Re: [patch] mm: memcontrol: clarify migration where old page is uncharged

[Michal Hocko]

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@xxxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  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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