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
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Date: Wed, 16 Nov 2005 16:49:45 -0500
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On Wed, 2005-11-16 at 13:31 -0800, Andrew Morton wrote:
> But block-backed filesytems have the same concern: we don't want to do a
> whole bunch of 4k I/Os.  Hence the writepages() interface, which is the
> appropriate place to be building up these large I/Os.
> 
> NFS does nfw_writepages->mpage_writepages->nfs_writepage and to build the
> large I/Os it leaves the I/O pending on return from nfs_writepage().  It
> appears to flush any pending pages on the exit path from nfs_writepages().
> 
> If that's a correct reading then there doesn't appear to be any way in
> which there's dangling I/O left to do after nfs_writepages() completes.

Agreed. AFAICS, nfs_writepages should be quite OK, however writepage()
on its own _is_ problematic.

Look at the usage in write_one_page(), which calls directly down to
->writepage(), and then immediately does a wait_on_page_writeback().

How is the filesystem supposed to distinguish between the cases
"VM->writepage()", and "VM->writepages->mpage_writepages->writepage()"?

Cheers,
  Trond

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