Re: [PATCH 2/7] mm/khugepaged: stop swapping in page when VM_FAULT_RETRY occurs

[Yang Shi <shy828301@gmail.com>]

On Wed, Jun 15, 2022 at 8:14 AM Zach O'Keefe <zokeefe@google.com> wrote:
>
> On 11 Jun 16:47, Miaohe Lin wrote:
> > When do_swap_page returns VM_FAULT_RETRY, we do not retry here and thus
> > swap entry will remain in pagetable. This will result in later failure.
> > So stop swapping in pages in this case to save cpu cycles.
> >
> > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> > ---
> >  mm/khugepaged.c | 19 ++++++++-----------
> >  1 file changed, 8 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 73570dfffcec..a8adb2d1e9c6 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1003,19 +1003,16 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
> >               swapped_in++;
> >               ret = do_swap_page(&vmf);
> >
> > -             /* do_swap_page returns VM_FAULT_RETRY with released mmap_lock */
> > +             /*
> > +              * do_swap_page returns VM_FAULT_RETRY with released mmap_lock.
> > +              * Note we treat VM_FAULT_RETRY as VM_FAULT_ERROR here because
> > +              * we do not retry here and swap entry will remain in pagetable
> > +              * resulting in later failure.
> > +              */
> >               if (ret & VM_FAULT_RETRY) {
> >                       mmap_read_lock(mm);
> > -                     if (hugepage_vma_revalidate(mm, haddr, &vma)) {
> > -                             /* vma is no longer available, don't continue to swapin */
> > -                             trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> > -                             return false;
> > -                     }
> > -                     /* check if the pmd is still valid */
> > -                     if (mm_find_pmd(mm, haddr) != pmd) {
> > -                             trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> > -                             return false;
> > -                     }
> > +                     trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> > +                     return false;
> >               }
> >               if (ret & VM_FAULT_ERROR) {
> >                       trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> > --
> > 2.23.0
> >
> >
>
> I've convinced myself this is correct, but don't understand how we got here.
> AFAICT, we've always continued to fault in pages, and, as you mention, don't
> retry ones that have failed with VM_FAULT_RETRY - so
> __collapse_huge_page_isolate() should fail. I don't think (?) there is any
> benefit to continuing to swap if we don't handle VM_FAULT_RETRY appropriately.
>
> So, I think this change looks good from that perspective. I suppose the only
> other question would be: should we handle the VM_FAULT_RETRY case? Maybe 1
> additional attempt then fail? AFAIK, this mostly (?) happens when the page is
> locked.  Maybe it's not worth the extra complexity though..

It should be unnecessary for khugepaged IMHO since it will scan all
the valid mm periodically, so it will come back eventually.

>