netdev - Re: [PATCH 08/17] net: Do not coalesce skbs belonging to PFMEMALLOC sockets

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Date:	Wed, 20 Jun 2012 14:36:56 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-Netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>, Neil Brown <neilb@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Christie <michaelc@...wisc.edu>,
	Eric B Munson <emunson@...bm.net>,
	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
Subject: Re: [PATCH 08/17] net: Do not coalesce skbs belonging to PFMEMALLOC
 sockets

On Wed, Jun 20, 2012 at 02:04:52PM +0200, Eric Dumazet wrote:
> On Wed, 2012-06-20 at 12:44 +0100, Mel Gorman wrote:
> > Commit [bad43ca8: net: introduce skb_try_coalesce()] introduced an
> > optimisation to coalesce skbs to reduce memory usage and cache line
> > misses. In the case where the socket is used for swapping this can result
> > in a warning like the following.
> > 
> > [  110.476565] nbd0: page allocation failure: order:0, mode:0x20
> > [  110.476568] Pid: 2714, comm: nbd0 Not tainted 3.5.0-rc2-swapnbd-v12r2-slab #3
> > [  110.476569] Call Trace:
> > [  110.476573]  [<ffffffff811042d3>] warn_alloc_failed+0xf3/0x160
> > [  110.476578]  [<ffffffff81107c92>] __alloc_pages_nodemask+0x6e2/0x930
> >
> > <SNIP
> >  
> 
> 
> This makes absolutely no sense to me.
> 
> This patch changes input path, while your stack trace is about output
> path and a packet being fragmented.
> 

The intention was to avoid any coalescing in the input path due to avoid
packets that "were held back due to TCP_CORK or attempt at coalescing
tiny packet". I recognise that it is clumsy and will take the approach
instead of having __tcp_push_pending_frames() use sk_gfp_atomic() in the
output path.

Thanks.

-- 
Mel Gorman
SUSE Labs
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