Re: [PATCH v2] mm: fix finish_fault() handling for large folios

[Brian Geffon <bgeffon@google.com>]

On Thu, Feb 27, 2025 at 2:34 AM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
>
> On 2025/2/27 00:23, 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 a single pte.
> >
> > Cc: stable@vger.kernel.org
> > Cc: Hugh Dickins <hughd@google.com>
> > Fixes: 43e027e41423 ("mm: memory: extend finish_fault() to support large folio")
> > Suggested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > Reported-by: Marek Maslanka <mmaslanka@google.com>
> > Signed-off-by: Brian Geffon <bgeffon@google.com>
> > ---
> >   mm/memory.c | 15 ++++++++++-----
> >   1 file changed, 10 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index b4d3d4893267..b6c467fdbfa4 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5183,7 +5183,11 @@ 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 needs_fallback = false;
> > +
> > +fallback:
> > +     addr = vmf->address;
> >
> >       /* Did we COW the page? */
> >       if (is_cow)
> > @@ -5222,7 +5226,8 @@ 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(needs_fallback)) {
>
> Nit: can you align the code? Otherwise look good to me.

I mailed a v3 with adjusted alignment, I'll let Andrew decide which
variation he prefers.

>
> >               nr_pages = 1;
> >       } else if (nr_pages > 1) {
> >               pgoff_t idx = folio_page_idx(folio, page);
> > @@ -5258,9 +5263,9 @@ 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;
> > +             needs_fallback = true;
> > +             pte_unmap_unlock(vmf->pte, vmf->ptl);
> > +             goto fallback;
> >       }
> >
> >       folio_ref_add(folio, nr_pages - 1);

Thanks for looking
Brian