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

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Message-ID: <20161129172854.GF9796@dhcp22.suse.cz>
Date:   Tue, 29 Nov 2016 18:28:54 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>, Tejun Heo <tj@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, linux-mm <linux-mm@...ck.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 29-11-16 09:17:37, Linus Torvalds wrote:
> On Tue, Nov 29, 2016 at 9:13 AM, Michal Hocko <mhocko@...nel.org> wrote:
> > How does this look like?
> 
> No.
> 
> I *really* want people to write out that "I am ok with the allocation failing".
> 
> It's not an "inconvenience". It's a sign that you are competent and
> that you know it will fail, and that you can handle it.
> 
> If you don't show that you know that, we warn about it.

How does this warning help those who are watching the logs? What are
they supposed to do about it? Unlike GFP_ATOMIC there is no tuning you
can possibly do.

>From my experience people tend to report those and worry about them
(quite often confusing them with the real OOM) and we usually only can
explain that this is nothing to worry about... And so then we sprinkle
GFP_NOWARN all over the place as we hit those. Is this really what we
want?

> And no, "GFP_NOWAIT" does *not* mean "I have a good fallback".

I am confused, how can anybody _rely_ on GFP_NOWAIT to succeed?

-- 
Michal Hocko
SUSE Labs