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madvise:madvise_cold_or_pageout_pte_range(): allow split while folio_estimated_sharers = 0
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 commit07e8c82b5e
("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. Link: https://lkml.kernel.org/r/20240221085036.105621-1-21cnbao@gmail.com Fixes:2f406263e3
("madvise:madvise_cold_or_pageout_pte_range(): don't use mapcount() against large folio for sharing check") Signed-off-by: Barry Song <v-songbaohua@oppo.com> Reviewed-by: David Hildenbrand <david@redhat.com> Reviewed-by: Vishal Moola (Oracle) <vishal.moola@gmail.com> Cc: Yin Fengwei <fengwei.yin@intel.com> Cc: Yu Zhao <yuzhao@google.com> Cc: Ryan Roberts <ryan.roberts@arm.com> Cc: Kefeng Wang <wangkefeng.wang@huawei.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Minchan Kim <minchan@kernel.org> Cc: Yang Shi <shy828301@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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@ -453,7 +453,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
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if (folio_test_large(folio)) {
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int err;
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if (folio_estimated_sharers(folio) != 1)
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if (folio_estimated_sharers(folio) > 1)
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break;
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if (pageout_anon_only_filter && !folio_test_anon(folio))
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break;
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