linux-kernel - Re: [PATCH] mm: respect the __GFP_NOWARN flag when warning about stalls

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Message-Id: <201709132314.BID39077.HMFOJSLFtVOFOQ@I-love.SAKURA.ne.jp>
Date:   Wed, 13 Sep 2017 23:14:43 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     vbabka@...e.cz, mhocko@...nel.org, mpatocka@...hat.com
Cc:     hannes@...xchg.org, mgorman@...e.de, dave.hansen@...el.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: respect the __GFP_NOWARN flag when warning about stalls

Vlastimil Babka wrote:
> On 09/13/2017 03:54 PM, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> >> Let's see what others think about this.
> > 
> > Whether __GFP_NOWARN should warn about stalls is not a topic to discuss.
> 
> It is the topic of this thread, which tries to address a concrete
> problem somebody has experienced. In that context, the rest of your
> concerns seem to me not related to this problem, IMHO.

I suggested replacing warn_alloc() with safe/useful one rather than tweaking
warn_alloc() about __GFP_NOWARN.

> 
> > I consider warn_alloc() for reporting stalls is broken. It fails to provide
> > backtrace of stalling location. For example, OOM lockup with oom_lock held
> > cannot be reported by warn_alloc(). It fails to provide readable output when
> > called concurrently. For example, concurrent calls can cause printk()/
> > schedule_timeout_killable() lockup with oom_lock held. printk() offloading is
> > not an option, for there will be situations where printk() offloading cannot
> > be used (e.g. queuing via printk() is faster than writing to serial consoles
> > which results in unreadable logs due to log_bug overflow).