netdev - Re: [PATCH 01/12] netvm: Prevent a stream-specific deadlock

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Date:	Mon, 14 May 2012 11:56:04 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	David Miller <davem@...emloft.net>
Cc:	akpm@...ux-foundation.org, linux-mm@...ck.org,
	netdev@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, Trond.Myklebust@...app.com,
	neilb@...e.de, hch@...radead.org, a.p.zijlstra@...llo.nl,
	michaelc@...wisc.edu, emunson@...bm.net
Subject: Re: [PATCH 01/12] netvm: Prevent a stream-specific deadlock

On Fri, May 11, 2012 at 01:10:34AM -0400, David Miller wrote:
> From: Mel Gorman <mgorman@...e.de>
> Date: Thu, 10 May 2012 14:54:14 +0100
> 
> > It could happen that all !SOCK_MEMALLOC sockets have buffered so
> > much data that we're over the global rmem limit. This will prevent
> > SOCK_MEMALLOC buffers from receiving data, which will prevent userspace
> > from running, which is needed to reduce the buffered data.
> > 
> > Fix this by exempting the SOCK_MEMALLOC sockets from the rmem limit.
> > 
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Signed-off-by: Mel Gorman <mgorman@...e.de>
> 
> This introduces an invariant which I am not so sure is enforced.
> 
> With this change it is absolutely required that once a socket
> becomes SOCK_MEMALLOC it must never _ever_ lose that attribute.
> 

This is effectively true. In the NFS case, the flag is cleared on
swapoff after all the entries have been paged in. In the NBD case,
SOCK_MEMALLOC is left set until the socket is destroyed. I'll update the
changelog.

-- 
Mel Gorman
SUSE Labs
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