[PATCH v4 09/15] userfaultfd: introduce vm_uffd_ops->alloc_folio()

Mike Rapoport
2026-04-02T04:13:07Z

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

and use it to refactor mfill_atomic_pte_zeroed_folio() and
mfill_atomic_pte_copy().

mfill_atomic_pte_zeroed_folio() and mfill_atomic_pte_copy() perform
almost identical actions:
* allocate a folio
* update folio contents (either copy from userspace of fill with zeros)
* update page tables with the new folio

Split a __mfill_atomic_pte() helper that handles both cases and uses newly
introduced vm_uffd_ops->alloc_folio() to allocate the folio.

Pass the ops structure from the callers to __mfill_atomic_pte() to later
allow using anon_uffd_ops for MAP_PRIVATE mappings of file-backed VMAs.

Note, that the new ops method is called alloc_folio() rather than
folio_alloc() to avoid clash with alloc_tag macro folio_alloc().

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: James Houghton <jthoughton@google.com>
---
include/linux/userfaultfd_k.h | 6 +++
mm/userfaultfd.c | 92 ++++++++++++++++++-----------------
2 files changed, 54 insertions(+), 44 deletions(-)

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 66dfc3c164e6..55a67421de0a 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -91,6 +91,12 @@ struct vm_uffd_ops {
* The returned folio is locked and with reference held.
*/
struct folio *(*get_folio_noalloc)(struct inode *inode, pgoff_t pgoff);
+ /*
+ * Called during resolution of UFFDIO_COPY request.
+ * Should allocate and return a folio or NULL if allocation fails.
+ */
+ struct folio *(*alloc_folio)(struct vm_area_struct *vma,
+ unsigned long addr);
};

/* A combined operation mode + behavior flags. */
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 832dbdde5868..771a1e607c4c 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -42,8 +42,26 @@ static bool anon_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags)
return true;
}

+static struct folio *anon_alloc_folio(struct vm_area_struct *vma,
+ unsigned long addr)
+{
+ struct folio *folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma,
+ addr);
+
+ if (!folio)
+ return NULL;
+
+ if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL)) {
+ folio_put(folio);
+ return NULL;
+ }
+
+ return folio;
+}
+
static const struct vm_uffd_ops anon_uffd_ops = {
.can_userfault = anon_can_userfault,
+ .alloc_folio = anon_alloc_folio,
};

static const struct vm_uffd_ops *vma_uffd_ops(struct vm_area_struct *vma)
@@ -456,7 +474,8 @@ static int mfill_copy_folio_retry(struct mfill_state *state, struct folio *folio
return 0;
}

-static int mfill_atomic_pte_copy(struct mfill_state *state)
+static int __mfill_atomic_pte(struct mfill_state *state,
+ const struct vm_uffd_ops *ops)
{
unsigned long dst_addr = state->dst_addr;
unsigned long src_addr = state->src_addr;
@@ -464,16 +483,12 @@ static int mfill_atomic_pte_copy(struct mfill_state *state)
struct folio *folio;
int ret;

- folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, state->vma, dst_addr);
+ folio = ops->alloc_folio(state->vma, state->dst_addr);
if (!folio)
return -ENOMEM;

- ret = -ENOMEM;
- if (mem_cgroup_charge(folio, state->vma->vm_mm, GFP_KERNEL))
- goto out_release;
-
- ret = mfill_copy_folio_locked(folio, src_addr);
- if (unlikely(ret)) {
+ if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY)) {
+ ret = mfill_copy_folio_locked(folio, src_addr);
/*
* Fallback to copy_from_user outside mmap_lock.
* If retry is successful, mfill_copy_folio_locked() returns
@@ -481,9 +496,15 @@ static int mfill_atomic_pte_copy(struct mfill_state *state)
* If there was an error, we must mfill_put_vma() anyway and it
* will take care of unlocking if needed.
*/
- ret = mfill_copy_folio_retry(state, folio);
- if (ret)
- goto out_release;
+ if (unlikely(ret)) {
+ ret = mfill_copy_folio_retry(state, folio);
+ if (ret)
+ goto err_folio_put;
+ }
+ } else if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
+ clear_user_highpage(&folio->page, state->dst_addr);
+ } else {
+ VM_WARN_ONCE(1, "Unknown UFFDIO operation, flags: %x", flags);
}

/*
@@ -496,47 +517,30 @@ static int mfill_atomic_pte_copy(struct mfill_state *state)
ret = mfill_atomic_install_pte(state->pmd, state->vma, dst_addr,
&folio->page, true, flags);
if (ret)
- goto out_release;
-out:
- return ret;
-out_release:
+ goto err_folio_put;
+
+ return 0;
+
+err_folio_put:
+ folio_put(folio);
/* Don't return -ENOENT so that our caller won't retry */
if (ret == -ENOENT)
ret = -EFAULT;
- folio_put(folio);
- goto out;
+ return ret;
}

-static int mfill_atomic_pte_zeroed_folio(pmd_t *dst_pmd,
- struct vm_area_struct *dst_vma,
- unsigned long dst_addr)
+static int mfill_atomic_pte_copy(struct mfill_state *state)
{
- struct folio *folio;
- int ret = -ENOMEM;
-
- folio = vma_alloc_zeroed_movable_folio(dst_vma, dst_addr);
- if (!folio)
- return ret;
-
- if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL))
- goto out_put;
+ const struct vm_uffd_ops *ops = vma_uffd_ops(state->vma);

- /*
- * The memory barrier inside __folio_mark_uptodate makes sure that
- * zeroing out the folio become visible before mapping the page
- * using set_pte_at(). See do_anonymous_page().
- */
- __folio_mark_uptodate(folio);
+ return __mfill_atomic_pte(state, ops);
+}

- ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
- &folio->page, true, 0);
- if (ret)
- goto out_put;
+static int mfill_atomic_pte_zeroed_folio(struct mfill_state *state)
+{
+ const struct vm_uffd_ops *ops = vma_uffd_ops(state->vma);

- return 0;
-out_put:
- folio_put(folio);
- return ret;
+ return __mfill_atomic_pte(state, ops);
}

static int mfill_atomic_pte_zeropage(struct mfill_state *state)
@@ -549,7 +553,7 @@ static int mfill_atomic_pte_zeropage(struct mfill_state *state)
int ret;

if (mm_forbids_zeropage(dst_vma->vm_mm))
- return mfill_atomic_pte_zeroed_folio(dst_pmd, dst_vma, dst_addr);
+ return mfill_atomic_pte_zeroed_folio(state);

_dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
dst_vma->vm_page_prot));
--
2.53.0