Re: [PATCH 3/4] memcg: punt high overage reclaim to return-to-userland path

[Tejun Heo <tj@kernel.org>]

Hello, Vladimir.

On Fri, Aug 28, 2015 at 07:36:11PM +0300, Vladimir Davydov wrote:
> > * try_charge() can be invoked from any in-kernel allocation site and
> >   reclaim path may use considerable amount of stack.  This can lead to
> >   stack overflows which are extremely difficult to reproduce.
> 
> IMO this paragraph does not justify this patch at all, because one will
> still invoke direct reclaim from try_charge() on hitting the hard limit.

Ah... right, and we can't defer direct reclaim for hard limit.

> > * If the allocation doesn't have __GFP_WAIT, direct reclaim is
> >   skipped.  If a process performs only speculative allocations, it can
> >   blow way past the high limit.  This is actually easily reproducible
> >   by simply doing "find /".  VFS tries speculative !__GFP_WAIT
> >   allocations first, so as long as there's memory which can be
> >   consumed without blocking, it can keep allocating memory regardless
> >   of the high limit.
> 
> I think there shouldn't normally occur a lot of !__GFP_WAIT allocations
> in a row - they should still alternate with normal __GFP_WAIT
> allocations. Yes, that means we can breach memory.high threshold for a
> short period of time, but it isn't a hard limit, so it looks perfectly
> fine to me.
> 
> I tried to run `find /` over ext4 in a cgroup with memory.high set to
> 32M and kmem accounting enabled. With such a setup memory.current never
> got higher than 33152K, which is only 384K greater than the memory.high.
> Which FS did you use?

ext4.  Here, it goes onto happily consuming hundreds of megabytes with
limit set at 32M.  We have quite a few places where !__GFP_WAIT
allocations are performed speculatively in hot paths with fallback
slow paths, so this is bound to happen somewhere.

Thanks.

-- 
tejun

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