Re: [PATCH v2 09/15] userfaultfd: introduce vm_uffd_ops->alloc_folio()

James Houghton
2026-03-27T00:07:50Z

On Fri, Mar 6, 2026 at 9:19 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> 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>

Feel free to add:

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..4d8b879eed91 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 return allocate a and return folio or NULL if allocation fails.

"Should allocate and return a folio or NULL if allocation fails."

I see this mistake is fixed in the next patch. :)

> + */
> + 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 98ade14eaa5b..31f3ab6a73e2 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)
> @@ -458,7 +476,8 @@ 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;
> @@ -466,16 +485,12 @@ 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
> @@ -483,9 +498,15 @@ 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");

"Unknown UFFDIO operation. flags=%x", flags

seems a little better to me.

> }
>
> /*
> @@ -498,47 +519,30 @@ 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)
> @@ -551,7 +555,7 @@ 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));
> --
> 2.51.0
>