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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