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

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Message-ID: <CA+55aFwEik1Q-D0d4pRTNq672RS2eHpT2ULzGfttaSWW69Tajw@mail.gmail.com>
Date:   Tue, 22 Nov 2016 14:13:47 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>, 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 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".

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.

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