[PATCH v2 2/5] userfaultfd, shmem: use a VMA callback to handle UFFDIO_CONTINUE

Mike Rapoport
2025-11-25T18:39:01Z

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
--
2.50.1