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

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Message-ID: <CAOJsxLH54aUjVE3b7queQMOJP1kb+bxtUTAUA=T=N378M5_hJA@mail.gmail.com>
Date:	Tue, 25 Oct 2011 22:39:34 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
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>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org
Subject: Re: [PATCH] mm: avoid livelock on !__GFP_FS allocations

Hi Mel,

On Tue, Oct 25, 2011 at 2:23 PM, Mel Gorman <mgorman@...e.de> wrote:
> I see what you mean with GFP_NOIO but there is an important difference
> between GFP_NOIO and suspend.  A GFP_NOIO low-order allocation currently
> implies __GFP_NOFAIL as commented on in should_alloc_retry(). If no progress
> is made, we call wait_iff_congested() and sleep for a bit. As the system
> is running, kswapd and other process activity will proceed and eventually
> reclaim enough pages for the GFP_NOIO allocation to succeed. In a running
> system, GFP_NOIO can stall for a period of time but your patch will cause
> the allocation to fail. While I expect callers return ENOMEM or handle
> the situation properly with a wait-and-retry loop, there will be
> operations that fail that used to succeed. This is why I'd prefer it was
> a suspend-specific fix unless we know there is a case where a machine
> livelocks due to a GFP_NOIO allocation looping forever and even then I'd
> wonder why kswapd was not helping.

I'm not that happy about your patch because it's going to the
direction where the page allocator is special-casing for suspension.
If you don't think it's a good idea to fix it for the general case
(i.e. Colin's patch), why don't we fix it up in a way that suspension
code passes sane GFP flags?

                        Pekka
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