linux-kernel - Re: [PATCH] mm: avoid livelock on !__GFP

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Date:	Tue, 25 Oct 2011 15:18:15 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Mel Gorman <mgorman@...e.de>
cc:	Colin Cross <ccross@...roid.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org
Subject: Re: [PATCH] mm: avoid livelock on !__GFP_FS allocations

On Tue, 25 Oct 2011, Mel Gorman wrote:

> That said, it will be difficult to remember why checking __GFP_NOFAIL in
> this case is necessary and someone might "optimitise" it away later. It
> would be preferable if it was self-documenting. Maybe something like
> this? (This is totally untested)
> 

__GFP_NOFAIL _should_ be optimized away in this case because all he's 
passing is __GFP_WAIT | __GFP_NOFAIL.  That doesn't make any sense unless 
all you want to do is livelock.

__GFP_NOFAIL doesn't mean the page allocator would infinitely loop in all 
conditions.  That's why GFP_ATOMIC | __GFP_NOFAIL actually fails, and I 
would argue that __GFP_WAIT | __GFP_NOFAIL should fail as well since it's 
the exact same condition except doesn't have access to the extra memory 
reserves.

Suspend needs to either set __GFP_NORETRY to avoid the livelock if it's 
going to disable all means of memory reclaiming or freeing in the page 
allocator.  Or, better yet, just make it GFP_NOWAIT.
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