Re: [PATCH v2] mm: hold PTL from the first PTE while reclaiming a large folio

[Barry Song]

On Wed, Mar 27, 2024 at 5:17 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 26/03/2024 16:10, David Hildenbrand wrote:
> > On 06.03.24 10:52, Barry Song wrote:
> >> From: Barry Song <v-songbaohua@xxxxxxxx>
> >>
> >> Within try_to_unmap_one(), page_vma_mapped_walk() races with other
> >> PTE modifications preceded by pte clear. While iterating over PTEs
> >> of a large folio, it only starts acquiring PTL from the first valid
> >> (present) PTE. PTE modifications can temporarily set PTEs to
> >> pte_none. Consequently, the initial PTEs of a large folio might
> >> be skipped in try_to_unmap_one().
> >> For example, for an anon folio, if we skip PTE0, we may have PTE0
> >> which is still present, while PTE1 ~ PTE(nr_pages - 1) are swap
> >> entries after try_to_unmap_one().
> >> So folio will be still mapped, the folio fails to be reclaimed
> >> and is put back to LRU in this round.
> >> This also breaks up PTEs optimization such as CONT-PTE on this
> >> large folio and may lead to accident folio_split() afterwards.
> >> And since a part of PTEs are now swap entries, accessing those
> >> parts will introduce overhead - do_swap_page.
> >> Although the kernel can withstand all of the above issues, the
> >> situation still seems quite awkward and warrants making it more
> >> ideal.
> >> The same race also occurs with small folios, but they have only
> >> one PTE, thus, it won't be possible for them to be partially
> >> unmapped.
> >> This patch holds PTL from PTE0, allowing us to avoid reading PTE
> >> values that are in the process of being transformed. With stable
> >> PTE values, we can ensure that this large folio is either
> >> completely reclaimed or that all PTEs remain untouched in this
> >> round.
> >> A corner case is that if we hold PTL from PTE0 and most initial
> >> PTEs have been really unmapped before that, we may increase the
> >> duration of holding PTL. Thus we only apply this optimization to
> >> folios which are still entirely mapped (not in deferred_split
> >> list).
> >>
> >> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> >> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> >> ---
> >>   v2:
> >>    * Refine commit message and code comment after reading all comments
> >>      from Ryan and David, thanks!
> >>    * Avoid increasing the duration of PTL by applying optimization
> >>      on folios not in deferred_split_list with respect to Ying's
> >>      comment, thanks!
> >>
> >>   mm/vmscan.c | 12 ++++++++++++
> >>   1 file changed, 12 insertions(+)
> >>
> >> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> index 0b888a2afa58..7106741387e8 100644
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
> >> @@ -1270,6 +1270,18 @@ static unsigned int shrink_folio_list(struct list_head
> >> *folio_list,
> >>                 if (folio_test_pmd_mappable(folio))
> >>                   flags |= TTU_SPLIT_HUGE_PMD;
> >> +            /*
> >> +             * Without TTU_SYNC, try_to_unmap will only begin to hold PTL
> >> +             * from the first present PTE within a large folio. Some initial
> >> +             * PTEs might be skipped due to races with parallel PTE writes
> >> +             * in which PTEs can be cleared temporarily before being written
> >> +             * new present values. This will lead to a large folio is still
> >> +             * mapped while some subpages have been partially unmapped after
> >> +             * try_to_unmap; TTU_SYNC helps try_to_unmap acquire PTL from the
> >> +             * first PTE, eliminating the influence of temporary PTE values.
> >> +             */
> >> +            if (folio_test_large(folio) && list_empty(&folio->_deferred_list))
> >> +                flags |= TTU_SYNC;
> >>                 try_to_unmap(folio, flags);
> >>               if (folio_mapped(folio)) {
> >
> > Hopefully this won't have unexpected performance "surprises".
> >
> > I do wonder if we should really care about the "_deferred_list" optimization
> > here, though, I'd just drop it.

this is for a corner case:  <0, nr_pages-2> of a large folio have been unmapped
but nr_pages - 1 is still mapped. we are holding PTL to skip <0, nr_pages-2> w/
the patch, otherwise, we are skipping those PTEs w/o PTL.

But Ryan's swap-out will anyway split the folio before try_to_unmap,
so I feel we can
drop it.

>
> I also concluded that we do need the data_race() annotation around list_empty()
> if you do wind up keeping it. But I agree with David about dropping it.
>
> >
> > In any case
> >
> > Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks!

> >