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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>
This commit is contained in:
Johannes Weiner 2014-12-10 15:43:46 -08:00 committed by Linus Torvalds
parent dfe0e773d0
commit 7d5e324573
1 changed files with 6 additions and 1 deletions
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7
mm/memcontrol.c
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@ -6157,7 +6157,12 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
if (PageCgroupUsed(pc))
return;
/* Re-entrant migration: old page already uncharged? */
/*
* Swapcache readahead pages can get migrated before being
* charged, and migration from compaction can happen to an
* uncharged page when the PFN walker finds a page that
* reclaim just put back on the LRU but has not released yet.
*/
pc = lookup_page_cgroup(oldpage);
if (!PageCgroupUsed(pc))
return;