Re: [PATCH v3 2/2] memcg: punt high overage reclaim to return-to-userland path

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Sep 15, 2015 at 11:53:55AM -0400, Tejun Heo wrote:
> Hello, Johannes.
> 
> On Tue, Sep 15, 2015 at 09:47:24AM +0200, Johannes Weiner wrote:
> > Why can't we simply fail NOWAIT allocations when the high limit is
> > breached? We do the same for the max limit.
> 
> Because that can lead to continued systematic failures of NOWAIT
> allocations.  For that to work, we'll have to add async reclaimaing.
> 
> > As I see it, NOWAIT allocations are speculative attempts on available
> > memory. We should be able to just fail them and have somebody that is
> > allowed to reclaim try again, just like with the max limit.
> 
> Yes, but the assumption is that even back-to-back NOWAIT allocations
> won't continue to fail indefinitely.

But they have been failing indefinitely forever once you hit the hard
limit in the past. There was never an async reclaim provision there.

I can definitely see that the unconstrained high limit breaching needs
to be fixed one way or another, I just don't quite understand why you
chose to go for new semantics. Is there a new or a specific usecase
you had in mind when you chose deferred reclaim over simply failing?

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