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

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Date:   Wed, 23 Nov 2016 09:50:12 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>
Cc:     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

On 11/22/2016 11:13 PM, Linus Torvalds wrote:
> On Tue, Nov 22, 2016 at 8:48 AM, Tejun Heo <tj@...nel.org> wrote:
>>
>> Hello,
>>
>> On Tue, Nov 22, 2016 at 04:47:49PM +0100, Vlastimil Babka wrote:
>>> Thanks. Makes me wonder whether we should e.g. add __GFP_NOWARN to
>>> GFP_NOWAIT globally at some point.
>>
>> Yeah, that makes sense.  The caller is explicitly saying that it's
>> okay to fail the allocation.
>
> I'm not so convinced about the "atomic automatically means you shouldn't warn".

Right, but atomic allocations should be using GFP_ATOMIC, which allows 
to use the atomic reserves. I meant here just GFP_NOWAIT which does not 
allow reserves, for allocations that are not in atomic context, but 
still don't want to reclaim for performance or whatever reasons, and 
have a suitable fallback. It's their choice to not spend any effort on 
the allocation and thus they shouldn't spew warnings IMHO.

> 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.

Vlastimil

> Because as it is, atomic allocations certainly get to dig deeper into
> our memory reserves, but they most definitely can fail, and I
> definitely see how some code has no fallback because it thinks that
> the deeper reserves mean that it will succeed.
>
>              Linus
>