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

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Date:	Tue, 15 May 2012 11:08:29 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	David Miller <davem@...emloft.net>, 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,
	michaelc@...wisc.edu, emunson@...bm.net
Subject: Re: [PATCH 01/12] netvm: Prevent a stream-specific deadlock

On Tue, May 15, 2012 at 11:47:14AM +0200, Peter Zijlstra wrote:
> On Tue, 2012-05-15 at 10:14 +0100, Mel Gorman wrote:
> > @@ -289,6 +289,18 @@ void sk_clear_memalloc(struct sock *sk)
> >         sock_reset_flag(sk, SOCK_MEMALLOC);
> >         sk->sk_allocation &= ~__GFP_MEMALLOC;
> >         static_key_slow_dec(&memalloc_socks);
> > +
> > +       /*
> > +        * SOCK_MEMALLOC is allowed to ignore rmem limits to ensure forward
> > +        * progress of swapping. However, if SOCK_MEMALLOC is cleared while
> > +        * it has rmem allocations there is a risk that the user of the
> > +        * socket cannot make forward progress due to exceeding the rmem
> > +        * limits. By rights, sk_clear_memalloc() should only be called
> > +        * on sockets being torn down but warn and reset the accounting if
> > +        * that assumption breaks.
> > +        */
> > +       if (WARN_ON(sk->sk_forward_alloc))
> 
> WARN_ON_ONCE() perhaps?
> 

I do not expect SOCK_MEMALLOC to be cleared frequently at all with the
possible exception of swapon/swapoff stress tests. If the flag is being
cleared regularly with rmem tokens then that is interesting in itself
but a WARN_ON_ONCE would miss it.

> > +               sk_mem_reclaim(sk);
> >  } 

-- 
Mel Gorman
SUSE Labs
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