Subject: Re: mmap over nfs leads to excessive system load
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Andrew Morton <akpm@osdl.org>
Cc: theonetruekenny@yahoo.com, linux-kernel@vger.kernel.org
In-Reply-To: <20051116110938.1bf54339.akpm@osdl.org>
References: <20051116150141.29549.qmail@web34113.mail.mud.yahoo.com>
	 <1132163057.8811.15.camel@lade.trondhjem.org>
	 <20051116100053.44d81ae2.akpm@osdl.org>
	 <1132166062.8811.30.camel@lade.trondhjem.org>
	 <20051116110938.1bf54339.akpm@osdl.org>
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Date: Wed, 16 Nov 2005 15:05:00 -0500
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On Wed, 2005-11-16 at 11:09 -0800, Andrew Morton wrote:
> Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> >
> > On Wed, 2005-11-16 at 10:00 -0800, Andrew Morton wrote:
> > 
> > > That will fix it, but the PageWriteback accounting is still wrong.
> > > 
> > > Is it not possible to use set_page_writeback()/end_page_writeback()?
> > 
> > Not really. The pages aren't flushed at this time. We the point is to
> > gather several pages and coalesce them into one over-the-wire RPC call.
> > That means we cannot really do it from inside ->writepage().
> > 
> 
> I still don't get it.
> 
> Once nfs_writepage() has been called, the page is conceptually "under
> writeback", yes?  In that, at some point in the future, it will be written
> to backing store.
> 
> Hence it's perfectly appropriate to run set_page_writepage() within
> nfs_writepage().  It's a matter of finding the right place for the
> end_page_writeback().

The point is that the process of flushing has not been started at that
time, so anybody that calls wait_on_page_writeback() immediately after
calling writepage() may end up waiting for a very long time indeed
(probably until the next pdflush).

Cheers,
  Trond

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