linux-ext4 - Re: How to handle TIF

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Date:	Sat, 21 Feb 2015 22:48:49 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	akpm@...ux-foundation.org
Cc:	tytso@....edu, david@...morbit.com, hannes@...xchg.org,
	mhocko@...e.cz, dchinner@...hat.com, linux-mm@...ck.org,
	rientjes@...gle.com, oleg@...hat.com, mgorman@...e.de,
	torvalds@...ux-foundation.org, xfs@....sgi.com,
	linux-ext4@...r.kernel.org
Subject: Re: How to handle TIF_MEMDIE stalls?

Andrew Morton wrote:
> On Fri, 20 Feb 2015 22:20:00 -0500 "Theodore Ts'o" <tytso@....edu> 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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