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Re: How to handle TIF_MEMDIE stalls?

To: akpm@xxxxxxxxxxxxxxxxxxxx
Subject: Re: How to handle TIF_MEMDIE stalls?
From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Date: Sat, 21 Feb 2015 22:48:49 +0900
Cc: tytso@xxxxxxx, david@xxxxxxxxxxxxx, hannes@xxxxxxxxxxx, mhocko@xxxxxxx, dchinner@xxxxxxxxxx, linux-mm@xxxxxxxxx, rientjes@xxxxxxxxxx, oleg@xxxxxxxxxx, mgorman@xxxxxxx, torvalds@xxxxxxxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20150221011907.2d26c979.akpm@xxxxxxxxxxxxxxxxxxxx>
References: <20150219225217.GY12722@dastard> <201502201936.HBH34799.SOLFFFQtHOMOJV@xxxxxxxxxxxxxxxxxxx> <20150220231511.GH12722@dastard> <20150221032000.GC7922@xxxxxxxxx> <20150221011907.2d26c979.akpm@xxxxxxxxxxxxxxxxxxxx>
Andrew Morton wrote:
> On Fri, 20 Feb 2015 22:20:00 -0500 "Theodore Ts'o" <tytso@xxxxxxx> wrote:
> 
> > +akpm
> 
> I was hoping not to have to read this thread ;)

Sorry for getting so complicated.

> What I'm not really understanding is why the pre-3.19 implementation
> actually worked.  We've exhausted the free pages, we're not succeeding
> at reclaiming anything, we aren't able to oom-kill anyone.  Yet it
> *does* work - we eventually find that memory and everything proceeds.
> 
> How come?  Where did that memory come from?
> 

Even without __GFP_NOFAIL, GFP_NOFS / GFP_NOIO allocations retried forever
(without invoking the OOM killer) if order <= PAGE_ALLOC_COSTLY_ORDER and
TIF_MEMDIE is not set. Somebody else volunteered that memory while retrying.
This implies silent hang-up forever if nobody volunteers memory.

> And yes, I agree that sites such as xfs's kmem_alloc() should be
> passing __GFP_NOFAIL to tell the page allocator what's going on.  I
> don't think it matters a lot whether kmem_alloc() retains its retry
> loop.  If __GFP_NOFAIL is working correctly then it will never loop
> anyway...

Commit 9879de7373fc ("mm: page_alloc: embed OOM killing naturally into
allocation slowpath") inadvertently changed GFP_NOFS / GFP_NOIO allocations
not to retry unless __GFP_NOFAIL is specified. Therefore, either applying
Johannes's akpm-doesnt-know-why-it-works patch or passing __GFP_NOFAIL
will restore the pre-3.19 behavior (with possibility of silent hang-up).

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