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SubjectRe: [patch] mm: memcontrol: clarify migration where old page is uncharged
On Sun 19-10-14 11:30:47, Johannes Weiner wrote:
> 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>

Acked-by: Michal Hocko <mhocko@suse.cz>

> ---
> mm/memcontrol.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index fc1d7ca96b9d..76892eb89d26 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6166,7 +6166,12 @@ 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;
> --
> 2.1.2
>

--
Michal Hocko
SUSE Labs


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