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

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

On Fri, May 11, 2012 at 01:10:34AM -0400, David Miller wrote:

quoted

From: Mel Gorman <mgorman@suse.de>
Date: Thu, 10 May 2012 14:54:14 +0100

quoted

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 <redacted>
Signed-off-by: Mel Gorman <mgorman@suse.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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