linux-kernel - Re: [PATCH] block,blkcg: use __GFP_NOWARN for best-effort allocations in blkcg

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Message-ID: <20161128171907.GA14754@htj.duckdns.org>
Date:   Mon, 28 Nov 2016 12:19:07 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, linux-mm <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Marc MERLIN <marc@...lins.org>
Subject: Re: [PATCH] block,blkcg: use __GFP_NOWARN for best-effort
 allocations in blkcg

Hello,

On Wed, Nov 23, 2016 at 09:50:12AM +0100, Vlastimil Babka wrote:
> > You'd certainly _hope_ that atomic allocations either have fallbacks
> > or are harmless if they fail, but I'd still rather see that
> > __GFP_NOWARN just to make that very much explicit.
> 
> A global change to GFP_NOWAIT would of course mean that we should audit its
> users (there don't seem to be many), whether they are using it consciously
> and should not rather be using GFP_ATOMIC.

A while ago, I thought about something like, say, GFP_MAYBE which is
combination of NOWAIT and NOWARN but couldn't really come up with
scenarios where one would want to use NOWAIT w/o NOWARN.  If an
allocation is important enough to warn the user of its failure, it
better be dipping into the atomic reserve pool; otherwise, it doesn't
make sense to make noise.

Maybe we can come up with a better name which signifies that this is
likely to fail every now and then but I still think it'd be beneficial
to make it quiet by default.  Linus, do you still think NOWARN should
be explicit?

Thanks.

-- 
tejun