Re: [PATCH] mm: memcontrol: fix the return in mem_cgroup_margin

[Vladimir Davydov <vdavydov@virtuozzo.com>]

On Thu, May 19, 2016 at 09:44:53AM +0800, Li RongQing wrote:
> On Wed, May 18, 2016 at 3:32 PM, Michal Hocko <mhocko@kernel.org> wrote:
> > count should always be smaller than memsw.limit (this is a hard limit).
> > Even if we have some temporary breach then the code should work as
> > expected because margin is initialized to 0 and memsw.limit >= limit.
> 
> is it possible for this case? for example
> 
> memory count is 500, memory limit is 600; the margin is set to 100 firstly,
> then check memory+swap limit, its count(1100) is bigger than its limit(1000),
> then the margin 100 is returned wrongly.

I guess it is possible, because try_charge forces charging __GFP_NOFAIL
allocations, which may result in memsw.limit excess. If we are below
memory.limit and there's nothing to reclaim to reduce memsw.usage, we
might end up looping in try_charge forever. I've never seen that happen
in practice, but I still think the patch is worth applying.

Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>

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