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

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Message-Id: <20120514.162634.1094732813264319951.davem@davemloft.net>
Date:	Mon, 14 May 2012 16:26:34 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	mgorman@...e.de
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

From: Mel Gorman <mgorman@...e.de>
Date: Mon, 14 May 2012 11:56:04 +0100

> 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.

Bugs happen, you need to find a way to assert that nobody every does
this.  Because if a bug is introduced which makes this happen, it will
otherwise be very difficult to debug.
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