linux-kernel - Re: System freezes after OOM

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Message-ID: <alpine.DEB.2.10.1607131659190.92037@chino.kir.corp.google.com>
Date:	Wed, 13 Jul 2016 17:01:37 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
cc:	Mikulas Patocka <mpatocka@...hat.com>,
	Michal Hocko <mhocko@...nel.org>,
	Ondrej Kozina <okozina@...hat.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Stanislav Kozina <skozina@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: System freezes after OOM

On Wed, 13 Jul 2016, Tetsuo Handa wrote:

> I wonder whether commit f9054c70d28bc214 ("mm, mempool: only set
> __GFP_NOMEMALLOC if there are free elements") is doing correct thing.
> It says
> 
>     If an oom killed thread calls mempool_alloc(), it is possible that it'll
>     loop forever if there are no elements on the freelist since
>     __GFP_NOMEMALLOC prevents it from accessing needed memory reserves in
>     oom conditions.
> 
> but we can allow mempool_alloc(__GFP_NOMEMALLOC) requests to access
> memory reserves via below change, can't we? The purpose of allowing
> ALLOC_NO_WATERMARKS via TIF_MEMDIE is to make sure current allocation
> request does not to loop forever inside the page allocator, isn't it?

This would defeat the purpose of __GFP_NOMEMALLOC for oom killed threads, 
so you'd need to demonstrate that isn't a problem for the current users 
and then change the semantics of the gfp flag.

> Why we need to allow mempool_alloc(__GFP_NOMEMALLOC) requests to use
> ALLOC_NO_WATERMARKS when TIF_MEMDIE is not set?
> 

mempool_alloc(__GFP_NOMEMALLOC) is forbidden.