linux-kernel - Re: [PATCH v2] mm/rmap: do not add fully unmapped large folio to deferred split list

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Message-ID: <CAHbLzkpOTtRViNRwZYCS+0wePFEQFep30AB77uJk-CvFU_0wwg@mail.gmail.com>
Date: Wed, 24 Apr 2024 15:53:49 -0700
From: Yang Shi <shy828301@...il.com>
To: Zi Yan <ziy@...dia.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, Ryan Roberts <ryan.roberts@....com>, 
	Barry Song <21cnbao@...il.com>, David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/rmap: do not add fully unmapped large folio to
 deferred split list

On Wed, Apr 24, 2024 at 3:39 PM Zi Yan <ziy@...dia.com> wrote:
>
> On 24 Apr 2024, at 18:32, Yang Shi wrote:
>
> > On Wed, Apr 24, 2024 at 2:10 PM Zi Yan <zi.yan@...t.com> wrote:
> >>
> >> From: Zi Yan <ziy@...dia.com>
> >>
> >> In __folio_remove_rmap(), a large folio is added to deferred split list
> >> if any page in a folio loses its final mapping. It is possible that
> >> the folio is unmapped fully, but it is unnecessary to add the folio
> >> to deferred split list at all. Fix it by checking folio->_nr_pages_mapped
> >> before adding a folio to deferred split list. If the folio is already
> >> on the deferred split list, it will be skipped.
> >>
> >> 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 not
> >> fix everything. A fully unmapped PTE-mapped order-9 THP was also 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(). However, this miscount
> >> was present even earlier due to implementation, since PTEs are unmapped
> >> individually and first PTE unmapping adds the THP into the deferred split
> >> list.
> >
> > Shall you mention the miscounting for mTHP too? There is another patch
> > series adding the counter support for mTHP.
>
> OK, will add it.
> >
> >>
> >> With commit b06dc281aa99 ("mm/rmap: introduce
> >> folio_remove_rmap_[pte|ptes|pmd]()"), kernel is able to unmap PTE-mapped
> >> folios in one shot without causing the miscount, hence this patch.
> >>
> >> Signed-off-by: Zi Yan <ziy@...dia.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 == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
> >> -                               deferred_split_folio(folio);
> >> +               if (folio_test_large(folio) && folio_test_anon(folio) &&
> >> +                   list_empty(&folio->_deferred_list) &&
> >
> > Do we really need this check? deferred_split_folio() does the same
> > check too. Bailing out earlier sounds ok too, but there may not be too
> > much gain.
>
> Sure, I can remove it.
>
> >
> >> +                   ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
> >> +                    (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
> >
> > IIUC, this line is used to cover the case which has both partial
> > PTE-mapping and PMD-mapping, then PMD mapping is unmapped fully. IIRC
> > this case was not handled correctly before, the THP actually skipped
> > deferred split queue. If so please add some description in the commit
> > log.
>
> It is properly handled before, since the original code is
> (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped), meaning
> if either level is RMAP_LEVEL_PTE or
> (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped), the folio
> is added to the deferred split list. So only level == RMAP_LEVEL_PTE
> part of logic needs to be fixed.

Oh, yes. I misread "||" to "&&". Thanks for correcting me and fixing
the problem.

>
> >
> > Otherwise the patch looks good to me. Reviewed-by: Yang Shi
> > <shy828301@...il.com>
> >
>
> Thanks.
> >> +                       deferred_split_folio(folio);
> >>         }
> >>
> >>         /*
> >>
> >> base-commit: 2541ee5668b019c486dd3e815114130e35c1495d
> >> --
> >> 2.43.0
> >>
>
>
> --
> Best Regards,
> Yan, Zi