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

[Tejun Heo <tj@kernel.org>]

Hello, Vladmir.

On Fri, Aug 28, 2015 at 11:45:54PM +0300, Vladimir Davydov wrote:
> Actually, memory.high by itself *is* the protection against GFP_NOWAIT
> allocations, similarly to zone watermarks. W/o it we would have no other
> choice but fail a GFP_NOWAIT allocation on hitting memory.max. One
> should just set it so that
> 
>   memory.max - memory.high > [max sum size of !__GFP_WAIT allocations
>                               that can normally occur in a row]

While this would be true in many cases, I don't think this is the
intention of the two knobs and the space between high and max can be
filled up by anything which can't be reclaimed - e.g. too many dirty /
writeback pages on a slow device or memlocked pages.  If it were
really the buffer for GFP_NOWAIT, there's no reason to even make it a
separate knob and we *may* change how over-high reclaim behaves in the
future, so let's please not dig ourselves into something too specific.

> That being said, currently I don't see any point in making memory.high
> !__GFP_WAIT-safe.

Yeah, as long as the blow up can't be triggered consistently, it
should be fine.

Thanks.

-- 
tejun

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