cc); diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 96d9eae5c7cc..5ce5e364373c 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -842,3 +842,593 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start, mmap_read_unlock(dst_mm); return err; } + + +void double_pt_lock(spinlock_t *ptl1, + spinlock_t *ptl2) + __acquires(ptl1) + __acquires(ptl2) +{ + spinlock_t *ptl_tmp; + + if (ptl1 > ptl2) { + /* exchange ptl1 and ptl2 */ + ptl_tmp = ptl1; + ptl1 = ptl2; + ptl2 = ptl_tmp; + } + /* lock in virtual address order to avoid lock inversion */ + spin_lock(ptl1); + if (ptl1 != ptl2) + spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING); + else + __acquire(ptl2); +} + +void double_pt_unlock(spinlock_t *ptl1, + spinlock_t *ptl2) + __releases(ptl1) + __releases(ptl2) +{ + spin_unlock(ptl1); + if (ptl1 != ptl2) + spin_unlock(ptl2); + else + __release(ptl2); +} + + +static int remap_anon_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, + struct vm_area_struct *dst_vma, + struct vm_area_struct *src_vma, + unsigned long dst_addr, unsigned long src_addr, + pte_t *dst_pte, pte_t *src_pte, + pte_t orig_dst_pte, pte_t orig_src_pte, + spinlock_t *dst_ptl, spinlock_t *src_ptl, + struct folio *src_folio) +{ + struct anon_vma *dst_anon_vma; + + double_pt_lock(dst_ptl, src_ptl); + + if (!pte_same(*src_pte, orig_src_pte) || + !pte_same(*dst_pte, orig_dst_pte) || + folio_test_large(src_folio) || + folio_estimated_sharers(src_folio) != 1) { + double_pt_unlock(dst_ptl, src_ptl); + return -EAGAIN; + } + + BUG_ON(!folio_test_anon(src_folio)); + + dst_anon_vma = (void *)dst_vma->anon_vma + PAGE_MAPPING_ANON; + WRITE_ONCE(src_folio->mapping, + (struct address_space *) dst_anon_vma); + WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, + dst_addr)); + + orig_src_pte = ptep_clear_flush(src_vma, src_addr, src_pte); + orig_dst_pte = mk_pte(&src_folio->page, dst_vma->vm_page_prot); + orig_dst_pte = maybe_mkwrite(pte_mkdirty(orig_dst_pte), + dst_vma); + + set_pte_at(dst_mm, dst_addr, dst_pte, orig_dst_pte); + + if (dst_mm != src_mm) { + inc_mm_counter(dst_mm, MM_ANONPAGES); + dec_mm_counter(src_mm, MM_ANONPAGES); + } + + double_pt_unlock(dst_ptl, src_ptl); + + return 0; +} + +static int remap_swap_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, + unsigned long dst_addr, unsigned long src_addr, + pte_t *dst_pte, pte_t *src_pte, + pte_t orig_dst_pte, pte_t orig_src_pte, + spinlock_t *dst_ptl, spinlock_t *src_ptl) +{ + if (!pte_swp_exclusive(orig_src_pte)) + return -EBUSY; + + double_pt_lock(dst_ptl, src_ptl); + + if (!pte_same(*src_pte, orig_src_pte) || + !pte_same(*dst_pte, orig_dst_pte)) { + double_pt_unlock(dst_ptl, src_ptl); + return -EAGAIN; + } + + orig_src_pte = ptep_get_and_clear(src_mm, src_addr, src_pte); + set_pte_at(dst_mm, dst_addr, dst_pte, orig_src_pte); + + if (dst_mm != src_mm) { + inc_mm_counter(dst_mm, MM_ANONPAGES); + dec_mm_counter(src_mm, MM_ANONPAGES); + } + + double_pt_unlock(dst_ptl, src_ptl); + + return 0; +} + +/* + * The mmap_lock for reading is held by the caller. Just move the page + * from src_pmd to dst_pmd if possible, and return true if succeeded + * in moving the page. + */ +static int remap_pages_pte(struct mm_struct *dst_mm, + struct mm_struct *src_mm, + pmd_t *dst_pmd, + pmd_t *src_pmd, + struct vm_area_struct *dst_vma, + struct vm_area_struct *src_vma, + unsigned long dst_addr, + unsigned long src_addr, + __u64 mode) +{ + swp_entry_t entry; + pte_t orig_src_pte, orig_dst_pte; + spinlock_t *src_ptl, *dst_ptl; + pte_t *src_pte = NULL; + pte_t *dst_pte = NULL; + + struct folio *src_folio = NULL; + struct anon_vma *src_anon_vma = NULL; + struct mmu_notifier_range range; + int err = 0; + + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, src_mm, + src_addr, src_addr + PAGE_SIZE); + mmu_notifier_invalidate_range_start(&range); +retry: + dst_pte = pte_offset_map_nolock(dst_mm, dst_pmd, dst_addr, &dst_ptl); + + /* If an huge pmd materialized from under us fail */ + if (unlikely(!dst_pte)) { + err = -EFAULT; + goto out; + } + + src_pte = pte_offset_map_nolock(src_mm, src_pmd, src_addr, &src_ptl); + + /* + * We held the mmap_lock for reading so MADV_DONTNEED + * can zap transparent huge pages under us, or the + * transparent huge page fault can establish new + * transparent huge pages under us. + */ + if (unlikely(!src_pte)) { + err = -EFAULT; + goto out; + } + + BUG_ON(pmd_none(*dst_pmd)); + BUG_ON(pmd_none(*src_pmd)); + BUG_ON(pmd_trans_huge(*dst_pmd)); + BUG_ON(pmd_trans_huge(*src_pmd)); + + spin_lock(dst_ptl); + orig_dst_pte = *dst_pte; + spin_unlock(dst_ptl); + if (!pte_none(orig_dst_pte)) { + err = -EEXIST; + goto out; + } + + spin_lock(src_ptl); + orig_src_pte = *src_pte; + spin_unlock(src_ptl); + if (pte_none(orig_src_pte)) { + if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES)) + err = -ENOENT; + else /* nothing to do to remap a hole */ + err = 0; + goto out; + } + + if (pte_present(orig_src_pte)) { + /* + * Pin and lock both source folio and anon_vma. Since we are in + * RCU read section, we can't block, so on contention have to + * unmap the ptes, obtain the lock and retry. + */ + if (!src_folio) { + struct folio *folio; + + /* + * Pin the page while holding the lock to be sure the + * page isn't freed under us + */ + spin_lock(src_ptl); + if (!pte_same(orig_src_pte, *src_pte)) { + spin_unlock(src_ptl); + err = -EAGAIN; + goto out; + } + + folio = vm_normal_folio(src_vma, src_addr, orig_src_pte); + if (!folio || !folio_test_anon(folio) || + folio_test_large(folio) || + folio_estimated_sharers(folio) != 1) { + spin_unlock(src_ptl); + err = -EBUSY; + goto out; + } + + folio_get(folio); + src_folio = folio; + spin_unlock(src_ptl); + + /* block all concurrent rmap walks */ + if (!folio_trylock(src_folio)) { + pte_unmap(&orig_src_pte); + pte_unmap(&orig_dst_pte); + src_pte = dst_pte = NULL; + /* now we can block and wait */ + folio_lock(src_folio); + goto retry; + } + } + + if (!src_anon_vma) { + /* + * folio_referenced walks the anon_vma chain + * without the folio lock. Serialize against it with + * the anon_vma lock, the folio lock is not enough. + */ + src_anon_vma = folio_get_anon_vma(src_folio); + if (!src_anon_vma) { + /* page was unmapped from under us */ + err = -EAGAIN; + goto out; + } + if (!anon_vma_trylock_write(src_anon_vma)) { + pte_unmap(&orig_src_pte); + pte_unmap(&orig_dst_pte); + src_pte = dst_pte = NULL; + /* now we can block and wait */ + anon_vma_lock_write(src_anon_vma); + goto retry; + } + } + + err = remap_anon_pte(dst_mm, src_mm, dst_vma, src_vma, + dst_addr, src_addr, dst_pte, src_pte, + orig_dst_pte, orig_src_pte, + dst_ptl, src_ptl, src_folio); + } else { + entry = pte_to_swp_entry(orig_src_pte); + if (non_swap_entry(entry)) { + if (is_migration_entry(entry)) { + pte_unmap(&orig_src_pte); + pte_unmap(&orig_dst_pte); + src_pte = dst_pte = NULL; + migration_entry_wait(src_mm, src_pmd, + src_addr); + err = -EAGAIN; + } else + err = -EFAULT; + goto out; + } + + err = remap_swap_pte(dst_mm, src_mm, dst_addr, src_addr, + dst_pte, src_pte, + orig_dst_pte, orig_src_pte, + dst_ptl, src_ptl); + } + +out: + if (src_anon_vma) { + anon_vma_unlock_write(src_anon_vma); + put_anon_vma(src_anon_vma); + } + if (src_folio) { + folio_unlock(src_folio); + folio_put(src_folio); + } + if (dst_pte) + pte_unmap(dst_pte); + if (src_pte) + pte_unmap(src_pte); + mmu_notifier_invalidate_range_end(&range); + + return err; +} + +static int validate_remap_areas(struct vm_area_struct *src_vma, + struct vm_area_struct *dst_vma) +{ + /* Only allow remapping if both have the same access and protection */ + if ((src_vma->vm_flags & VM_ACCESS_FLAGS) != (dst_vma->vm_flags & VM_ACCESS_FLAGS) || + pgprot_val(src_vma->vm_page_prot) != pgprot_val(dst_vma->vm_page_prot)) + return -EINVAL; + + /* Only allow remapping if both are mlocked or both aren't */ + if ((src_vma->vm_flags & VM_LOCKED) != (dst_vma->vm_flags & VM_LOCKED)) + return -EINVAL; + + /* + * Be strict and only allow remap_pages if either the src or + * dst range is registered in the userfaultfd to prevent + * userland errors going unnoticed. As far as the VM + * consistency is concerned, it would be perfectly safe to + * remove this check, but there's no useful usage for + * remap_pages ouside of userfaultfd registered ranges. This + * is after all why it is an ioctl belonging to the + * userfaultfd and not a syscall. + * + * Allow both vmas to be registered in the userfaultfd, just + * in case somebody finds a way to make such a case useful. + * Normally only one of the two vmas would be registered in + * the userfaultfd. + */ + if (!dst_vma->vm_userfaultfd_ctx.ctx && + !src_vma->vm_userfaultfd_ctx.ctx) + return -EINVAL; + + /* + * FIXME: only allow remapping across anonymous vmas, + * tmpfs should be added. + */ + if (!vma_is_anonymous(src_vma) || !vma_is_anonymous(dst_vma)) + return -EINVAL; + + /* + * Ensure the dst_vma has a anon_vma or this page + * would get a NULL anon_vma when moved in the + * dst_vma. + */ + if (unlikely(anon_vma_prepare(dst_vma))) + return -ENOMEM; + + return 0; +} + +/** + * remap_pages - remap arbitrary anonymous pages of an existing vma + * @dst_start: start of the destination virtual memory range + * @src_start: start of the source virtual memory range + * @len: length of the virtual memory range + * + * remap_pages() remaps arbitrary anonymous pages atomically in zero + * copy. It only works on non shared anonymous pages because those can + * be relocated without generating non linear anon_vmas in the rmap + * code. + * + * It provides a zero copy mechanism to handle userspace page faults. + * The source vma pages should have mapcount == 1, which can be + * enforced by using madvise(MADV_DONTFORK) on src vma. + * + * The thread receiving the page during the userland page fault + * will receive the faulting page in the source vma through the network, + * storage or any other I/O device (MADV_DONTFORK in the source vma + * avoids remap_pages() to fail with -EBUSY if the process forks before + * remap_pages() is called), then it will call remap_pages() to map the + * page in the faulting address in the destination vma. + * + * This userfaultfd command works purely via pagetables, so it's the + * most efficient way to move physical non shared anonymous pages + * across different virtual addresses. Unlike mremap()/mmap()/munmap() + * it does not create any new vmas. The mapping in the destination + * address is atomic. + * + * It only works if the vma protection bits are identical from the + * source and destination vma. + * + * It can remap non shared anonymous pages within the same vma too. + * + * If the source virtual memory range has any unmapped holes, or if + * the destination virtual memory range is not a whole unmapped hole, + * remap_pages() will fail respectively with -ENOENT or -EEXIST. This + * provides a very strict behavior to avoid any chance of memory + * corruption going unnoticed if there are userland race conditions. + * Only one thread should resolve the userland page fault at any given + * time for any given faulting address. This means that if two threads + * try to both call remap_pages() on the same destination address at the + * same time, the second thread will get an explicit error from this + * command. + * + * The command retval will return "len" is successful. The command + * however can be interrupted by fatal signals or errors. If + * interrupted it will return the number of bytes successfully + * remapped before the interruption if any, or the negative error if + * none. It will never return zero. Either it will return an error or + * an amount of bytes successfully moved. If the retval reports a + * "short" remap, the remap_pages() command should be repeated by + * userland with src+retval, dst+reval, len-retval if it wants to know + * about the error that interrupted it. + * + * The UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES flag can be specified to + * prevent -ENOENT errors to materialize if there are holes in the + * source virtual range that is being remapped. The holes will be + * accounted as successfully remapped in the retval of the + * command. This is mostly useful to remap hugepage naturally aligned + * virtual regions without knowing if there are transparent hugepage + * in the regions or not, but preventing the risk of having to split + * the hugepmd during the remap. + * + * If there's any rmap walk that is taking the anon_vma locks without + * first obtaining the folio lock (for example split_huge_page and + * folio_referenced), they will have to verify if the folio->mapping + * has changed after taking the anon_vma lock. If it changed they + * should release the lock and retry obtaining a new anon_vma, because + * it means the anon_vma was changed by remap_pages() before the lock + * could be obtained. This is the only additional complexity added to + * the rmap code to provide this anonymous page remapping functionality. + */ +ssize_t remap_pages(struct mm_struct *dst_mm, struct mm_struct *src_mm, + unsigned long dst_start, unsigned long src_start, + unsigned long len, __u64 mode) +{ + struct vm_area_struct *src_vma, *dst_vma; + unsigned long src_addr, dst_addr; + pmd_t *src_pmd, *dst_pmd; + long err = -EINVAL; + ssize_t moved = 0; + + /* + * Sanitize the command parameters: + */ + BUG_ON(src_start & ~PAGE_MASK); + BUG_ON(dst_start & ~PAGE_MASK); + BUG_ON(len & ~PAGE_MASK); + + /* Does the address range wrap, or is the span zero-sized? */ + BUG_ON(src_start + len <= src_start); + BUG_ON(dst_start + len <= dst_start); + + /* + * Because these are read sempahores there's no risk of lock + * inversion. + */ + mmap_read_lock(dst_mm); + if (dst_mm != src_mm) + mmap_read_lock(src_mm); + + /* + * Make sure the vma is not shared, that the src and dst remap + * ranges are both valid and fully within a single existing + * vma. + */ + src_vma = find_vma(src_mm, src_start); + if (!src_vma || (src_vma->vm_flags & VM_SHARED)) + goto out; + if (src_start < src_vma->vm_start || + src_start + len > src_vma->vm_end) + goto out; + + dst_vma = find_vma(dst_mm, dst_start); + if (!dst_vma || (dst_vma->vm_flags & VM_SHARED)) + goto out; + if (dst_start < dst_vma->vm_start || + dst_start + len > dst_vma->vm_end) + goto out; + + err = validate_remap_areas(src_vma, dst_vma); + if (err) + goto out; + + for (src_addr = src_start, dst_addr = dst_start; + src_addr < src_start + len;) { + spinlock_t *ptl; + pmd_t dst_pmdval; + unsigned long step_size; + + BUG_ON(dst_addr >= dst_start + len); + /* + * Below works because anonymous area would not have a + * transparent huge PUD. If file-backed support is added, + * that case would need to be handled here. + */ + src_pmd = mm_find_pmd(src_mm, src_addr); + if (unlikely(!src_pmd)) { + if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES)) { + err = -ENOENT; + break; + } + src_pmd = mm_alloc_pmd(src_mm, src_addr); + if (unlikely(!src_pmd)) { + err = -ENOMEM; + break; + } + } + dst_pmd = mm_alloc_pmd(dst_mm, dst_addr); + if (unlikely(!dst_pmd)) { + err = -ENOMEM; + break; + } + + dst_pmdval = pmdp_get_lockless(dst_pmd); + /* + * If the dst_pmd is mapped as THP don't override it and just + * be strict. If dst_pmd changes into TPH after this check, the + * remap_pages_huge_pmd() will detect the change and retry + * while remap_pages_pte() will detect the change and fail. + */ + if (unlikely(pmd_trans_huge(dst_pmdval))) { + err = -EEXIST; + break; + } + + ptl = pmd_trans_huge_lock(src_pmd, src_vma); + if (ptl && !pmd_trans_huge(*src_pmd)) { + spin_unlock(ptl); + ptl = NULL; + } + + if (ptl) { + /* + * Check if we can move the pmd without + * splitting it. First check the address + * alignment to be the same in src/dst. These + * checks don't actually need the PT lock but + * it's good to do it here to optimize this + * block away at build time if + * CONFIG_TRANSPARENT_HUGEPAGE is not set. + */ + if ((src_addr & ~HPAGE_PMD_MASK) || (dst_addr & ~HPAGE_PMD_MASK) || + src_start + len - src_addr < HPAGE_PMD_SIZE || !pmd_none(dst_pmdval)) { + spin_unlock(ptl); + split_huge_pmd(src_vma, src_pmd, src_addr); + continue; + } + + err = remap_pages_huge_pmd(dst_mm, src_mm, + dst_pmd, src_pmd, + dst_pmdval, + dst_vma, src_vma, + dst_addr, src_addr); + step_size = HPAGE_PMD_SIZE; + } else { + if (pmd_none(*src_pmd)) { + if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES)) { + err = -ENOENT; + break; + } + if (unlikely(__pte_alloc(src_mm, src_pmd))) { + err = -ENOMEM; + break; + } + } + + if (unlikely(pte_alloc(dst_mm, dst_pmd))) { + err = -ENOMEM; + break; + } + + err = remap_pages_pte(dst_mm, src_mm, + dst_pmd, src_pmd, + dst_vma, src_vma, + dst_addr, src_addr, + mode); + step_size = PAGE_SIZE; + } + + cond_resched(); + + if (!err) { + dst_addr += step_size; + src_addr += step_size; + moved += step_size; + } + + if ((!err || err == -EAGAIN) && + fatal_signal_pending(current)) + err = -EINTR; + + if (err && err != -EAGAIN) + break; + } + +out: + mmap_read_unlock(dst_mm); + if (dst_mm != src_mm) + mmap_read_unlock(src_mm); + BUG_ON(moved < 0); + BUG_ON(err > 0); + BUG_ON(!moved && !err); + return moved ? moved : err; +}