mm: memcontrol: clarify migration where old page is uncharged
Better explain re-entrant migration when compaction races with reclaim, and also mention swapcache readahead pages as possible uncharged migration sources. Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Cc: Hugh Dickins <hughd@google.com> Acked-by: Michal Hocko <mhocko@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -6157,7 +6157,12 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
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if (PageCgroupUsed(pc))
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return;
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/* Re-entrant migration: old page already uncharged? */
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/*
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* Swapcache readahead pages can get migrated before being
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* charged, and migration from compaction can happen to an
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* uncharged page when the PFN walker finds a page that
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* reclaim just put back on the LRU but has not released yet.
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*/
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pc = lookup_page_cgroup(oldpage);
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if (!PageCgroupUsed(pc))
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return;
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