Re: [PATCH mmotm] memcg: Ignore partial THP when moving task

[Johannes Weiner]

On Tue, Dec 08, 2015 at 06:13:09PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> After "mm: rework mapcount accounting to enable 4k mapping of THPs"
> it is possible to have a partial THP accessible via ptes. Memcg task
> migration code is not prepared for this situation and uncharges the tail
> page from the original memcg while the original THP is still charged via
> the head page which is not mapped to the moved task. The page counter
> of the origin memcg will underflow when the whole THP is uncharged later
> on and lead to:
> WARNING: CPU: 0 PID: 1340 at mm/page_counter.c:26 page_counter_cancel+0x34/0x40()
> reported by Minchan Kim.
> 
> This patch prevents from the underflow by skipping any partial THP pages
> in mem_cgroup_move_charge_pte_range. PageTransCompound is checked when
> we do pte walk. This means that a process might leave a partial THP
> behind in the original memcg if there is no other process mapping it via
> pmd but this is considered acceptable because it shouldn't happen often
> and this is not considered a memory leak because the original THP is
> still accessible and reclaimable. Moreover the task migration has always
> been racy and never guaranteed to move all pages.
> 
> Reported-by: Minchan Kim <minchan@xxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> 
> Hi,
> this is a patch tested by Minchan in the original thread [1]. I have
> only replaced PageCompound with PageTransCompound because other similar
> fixes in mmotm used this one. The underlying implementation is the same.
> Johannes, I have kept your a-b but let me know if you are not OK with the
> changelog.

Looks good to me, thanks Michal. Please keep my Acked-by.
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