Message-ID: <20111115103007.GB27150@suse.de>
Date:	Tue, 15 Nov 2011 10:30:07 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Colin Cross <ccross@...roid.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: avoid livelock on !__GFP_FS allocations

On Mon, Nov 14, 2011 at 07:38:12PM +0100, Andrea Arcangeli wrote:
> On Mon, Nov 14, 2011 at 02:04:21PM +0000, Mel Gorman wrote:
> > In his fix, he avoided retrying the allocation if reclaim made no
> > progress and __GFP_FS was not set. The problem is that this would
> > result in GFP_NOIO allocations failing that previously succeeded
> > which would be very unfortunate.
> 
> GFP_NOFS are made by filesystems/buffers to avoid locking up on fs/vfs
> locking. Those also should be able to handle failure gracefully but
> userland is more likely to get a -ENOMEM from these (for example
> during direct-io) if those fs allocs fails.

I was also vaguely recalling Roland's talk at day 2 of kernel summit
(reported at http://lwn.net/Articles/464500/) where he talked about
error handling. One point he made was that some filesystems ran into
problems in the event of memory allocation failure. I didn't audit
if the block layer handles it better but one way or the other I did
not want to throw the the block or filesystem layers curve balls.

> So clearly it sounds risky
> to apply the modification quoted above and risk having any GFP_NOFS
> fail. Said that I'm afraid we're not deadlock safe with current code
> that cannot fail but there's no easy solution and no way to fix it in
> the short term, and it's only a theoretical concern.

It's still a valid concern. The expectation is that we are protected
from deadlocks a combination of mempools and the watermarks forcing
processes to stall in direct reclaim leaving a cushion of pages for
reclaim using PF_MEMALLOC to always make forward progress. Patches
that break how watermarks work tend to lead to deadlock.

> For !__GFP_FS allocations, __GFP_NOFAIL is the default for order <=
> PAGE_ALLOC_COSTLY_ORDER and __GFP_NORETRY is the default for order >
> PAGE_ALLOC_COSTLY_ORDER. This inconsistency is not so clean in my
> view.

Is your concern that the behaviour of the allocator changes quite
significantly for orders < PAGE_ALLOC_COSTLY_ORDER?

I agree with you that it would be nicer if there was a gradual scaling
back of how much work the allocator did that depended on order. To
date there has not been much pressure or motivation to implement it.

> Also for GFP_KERNEL/USER/__GFP_FS regular allocations the
> __GFP_NOFAIL looks more like a __GFP_MAY_OOM.  But if we fix that and
> we drop __GFP_NORETRY, and we set __GFP_NOFAIL within the
> GFP_NOFS/NOIO #defines (to remove the magic PAGE_ALLOC_COSTLY_ORDER
> check in should_alloc_retry) we may loop forever if somebody allocates
> several mbytes of huge contiguous RAM with GFP_NOIO. So at least
> there's a practical explanation for the current code.
> 

Yep.

> Patch looks good to me (and safer) even if I don't like keeping
> infinite loops from a purely theoretical standpoint.

>From a more practical point of view, I am generally more concerned
with abnormally large stalls from within the page allocator which
is what patches like "Do not stall in synchronous compaction for THP
allocations" address.

-- 
Mel Gorman
SUSE Labs
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