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From: Barry Song <21cnbao@gmail.com>
Date: Fri, 26 Apr 2024 11:36:19 +0800
Message-ID: <CAGsJ_4wkDOvwru9pGLHUu_NPzwatX2UyizXKALicJ8qY2UXqHQ@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 11:28=E2=80=AFAM Barry Song <21cnbao@gmail.com> wro=
te:
>
> On Fri, Apr 26, 2024 at 10:50=E2=80=AFAM Zi Yan <ziy@nvidia.com> wrote:
> >
> > On 25 Apr 2024, at 22:23, Barry Song wrote:
> >
> > > 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> wr=
ote:
> > >>>>
> > >>>> From: Zi Yan <ziy@nvidia.com>
> > >>>>
> > >>>> In __folio_remove_rmap(), a large folio is added to deferred split=
 list
> > >>>> if any page in a folio loses its final mapping. But it is possible=
 that
> > >>>> the folio is fully unmapped and adding it to deferred split list i=
s
> > >>>> unnecessary.
> > >>>>
> > >>>> For PMD-mapped THPs, that was not really an issue, because removin=
g the
> > >>>> last PMD mapping in the absence of PTE mappings would not have add=
ed the
> > >>>> 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 ca=
n be
> > >>>> unintentionally increased, making it look like there are many part=
ially
> > >>>> mapped folios -- although the whole folio is fully unmapped stepwi=
se.
> > >>>>
> > >>>> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped T=
HPs
> > >>>> where possible starting from commit b06dc281aa99 ("mm/rmap: introd=
uce
> > >>>> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE=
-mapped
> > >>>> folio is unmapped in one go and can avoid being added to deferred =
split
> > >>>> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will s=
till be
> > >>>> noise when we cannot batch-unmap a complete PTE-mapped folio in on=
e go
> > >>>> -- or where this type of batching is not implemented yet, e.g., mi=
gration.
> > >>>>
> > >>>> To avoid the unnecessary addition, folio->_nr_pages_mapped is chec=
ked
> > >>>> 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 excl=
ude
> > >>>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it doe=
s not
> > >>>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was s=
till
> > >>>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAG=
E,
> > >>>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
> > >>>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappabl=
e().
> > >>>>
> > >>>> 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(fol=
io))
> > >>>> -                       if (level =3D=3D RMAP_LEVEL_PTE || nr < nr=
_pmdmapped)
> > >>>> -                               deferred_split_folio(folio);
> > >>>> +               if (folio_test_large(folio) && folio_test_anon(fol=
io) &&
> > >>>> +                   list_empty(&folio->_deferred_list) &&
> > >>>> +                   ((level =3D=3D RMAP_LEVEL_PTE && atomic_read(m=
apped)) ||
> > >>>> +                    (level =3D=3D RMAP_LEVEL_PMD && nr < nr_pmdma=
pped)))
> > >>>> +                       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 stil=
l adding 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 on=
ly increased
> > >> from 0 when one of the subpages in the mTHP has no mapping, namely p=
age->_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;
> >
> > Right, because for every subpage its corresponding
> > last =3D atomic_add_negative(-1, &page->_mapcount); is not true after t=
he unmapping.2
>
> if a mTHP is mapped only by one process, and we unmap it entirely, we wil=
l
> get nr > 0, then we are executing adding it into deferred_list? so it see=
ms
> atomic_read(mapped) is preventing this case from adding deferred_list?
>
> I wonder if  it is possible to fixup nr to 0 from the first place?
> for example
> /* we are doing an entire unmapping */
> if (page=3D=3D&folio->page && nr_pages =3D=3D  folio_nr_pages(folio))

or maybe
 case RMAP_LEVEL_PTE:
..
+ if (!atomic_read(mapped))
+     nr =3D 0;
break;

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