[PATCH v4 08/15] shmem, userfaultfd: use a VMA callback to handle UFFDIO_CONTINUE

Mike Rapoport
2026-04-02T04:13:04Z

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_uffd_ops' 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_get_vma() and mfill_atomic_pte_continue() to support
this new API.

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: James Houghton <jthoughton@google.com>
---
include/linux/userfaultfd_k.h | 7 +++++++
mm/shmem.c | 15 ++++++++++++++-
mm/userfaultfd.c | 34 ++++++++++++++++++----------------
3 files changed, 39 insertions(+), 17 deletions(-)

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 56e85ab166c7..66dfc3c164e6 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -84,6 +84,13 @@ extern vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason);
struct vm_uffd_ops {
/* Checks if a VMA can support userfaultfd */
bool (*can_userfault)(struct vm_area_struct *vma, vm_flags_t vm_flags);
+ /*
+ * Called 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);
};

/* A combined operation mode + behavior flags. */
diff --git a/mm/shmem.c b/mm/shmem.c
index f2a25805b9bf..7bd887b64f62 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3295,13 +3295,26 @@ int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
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;
+}
+
static bool shmem_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags)
{
return true;
}

static const struct vm_uffd_ops shmem_uffd_ops = {
- .can_userfault = shmem_can_userfault,
+ .can_userfault = shmem_can_userfault,
+ .get_folio_noalloc = shmem_get_folio_noalloc,
};
#endif /* CONFIG_USERFAULTFD */

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index e3024a39c19d..832dbdde5868 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -191,6 +191,7 @@ static int mfill_get_vma(struct mfill_state *state)
struct userfaultfd_ctx *ctx = state->ctx;
uffd_flags_t flags = state->flags;
struct vm_area_struct *dst_vma;
+ const struct vm_uffd_ops *ops;
int err;

/*
@@ -232,10 +233,12 @@ static int mfill_get_vma(struct mfill_state *state)
if (is_vm_hugetlb_page(dst_vma))
return 0;

- if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
+ ops = vma_uffd_ops(dst_vma);
+ if (!ops)
goto out_unlock;
- if (!vma_is_shmem(dst_vma) &&
- uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
+
+ if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE) &&
+ !ops->get_folio_noalloc)
goto out_unlock;

return 0;
@@ -575,6 +578,7 @@ static int mfill_atomic_pte_zeropage(struct mfill_state *state)
static int mfill_atomic_pte_continue(struct mfill_state *state)
{
struct vm_area_struct *dst_vma = state->vma;
+ const struct vm_uffd_ops *ops = vma_uffd_ops(dst_vma);
unsigned long dst_addr = state->dst_addr;
pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
struct inode *inode = file_inode(dst_vma->vm_file);
@@ -584,17 +588,16 @@ static int mfill_atomic_pte_continue(struct mfill_state *state)
struct page *page;
int ret;

- ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
- /* 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 (!ops) {
+ VM_WARN_ONCE(1, "UFFDIO_CONTINUE for unsupported VMA");
+ return -EOPNOTSUPP;
}

+ folio = ops->get_folio_noalloc(inode, pgoff);
+ /* Our caller expects us to return -EFAULT if we failed to find folio */
+ if (IS_ERR_OR_NULL(folio))
+ return -EFAULT;
+
page = folio_file_page(folio, pgoff);
if (PageHWPoison(page)) {
ret = -EIO;
@@ -607,13 +610,12 @@ static int mfill_atomic_pte_continue(struct mfill_state *state)
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. */
--
2.53.0