Re: [PATCH v2 12/15] mm: generalize handling of userfaults in __do_fault()

James Houghton
2026-03-27T01:55:53Z

On Fri, Mar 6, 2026 at 9:19 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> From: Peter Xu <peterx@redhat.com>
>
> When a VMA is registered with userfaulfd, its ->fault()
> method should check if a folio exists in the page cache and call
> handle_userfault() with appropriate mode:
>
> - VM_UFFD_MINOR if VMA is registered in minor mode and the folio exists
> - VM_UFFD_MISSING if VMA is registered in missing mode and the folio
> does not exist
>
> Instead of calling handle_userfault() directly from a specific ->fault()
> handler, call __do_userfault() helper from the generic __do_fault().
>
> For VMAs registered with userfaultfd the new __do_userfault() helper
> will check if the folio is found in the page cache using
> vm_uffd_ops->get_folio_noalloc() and call handle_userfault() with the
> appropriate mode.
>
> Make vm_uffd_ops->get_folio_noalloc() required method for non-anonymous
> VMAs mapped at PTE level.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> Co-developed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
> mm/memory.c | 43 +++++++++++++++++++++++++++++++++++++++++++
> mm/shmem.c | 12 ------------
> mm/userfaultfd.c | 8 ++++++++
> 3 files changed, 51 insertions(+), 12 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 07778814b4a8..e2183c44d70b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5328,6 +5328,41 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> return VM_FAULT_OOM;
> }
>
> +#ifdef CONFIG_USERFAULTFD
> +static vm_fault_t __do_userfault(struct vm_fault *vmf)
> +{
> + struct vm_area_struct *vma = vmf->vma;
> + struct inode *inode;
> + struct folio *folio;
> +
> + if (!(userfaultfd_missing(vma) || userfaultfd_minor(vma)))
> + return 0;
> +
> + inode = file_inode(vma->vm_file);
> + folio = vma->vm_ops->uffd_ops->get_folio_noalloc(inode, vmf->pgoff);

If you do away with the change you made to vma_can_userfault(), please
add a WARN_ON_ONCE() here if uffd_ops or uffd_ops->get_folio_noalloc
are not present.

> + if (!IS_ERR_OR_NULL(folio)) {
> + /*
> + * TODO: provide a flag for get_folio_noalloc() to avoid
> + * locking (or even the extra reference?)
> + */

I think a whole new op is better than adding a flag. Something like
uffd_ops->folio_present().

> + folio_unlock(folio);
> + folio_put(folio);
> + if (userfaultfd_minor(vma))
> + return handle_userfault(vmf, VM_UFFD_MINOR);
> + } else {
> + if (userfaultfd_missing(vma))
> + return handle_userfault(vmf, VM_UFFD_MISSING);
> + }
> +
> + return 0;
> +}
> +#else
> +static inline vm_fault_t __do_userfault(struct vm_fault *vmf)
> +{
> + return 0;
> +}
> +#endif
> +
> /*
> * The mmap_lock must have been held on entry, and may have been
> * released depending on flags and vma->vm_ops->fault() return value.
> @@ -5360,6 +5395,14 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
> return VM_FAULT_OOM;
> }
>
> + /*
> + * If this is an userfaultfd trap, process it in advance before

"If this is a userfault"

> + * triggering the genuine fault handler.
> + */
> + ret = __do_userfault(vmf);
> + if (ret)
> + return ret;
> +
> ret = vma->vm_ops->fault(vmf);
> if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
> VM_FAULT_DONE_COW)))
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 68620caaf75f..239545352cd2 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2489,13 +2489,6 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
> fault_mm = vma ? vma->vm_mm : NULL;
>
> folio = filemap_get_entry(inode->i_mapping, index);
> - if (folio && vma && userfaultfd_minor(vma)) {
> - if (!xa_is_value(folio))
> - folio_put(folio);
> - *fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
> - return 0;
> - }
> -
> if (xa_is_value(folio)) {
> error = shmem_swapin_folio(inode, index, &folio,
> sgp, gfp, vma, fault_type);
> @@ -2540,11 +2533,6 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
> * Fast cache lookup and swap lookup did not find it: allocate.
> */
>
> - if (vma && userfaultfd_missing(vma)) {
> - *fault_type = handle_userfault(vmf, VM_UFFD_MISSING);
> - return 0;
> - }
> -
> /* Find hugepage orders that are allowed for anonymous shmem and tmpfs. */
> orders = shmem_allowable_huge_orders(inode, vma, index, write_end, false);
> if (orders > 0) {
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 7cd7c5d1ce84..2ac5fad0ed6c 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -2045,6 +2045,14 @@ bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
> !vma_is_anonymous(vma))
> return false;
>
> + /*
> + * File backed memory with PTE level mappigns must implement

"File-backed VMAs (except HugeTLB VMAs) must implement
ops->get_folio_alloc() to support userfaults, as all userfaults in
file-backed VMAs wlll call ops->get_folio_alloc() to determine the
userfault type."

> + * ops->get_folio_noalloc()
> + */
> + if (!vma_is_anonymous(vma) && !is_vm_hugetlb_page(vma) &&
> + !ops->get_folio_noalloc)

With the separate folio_present() (or whatever) op, this check becomes
more obvious. (Looking at this right now, it looks like we're checking
for UFFDIO_CONTINUE/minor fault support here, but that's not really
what's going on.)

Honestly I'm not 100% sure if this check should be here. If it's going
to be here, then we should probably also have checks like these here:

if (vm_flags & VM_UFFD_MINOR && !ops->get_folio_noalloc)

and

if (vm_flags & VM_UFFD_MISSING && !ops->alloc_folio)

So, maybe we should just leave it up to ops->can_userfault() to make
sure we're doing the right thing?

> + return false;
> +
> return ops->can_userfault(vma, vm_flags);
> }
>
> --
> 2.51.0
>