Subject: Re: nfs unhappiness with memory pressure
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Kenny Simpson <theonetruekenny@yahoo.com>, Andrew Morton <akpm@osdl.org>,
       linux-kernel@vger.kernel.org
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Date: Mon, 05 Dec 2005 19:48:19 -0500
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On Tue, 2005-12-06 at 10:40 +1100, Nick Piggin wrote:

> > ...and most important of all: 'unstable' does _not_ mean that I/O is
> > active on those pages (unlike the apparent assumption in
> > vm_throttle_write.
> > That is why the choice is either to kick pdflush there, or to remove
> > nr_unstable from the accounting in that loop.
> > 
> 
> Doesn't matter if IO is actually active or not, if you've allocated
> memory for these unstable pages, then page reclaim can scan itself
> to death, which is what seems to have happened here. And which is
> what vm_throttle_write is supposed to help.

Unless someone somehow triggers an NFS commit, then nr_unstable is not
ever going to decrease, and your process will end up looping forever. In
fact, those nr_writeback that refer to NFS pages, will end up being
added to nr_unstable (because they have been written to the server, but
not committed to disk).

Cheers,
  Trond

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