Re: [PATCH] mm: fix finish_fault() handling for large folios

[Brian Geffon <bgeffon@google.com>]

On Wed, Feb 26, 2025 at 10:17 AM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
>
> On 2025/2/26 19:48, Brian Geffon wrote:
> > When handling faults for anon shmem finish_fault() will attempt to install
> > ptes for the entire folio. Unfortunately if it encounters a single
> > non-pte_none entry in that range it will bail, even if the pte that
> > triggered the fault is still pte_none. When this situation happens the
> > fault will be retried endlessly never making forward progress.
> >
> > This patch fixes this behavior and if it detects that a pte in the range
> > is not pte_none it will fall back to setting just the pte for the
> > address that triggered the fault.
>
> Could you describe in detail how this situation occurs? How is the none
> pte inserted within the range of the large folio? Because we have checks
> in shmem to determine if a large folio is suitable.

We're seeing it because of racing shmem_undo_range() calls, basically
we have a portion of the folio is zapped and that prevents
finish_fault() from ever completing because it has an assumption that
the entire range must be pte_none.

>
> Anyway, if we find the pte_range_none() is false, we can fallback to
> per-page fault as the following code shows (untested), which seems more
> simple?

Yah, I like this approach. I'll send a v2 with you as a Suggested-by.

>
> diff --git a/mm/memory.c b/mm/memory.c
> index a8196ae72e9a..8a2a9fda5410 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5219,7 +5219,12 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>          bool is_cow = (vmf->flags & FAULT_FLAG_WRITE) &&
>                        !(vma->vm_flags & VM_SHARED);
>          int type, nr_pages;
> -       unsigned long addr = vmf->address;
> +       unsigned long addr;
> +       bool fallback_per_page = false;
> +
> +
> +fallback:
> +       addr = vmf->address;
>
>          /* Did we COW the page? */
>          if (is_cow)
> @@ -5258,7 +5263,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>           * approach also applies to non-anonymous-shmem faults to avoid
>           * inflating the RSS of the process.
>           */
> -       if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma))) {
> +       if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma))
> +           || unlikely(fallback_per_page)) {
>                  nr_pages = 1;
>          } else if (nr_pages > 1) {
>                  pgoff_t idx = folio_page_idx(folio, page);
> @@ -5294,9 +5300,9 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>                  ret = VM_FAULT_NOPAGE;
>                  goto unlock;
>          } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> -               update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
> -               ret = VM_FAULT_NOPAGE;
> -               goto unlock;
> +               fallback_per_page = true;
> +               pte_unmap_unlock(vmf->pte, vmf->ptl);
> +               goto fallback;
>          }
>
>          folio_ref_add(folio, nr_pages - 1);
>
> >
> > Cc: stable@vger.kernel.org
> > Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> > Cc: Hugh Dickins <hughd@google.com>
> > Fixes: 43e027e41423 ("mm: memory: extend finish_fault() to support large folio")
> > Reported-by: Marek Maslanka <mmaslanka@google.com>
> > Signed-off-by: Brian Geffon <bgeffon@google.com>
> > ---
> >   mm/memory.c | 19 ++++++++++++++++---
> >   1 file changed, 16 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index b4d3d4893267..32de626ec1da 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5258,9 +5258,22 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> >               ret = VM_FAULT_NOPAGE;
> >               goto unlock;
> >       } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> > -             update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
> > -             ret = VM_FAULT_NOPAGE;
> > -             goto unlock;
> > +             /*
> > +              * We encountered a set pte, let's just try to install the
> > +              * pte for the original fault if that pte is still pte none.
> > +              */
> > +             pgoff_t idx = (vmf->address - addr) / PAGE_SIZE;
> > +
> > +             if (!pte_none(ptep_get_lockless(vmf->pte + idx))) {
> > +                     update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
> > +                     ret = VM_FAULT_NOPAGE;
> > +                     goto unlock;
> > +             }
> > +
> > +             vmf->pte = vmf->pte + idx;
> > +             page = folio_page(folio, idx);
> > +             addr = vmf->address;
> > +             nr_pages = 1;
> >       }
> >
> >       folio_ref_add(folio, nr_pages - 1);