[PATCH v2 10/15] shmem, userfaultfd: implement shmem uffd operations using vm_uffd_ops

Mike Rapoport
2026-03-06T17:19:28Z

From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>

Add filemap_add() and filemap_remove() methods to vm_uffd_ops and use
them in __mfill_atomic_pte() to add shmem folios to page cache and
remove them in case of error.

Implement these methods in shmem along with vm_uffd_ops->alloc_folio()
and drop shmem_mfill_atomic_pte().

Since userfaultfd now does not reference any functions from shmem, drop
include if linux/shmem_fs.h from mm/userfaultfd.c

mfill_atomic_install_pte() is not used anywhere outside of
mm/userfaultfd, make it static.

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
include/linux/shmem_fs.h | 14 ----
include/linux/userfaultfd_k.h | 21 +++--
mm/shmem.c | 148 ++++++++++++----------------------
mm/userfaultfd.c | 79 +++++++++---------
4 files changed, 106 insertions(+), 156 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index a8273b32e041..1a345142af7d 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -221,20 +221,6 @@ static inline pgoff_t shmem_fallocend(struct inode *inode, pgoff_t eof)

extern bool shmem_charge(struct inode *inode, long pages);

-#ifdef CONFIG_USERFAULTFD
-#ifdef CONFIG_SHMEM
-extern int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
- struct vm_area_struct *dst_vma,
- unsigned long dst_addr,
- unsigned long src_addr,
- uffd_flags_t flags,
- struct folio **foliop);
-#else /* !CONFIG_SHMEM */
-#define shmem_mfill_atomic_pte(dst_pmd, dst_vma, dst_addr, \
- src_addr, flags, foliop) ({ BUG(); 0; })
-#endif /* CONFIG_SHMEM */
-#endif /* CONFIG_USERFAULTFD */
-
/*
* Used space is stored as unsigned 64-bit value in bytes but
* quota core supports only signed 64-bit values so use that
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 4d8b879eed91..bf4e595ac914 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -93,10 +93,24 @@ struct vm_uffd_ops {
struct folio *(*get_folio_noalloc)(struct inode *inode, pgoff_t pgoff);
/*
* Called during resolution of UFFDIO_COPY request.
- * Should return allocate a and return folio or NULL if allocation fails.
+ * Should allocate and return a folio or NULL if allocation
+ * fails.
*/
struct folio *(*alloc_folio)(struct vm_area_struct *vma,
unsigned long addr);
+ /*
+ * Called during resolution of UFFDIO_COPY request.
+ * Should lock the folio and add it to VMA's page cache.
+ * Returns 0 on success, error code on failure.
+ */
+ int (*filemap_add)(struct folio *folio, struct vm_area_struct *vma,
+ unsigned long addr);
+ /*
+ * Called during resolution of UFFDIO_COPY request on the error
+ * handling path.
+ * Should revert the operation of ->filemap_add().
+ */
+ void (*filemap_remove)(struct folio *folio, struct vm_area_struct *vma);
};

/* A combined operation mode + behavior flags. */
@@ -130,11 +144,6 @@ static inline uffd_flags_t uffd_flags_set_mode(uffd_flags_t flags, enum mfill_at
/* Flags controlling behavior. These behavior changes are mode-independent. */
#define MFILL_ATOMIC_WP MFILL_ATOMIC_FLAG(0)

-extern int mfill_atomic_install_pte(pmd_t *dst_pmd,
- struct vm_area_struct *dst_vma,
- unsigned long dst_addr, struct page *page,
- bool newly_allocated, uffd_flags_t flags);
-
extern ssize_t mfill_atomic_copy(struct userfaultfd_ctx *ctx, unsigned long dst_start,
unsigned long src_start, unsigned long len,
uffd_flags_t flags);
diff --git a/mm/shmem.c b/mm/shmem.c
index 7bd887b64f62..68620caaf75f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3181,118 +3181,73 @@ static struct inode *shmem_get_inode(struct mnt_idmap *idmap,
#endif /* CONFIG_TMPFS_QUOTA */

#ifdef CONFIG_USERFAULTFD
-int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
- struct vm_area_struct *dst_vma,
- unsigned long dst_addr,
- unsigned long src_addr,
- uffd_flags_t flags,
- struct folio **foliop)
-{
- struct inode *inode = file_inode(dst_vma->vm_file);
- struct shmem_inode_info *info = SHMEM_I(inode);
+static struct folio *shmem_mfill_folio_alloc(struct vm_area_struct *vma,
+ unsigned long addr)
+{
+ struct inode *inode = file_inode(vma->vm_file);
struct address_space *mapping = inode->i_mapping;
+ struct shmem_inode_info *info = SHMEM_I(inode);
+ pgoff_t pgoff = linear_page_index(vma, addr);
gfp_t gfp = mapping_gfp_mask(mapping);
- pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
- void *page_kaddr;
struct folio *folio;
- int ret;
- pgoff_t max_off;
-
- if (shmem_inode_acct_blocks(inode, 1)) {
- /*
- * We may have got a page, returned -ENOENT triggering a retry,
- * and now we find ourselves with -ENOMEM. Release the page, to
- * avoid a BUG_ON in our caller.
- */
- if (unlikely(*foliop)) {
- folio_put(*foliop);
- *foliop = NULL;
- }
- return -ENOMEM;
- }

- if (!*foliop) {
- ret = -ENOMEM;
- folio = shmem_alloc_folio(gfp, 0, info, pgoff);
- if (!folio)
- goto out_unacct_blocks;
+ if (unlikely(pgoff >= DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE)))
+ return NULL;

- if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY)) {
- page_kaddr = kmap_local_folio(folio, 0);
- /*
- * The read mmap_lock is held here. Despite the
- * mmap_lock being read recursive a deadlock is still
- * possible if a writer has taken a lock. For example:
- *
- * process A thread 1 takes read lock on own mmap_lock
- * process A thread 2 calls mmap, blocks taking write lock
- * process B thread 1 takes page fault, read lock on own mmap lock
- * process B thread 2 calls mmap, blocks taking write lock
- * process A thread 1 blocks taking read lock on process B
- * process B thread 1 blocks taking read lock on process A
- *
- * Disable page faults to prevent potential deadlock
- * and retry the copy outside the mmap_lock.
- */
- pagefault_disable();
- ret = copy_from_user(page_kaddr,
- (const void __user *)src_addr,
- PAGE_SIZE);
- pagefault_enable();
- kunmap_local(page_kaddr);
-
- /* fallback to copy_from_user outside mmap_lock */
- if (unlikely(ret)) {
- *foliop = folio;
- ret = -ENOENT;
- /* don't free the page */
- goto out_unacct_blocks;
- }
+ folio = shmem_alloc_folio(gfp, 0, info, pgoff);
+ if (!folio)
+ return NULL;

- flush_dcache_folio(folio);
- } else { /* ZEROPAGE */
- clear_user_highpage(&folio->page, dst_addr);
- }
- } else {
- folio = *foliop;
- VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
- *foliop = NULL;
+ if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL)) {
+ folio_put(folio);
+ return NULL;
}

- VM_BUG_ON(folio_test_locked(folio));
- VM_BUG_ON(folio_test_swapbacked(folio));
+ return folio;
+}
+
+static int shmem_mfill_filemap_add(struct folio *folio,
+ struct vm_area_struct *vma,
+ unsigned long addr)
+{
+ struct inode *inode = file_inode(vma->vm_file);
+ struct address_space *mapping = inode->i_mapping;
+ pgoff_t pgoff = linear_page_index(vma, addr);
+ gfp_t gfp = mapping_gfp_mask(mapping);
+ int err;
+
__folio_set_locked(folio);
__folio_set_swapbacked(folio);
- __folio_mark_uptodate(folio);
-
- ret = -EFAULT;
- max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
- if (unlikely(pgoff >= max_off))
- goto out_release;

- ret = mem_cgroup_charge(folio, dst_vma->vm_mm, gfp);
- if (ret)
- goto out_release;
- ret = shmem_add_to_page_cache(folio, mapping, pgoff, NULL, gfp);
- if (ret)
- goto out_release;
+ err = shmem_add_to_page_cache(folio, mapping, pgoff, NULL, gfp);
+ if (err)
+ goto err_unlock;

- ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
- &folio->page, true, flags);
- if (ret)
- goto out_delete_from_cache;
+ if (shmem_inode_acct_blocks(inode, 1)) {
+ err = -ENOMEM;
+ goto err_delete_from_cache;
+ }

+ folio_add_lru(folio);
shmem_recalc_inode(inode, 1, 0);
- folio_unlock(folio);
+
return 0;
-out_delete_from_cache:
+
+err_delete_from_cache:
filemap_remove_folio(folio);
-out_release:
+err_unlock:
+ folio_unlock(folio);
+ return err;
+}
+
+static void shmem_mfill_filemap_remove(struct folio *folio,
+ struct vm_area_struct *vma)
+{
+ struct inode *inode = file_inode(vma->vm_file);
+
+ filemap_remove_folio(folio);
+ shmem_recalc_inode(inode, 0, 0);
folio_unlock(folio);
- folio_put(folio);
-out_unacct_blocks:
- shmem_inode_unacct_blocks(inode, 1);
- return ret;
}

static struct folio *shmem_get_folio_noalloc(struct inode *inode, pgoff_t pgoff)
@@ -3315,6 +3270,9 @@ static bool shmem_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags)
static const struct vm_uffd_ops shmem_uffd_ops = {
.can_userfault = shmem_can_userfault,
.get_folio_noalloc = shmem_get_folio_noalloc,
+ .alloc_folio = shmem_mfill_folio_alloc,
+ .filemap_add = shmem_mfill_filemap_add,
+ .filemap_remove = shmem_mfill_filemap_remove,
};
#endif /* CONFIG_USERFAULTFD */

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 31f3ab6a73e2..a0f8e67006d6 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -14,7 +14,6 @@
#include <linux/userfaultfd_k.h>
#include <linux/mmu_notifier.h>
#include <linux/hugetlb.h>
-#include <linux/shmem_fs.h>
#include <asm/tlbflush.h>
#include <asm/tlb.h>
#include "internal.h"
@@ -340,10 +339,10 @@ static bool mfill_file_over_size(struct vm_area_struct *dst_vma,
* This function handles both MCOPY_ATOMIC_NORMAL and _CONTINUE for both shmem
* and anon, and for both shared and private VMAs.
*/
-int mfill_atomic_install_pte(pmd_t *dst_pmd,
- struct vm_area_struct *dst_vma,
- unsigned long dst_addr, struct page *page,
- bool newly_allocated, uffd_flags_t flags)
+static int mfill_atomic_install_pte(pmd_t *dst_pmd,
+ struct vm_area_struct *dst_vma,
+ unsigned long dst_addr, struct page *page,
+ uffd_flags_t flags)
{
int ret;
struct mm_struct *dst_mm = dst_vma->vm_mm;
@@ -387,9 +386,6 @@ int mfill_atomic_install_pte(pmd_t *dst_pmd,
goto out_unlock;

if (page_in_cache) {
- /* Usually, cache pages are already added to LRU */
- if (newly_allocated)
- folio_add_lru(folio);
folio_add_file_rmap_pte(folio, page, dst_vma);
} else {
folio_add_new_anon_rmap(folio, dst_vma, dst_addr, RMAP_EXCLUSIVE);
@@ -404,6 +400,9 @@ int mfill_atomic_install_pte(pmd_t *dst_pmd,

set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);

+ if (page_in_cache)
+ folio_unlock(folio);
+
/* No need to invalidate - it was non-present before */
update_mmu_cache(dst_vma, dst_addr, dst_pte);
ret = 0;
@@ -516,13 +515,22 @@ static int __mfill_atomic_pte(struct mfill_state *state,
*/
__folio_mark_uptodate(folio);

+ if (ops->filemap_add) {
+ ret = ops->filemap_add(folio, state->vma, state->dst_addr);
+ if (ret)
+ goto err_folio_put;
+ }
+
ret = mfill_atomic_install_pte(state->pmd, state->vma, dst_addr,
- &folio->page, true, flags);
+ &folio->page, flags);
if (ret)
- goto err_folio_put;
+ goto err_filemap_remove;

return 0;

+err_filemap_remove:
+ if (ops->filemap_remove)
+ ops->filemap_remove(folio, state->vma);
err_folio_put:
folio_put(folio);
/* Don't return -ENOENT so that our caller won't retry */
@@ -535,6 +543,18 @@ static int mfill_atomic_pte_copy(struct mfill_state *state)
{
const struct vm_uffd_ops *ops = vma_uffd_ops(state->vma);

+ /*
+ * The normal page fault path for a MAP_PRIVATE mapping in a
+ * file-backed VMA will invoke the fault, fill the hole in the file and
+ * COW it right away. The result generates plain anonymous memory.
+ * So when we are asked to fill a hole in a MAP_PRIVATE mapping, we'll
+ * generate anonymous memory directly without actually filling the
+ * hole. For the MAP_PRIVATE case the robustness check only happens in
+ * the pagetable (to verify it's still none) and not in the page cache.
+ */
+ if (!(state->vma->vm_flags & VM_SHARED))
+ ops = &anon_uffd_ops;
+
return __mfill_atomic_pte(state, ops);
}

@@ -554,7 +574,8 @@ static int mfill_atomic_pte_zeropage(struct mfill_state *state)
spinlock_t *ptl;
int ret;

- if (mm_forbids_zeropage(dst_vma->vm_mm))
+ if (mm_forbids_zeropage(dst_vma->vm_mm) ||
+ (dst_vma->vm_flags & VM_SHARED))
return mfill_atomic_pte_zeroed_folio(state);

_dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
@@ -609,11 +630,10 @@ static int mfill_atomic_pte_continue(struct mfill_state *state)
}

ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
- page, false, flags);
+ page, flags);
if (ret)
goto out_release;

- folio_unlock(folio);
return 0;

out_release:
@@ -836,41 +856,18 @@ extern ssize_t mfill_atomic_hugetlb(struct userfaultfd_ctx *ctx,

static __always_inline ssize_t mfill_atomic_pte(struct mfill_state *state)
{
- struct vm_area_struct *dst_vma = state->vma;
- unsigned long src_addr = state->src_addr;
- unsigned long dst_addr = state->dst_addr;
- struct folio **foliop = &state->folio;
uffd_flags_t flags = state->flags;
- pmd_t *dst_pmd = state->pmd;
- ssize_t err;

if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
return mfill_atomic_pte_continue(state);
if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON))
return mfill_atomic_pte_poison(state);
+ if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY))
+ return mfill_atomic_pte_copy(state);
+ if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE))
+ return mfill_atomic_pte_zeropage(state);

- /*
- * The normal page fault path for a shmem will invoke the
- * fault, fill the hole in the file and COW it right away. The
- * result generates plain anonymous memory. So when we are
- * asked to fill an hole in a MAP_PRIVATE shmem mapping, we'll
- * generate anonymous memory directly without actually filling
- * the hole. For the MAP_PRIVATE case the robustness check
- * only happens in the pagetable (to verify it's still none)
- * and not in the radix tree.
- */
- if (!(dst_vma->vm_flags & VM_SHARED)) {
- if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY))
- err = mfill_atomic_pte_copy(state);
- else
- err = mfill_atomic_pte_zeropage(state);
- } else {
- err = shmem_mfill_atomic_pte(dst_pmd, dst_vma,
- dst_addr, src_addr,
- flags, foliop);
- }
-
- return err;
+ return -EOPNOTSUPP;
}

static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
--
2.51.0