Messages in this thread | | | Date | Wed, 21 Feb 2024 10:21:52 +0100 | Subject | Re: [PATCH] madvise:madvise_cold_or_pageout_pte_range(): allow split while folio_estimated_sharers = 0 | From | David Hildenbrand <> |
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On 21.02.24 10:15, David Hildenbrand wrote: > On 21.02.24 09:50, Barry Song wrote: >> From: Barry Song <v-songbaohua@oppo.com> >> >> The purpose is stopping splitting large folios whose mapcount are 2 or >> above. Folios whose estimated_shares = 0 should be still perfect and >> even better candidates than estimated_shares = 1. >> >> Consider a pte-mapped large folio with 16 subpages, if we unmap 1-15, >> the current code will split folios and reclaim them while madvise goes >> on this folio; but if we unmap subpage 0, we will keep this folio and >> break. This is weird. >> >> For pmd-mapped large folios, we can still use "= 1" as the condition >> as anyway we have the entire map for it. So this patch doesn't change >> the condition for pmd-mapped large folios. >> This also explains why we had been using "= 1" for both pmd-mapped and >> pte-mapped large folios before commit 07e8c82b5eff ("madvise: convert >> madvise_cold_or_pageout_pte_range() to use folios"), because in the >> past, we used the mapcount of the specific subpage, since the subpage >> had pte present, its mapcount wouldn't be 0. >> >> The problem can be quite easily reproduced by writing a small program, >> unmapping the first subpage of a pte-mapped large folio vs. unmapping >> anyone other than the first subpage. >> >> Fixes: 2f406263e3e9 ("madvise:madvise_cold_or_pageout_pte_range(): don't use mapcount() against large folio for sharing check") >> Cc: Yin Fengwei <fengwei.yin@intel.com> >> Cc: Yu Zhao <yuzhao@google.com> >> Cc: Ryan Roberts <ryan.roberts@arm.com> >> Cc: David Hildenbrand <david@redhat.com> >> Cc: Kefeng Wang <wangkefeng.wang@huawei.com> >> Cc: Matthew Wilcox <willy@infradead.org> >> Cc: Minchan Kim <minchan@kernel.org> >> Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com> >> Cc: Yang Shi <shy828301@gmail.com> >> Signed-off-by: Barry Song <v-songbaohua@oppo.com> >> --- >> mm/madvise.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/mm/madvise.c b/mm/madvise.c >> index cfa5e7288261..abde3edb04f0 100644 >> --- a/mm/madvise.c >> +++ b/mm/madvise.c >> @@ -453,7 +453,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd, >> if (folio_test_large(folio)) { >> int err; >> >> - if (folio_estimated_sharers(folio) != 1) >> + if (folio_estimated_sharers(folio) > 1) >> break; >> if (pageout_anon_only_filter && !folio_test_anon(folio)) >> break; > > That's also what I do in > > https://lkml.kernel.org/r/20231124132626.235350-4-david@redhat.com > > I'll revive that soon.
Forgot to add: we can pull this in early.
Reviewed-by: David Hildenbrand <david@redhat.com>
(I'll do the simple folio_estimated_sharers() to folio_mapped_shared() conversion first and optimize with total mapcount separately)
-- Cheers,
David / dhildenb
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