linux-kernel - Re: [PATCH 0/2 v8] oom: capture unreclaimable slab info in oom message

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Message-Id: <201709290457.CAC30283.VFtMFOFOJLQHOS@I-love.SAKURA.ne.jp>
Date:   Fri, 29 Sep 2017 04:57:53 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     yang.s@...baba-inc.com, mhocko@...nel.org
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2 v8] oom: capture unreclaimable slab info in oom message

Yang Shi wrote:
> On 9/27/17 9:36 PM, Tetsuo Handa wrote:
> > On 2017/09/28 6:46, Yang Shi wrote:
> >> Changelog v7 -> v8:
> >> * Adopted Michal’s suggestion to dump unreclaim slab info when unreclaimable slabs amount > total user memory. Not only in oom panic path.
> > 
> > Holding slab_mutex inside dump_unreclaimable_slab() was refrained since V2
> > because there are
> > 
> > 	mutex_lock(&slab_mutex);
> > 	kmalloc(GFP_KERNEL);
> > 	mutex_unlock(&slab_mutex);
> > 
> > users. If we call dump_unreclaimable_slab() for non OOM panic path, aren't we
> > introducing a risk of crash (i.e. kernel panic) for regular OOM path?
> 
> I don't see the difference between regular oom path and oom path other 
> than calling panic() at last.
> 
> And, the slab dump may be called by panic path too, it is for both 
> regular and panic path.

Calling a function that might cause kerneloops immediately before calling panic()
would be tolerable, for the kernel will panic after all. But calling a function
that might cause kerneloops when there is no plan to call panic() is a bug.

> 
> Thanks,
> Yang
> 
> > 
> > We can try mutex_trylock() from dump_unreclaimable_slab() at best.
> > But it is still remaining unsafe, isn't it?
> > 
>