linux-kernel - Re: [PATCH RFC 02/10] mm: Make shrink

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Date:   Wed, 8 Aug 2018 21:36:18 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     mhocko@...e.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH RFC 02/10] mm: Make shrink_slab() lockless

On 2018/08/08 20:51, Kirill Tkhai wrote:
> @@ -192,7 +193,6 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
>  	int id, ret = -ENOMEM;
>  
>  	down_write(&shrinker_rwsem);
> -	/* This may call shrinker, so it must use down_read_trylock() */
>  	id = idr_alloc(&shrinker_idr, SHRINKER_REGISTERING, 0, 0, GFP_KERNEL);
>  	if (id < 0)
>  		goto unlock;

I don't know why perf reports down_read_trylock(&shrinker_rwsem). But
above code is already bad. GFP_KERNEL allocation involves shrinkers and
the OOM killer would be invoked because shrinkers are defunctional due to
this down_write(&shrinker_rwsem). Please avoid blocking memory allocation
with shrinker_rwsem held.