Re: [PATCH v2 1/5] mm: userfaultfd: generic continue for non hugetlbfs

[James Houghton <jthoughton@google.com>]

On Wed, Apr 2, 2025 at 9:07 AM Nikita Kalyazin <kalyazin@amazon.com> wrote:
>
> Remove shmem-specific code from UFFDIO_CONTINUE implementation for
> non-huge pages by calling vm_ops->fault().  A new VMF flag,
> FAULT_FLAG_NO_USERFAULT_MINOR, is introduced to avoid recursive call to
> handle_userfault().
>
> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
> ---
>  include/linux/mm_types.h |  3 +++
>  mm/hugetlb.c             |  2 +-
>  mm/shmem.c               |  3 ++-
>  mm/userfaultfd.c         | 25 ++++++++++++++++++-------
>  4 files changed, 24 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 0234f14f2aa6..91a00f2cd565 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1429,6 +1429,8 @@ enum tlb_flush_reason {
>   * @FAULT_FLAG_ORIG_PTE_VALID: whether the fault has vmf->orig_pte cached.
>   *                        We should only access orig_pte if this flag set.
>   * @FAULT_FLAG_VMA_LOCK: The fault is handled under VMA lock.
> + * @FAULT_FLAG_NO_USERFAULT_MINOR: The fault handler must not call userfaultfd
> + *                                 minor handler.

Perhaps instead a flag that says to avoid the userfaultfd minor fault
handler, maybe we should have a flag to indicate that vm_ops->fault()
has been called by UFFDIO_CONTINUE. See below.

>   *
>   * About @FAULT_FLAG_ALLOW_RETRY and @FAULT_FLAG_TRIED: we can specify
>   * whether we would allow page faults to retry by specifying these two
> @@ -1467,6 +1469,7 @@ enum fault_flag {
>         FAULT_FLAG_UNSHARE =            1 << 10,
>         FAULT_FLAG_ORIG_PTE_VALID =     1 << 11,
>         FAULT_FLAG_VMA_LOCK =           1 << 12,
> +       FAULT_FLAG_NO_USERFAULT_MINOR = 1 << 13,
>  };
>
>  typedef unsigned int __bitwise zap_flags_t;
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 97930d44d460..ba90d48144fc 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6228,7 +6228,7 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping,
>                 }
>
>                 /* Check for page in userfault range. */
> -               if (userfaultfd_minor(vma)) {
> +               if (userfaultfd_minor(vma) && !(vmf->flags & FAULT_FLAG_NO_USERFAULT_MINOR)) {
>                         folio_unlock(folio);
>                         folio_put(folio);
>                         /* See comment in userfaultfd_missing() block above */
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 1ede0800e846..5e1911e39dec 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2467,7 +2467,8 @@ 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 (folio && vma && userfaultfd_minor(vma) &&
> +           !(vmf->flags & FAULT_FLAG_NO_USERFAULT_MINOR)) {
>                 if (!xa_is_value(folio))
>                         folio_put(folio);
>                 *fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index d06453fa8aba..68a995216789 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -386,24 +386,35 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
>                                      unsigned long dst_addr,
>                                      uffd_flags_t flags)
>  {
> -       struct inode *inode = file_inode(dst_vma->vm_file);
> -       pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
>         struct folio *folio;
>         struct page *page;
>         int ret;
> +       struct vm_fault vmf = {
> +               .vma = dst_vma,
> +               .address = dst_addr,
> +               .flags = FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE |
> +                   FAULT_FLAG_NO_USERFAULT_MINOR,
> +               .pte = NULL,
> +               .page = NULL,
> +               .pgoff = linear_page_index(dst_vma, dst_addr),
> +       };
> +
> +       if (!dst_vma->vm_ops || !dst_vma->vm_ops->fault)
> +               return -EINVAL;
>
> -       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 = dst_vma->vm_ops->fault(&vmf);

shmem_get_folio() was being called with SGP_NOALLOC, and now it is
being called with SGP_CACHE (by shmem_fault()). This will result in a
UAPI change: UFFDIO_CONTINUE for a VA without a page in the page cache
should result in EFAULT, but now the page will be allocated.
SGP_NOALLOC was carefully chosen[1], so I think a better way to do
this will be to:

1. Have a FAULT_FLAG_USERFAULT_CONTINUE (or something)
2. In shmem_fault(), if FAULT_FLAG_USERFAULT_CONTINUE, use SGP_NOALLOC
instead of SGP_CACHE (and make sure not to drop into
handle_userfault(), of course)

[1]: https://lore.kernel.org/linux-mm/20220610173812.1768919-1-axelrasmussen@google.com/

> +       if (ret & VM_FAULT_ERROR) {
>                 ret = -EFAULT;
> -       if (ret)
>                 goto out;
> +       }
> +
> +       page = vmf.page;
> +       folio = page_folio(page);
>         if (!folio) {

What if ret == VM_FAULT_RETRY? I think we should retry instead instead
of returning -EFAULT.

And I'm not sure how VM_FAULT_NOPAGE should be handled, like if we
need special logic for it or not.

>                 ret = -EFAULT;
>                 goto out;
>         }
>
> -       page = folio_file_page(folio, pgoff);
>         if (PageHWPoison(page)) {
>                 ret = -EIO;
>                 goto out_release;
> --
> 2.47.1
>