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From: Barry Song <21cnbao@gmail.com>
Date: Fri, 26 Apr 2024 10:23:54 +0800
Message-ID: <CAGsJ_4xzb8RrEuPEbnvR4GbDWuoGCYL4FsC3TObOifAZ4CHGOA@mail.gmail.com>
Subject: Re: [PATCH v4] mm/rmap: do not add fully unmapped large folio to
 deferred split list
To: Zi Yan <ziy@nvidia.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, 
	"Matthew Wilcox (Oracle)" <willy@infradead.org>, Yang Shi <shy828301@gmail.com>, 
	Ryan Roberts <ryan.roberts@arm.com>, David Hildenbrand <david@redhat.com>, 
	Lance Yang <ioworker0@gmail.com>, linux-kernel@vger.kernel.org
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On Fri, Apr 26, 2024 at 9:55=E2=80=AFAM Zi Yan <ziy@nvidia.com> wrote:
>
> On 25 Apr 2024, at 21:45, Barry Song wrote:
>
> > On Fri, Apr 26, 2024 at 5:11=E2=80=AFAM Zi Yan <zi.yan@sent.com> wrote:
> >>
> >> From: Zi Yan <ziy@nvidia.com>
> >>
> >> In __folio_remove_rmap(), a large folio is added to deferred split lis=
t
> >> if any page in a folio loses its final mapping. But it is possible tha=
t
> >> the folio is fully unmapped and adding it to deferred split list is
> >> unnecessary.
> >>
> >> For PMD-mapped THPs, that was not really an issue, because removing th=
e
> >> last PMD mapping in the absence of PTE mappings would not have added t=
he
> >> folio to the deferred split queue.
> >>
> >> However, for PTE-mapped THPs, which are now more prominent due to mTHP=
,
> >> they are always added to the deferred split queue. One side effect
> >> is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can be
> >> unintentionally increased, making it look like there are many partiall=
y
> >> mapped folios -- although the whole folio is fully unmapped stepwise.
> >>
> >> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs
> >> where possible starting from commit b06dc281aa99 ("mm/rmap: introduce
> >> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-map=
ped
> >> folio is unmapped in one go and can avoid being added to deferred spli=
t
> >> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still=
 be
> >> noise when we cannot batch-unmap a complete PTE-mapped folio in one go
> >> -- or where this type of batching is not implemented yet, e.g., migrat=
ion.
> >>
> >> To avoid the unnecessary addition, folio->_nr_pages_mapped is checked
> >> to tell if the whole folio is unmapped. If the folio is already on
> >> deferred split list, it will be skipped, too.
> >>
> >> Note: commit 98046944a159 ("mm: huge_memory: add the missing
> >> folio_test_pmd_mappable() for THP split statistics") tried to exclude
> >> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does no=
t
> >> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still
> >> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,
> >> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
> >> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable().
> >>
> >> Signed-off-by: Zi Yan <ziy@nvidia.com>
> >> Reviewed-by: Yang Shi <shy828301@gmail.com>
> >> ---
> >>  mm/rmap.c | 8 +++++---
> >>  1 file changed, 5 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index a7913a454028..220ad8a83589 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_rmap=
(struct folio *folio,
> >>                  * page of the folio is unmapped and at least one page
> >>                  * is still mapped.
> >>                  */
> >> -               if (folio_test_large(folio) && folio_test_anon(folio))
> >> -                       if (level =3D=3D RMAP_LEVEL_PTE || nr < nr_pmd=
mapped)
> >> -                               deferred_split_folio(folio);
> >> +               if (folio_test_large(folio) && folio_test_anon(folio) =
&&
> >> +                   list_empty(&folio->_deferred_list) &&
> >> +                   ((level =3D=3D RMAP_LEVEL_PTE && atomic_read(mappe=
d)) ||
> >> +                    (level =3D=3D RMAP_LEVEL_PMD && nr < nr_pmdmapped=
)))
> >> +                       deferred_split_folio(folio);
> >
> > Hi Zi Yan,
> > in case a mTHP is mapped by two processed (forked but not CoW yet), if =
we
> > unmap the whole folio by pte level in one process only, are we still ad=
ding this
> > folio into deferred list?
>
> No. Because the mTHP is still fully mapped by the other process. In terms=
 of code,
> nr will be 0 in that case and this if condition is skipped. nr is only in=
creased
> from 0 when one of the subpages in the mTHP has no mapping, namely page->=
_mapcount
> becomes negative and last is true in the case RMAP_LEVEL_PTE.

Ok. i see, so "last" won't be true?

case RMAP_LEVEL_PTE:
do {
last =3D atomic_add_negative(-1, &page->_mapcount);
   if (last && folio_test_large(folio)) {
       last =3D atomic_dec_return_relaxed(mapped);
       last =3D (last < ENTIRELY_MAPPED);
}

if (last)
     nr++;
} while (page++, --nr_pages > 0);
break;


>
>
> --
> Best Regards,
> Yan, Zi