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 17:23:16 -0500
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On Wed, 2005-11-16 at 14:10 -0800, Andrew Morton wrote:

> > How is the filesystem supposed to distinguish between the cases
> > "VM->writepage()", and "VM->writepages->mpage_writepages->writepage()"?
> > 
> 
> Via the writeback_control, hopefully.
> 
> For write_one_page(), sync_mode==WB_SYNC_ALL, so NFS should start the I/O
> immediately (it appears to not do so).

Sorry, but so does filemap_fdatawrite(). WB_SYNC_ALL clearly does not
discriminate between a writepages() and a single writepage() situation,
whatever the original intention was.

> For vmscan->writepage, wbc->for_reclaim is set, so we know that the IO
> should be pushed immediately.  nfs_writepage() seems to dtrt here.
> 
> With the proposed changes, we don't need that iput() in nfs_writepage(). 
> That worries me because I recall from a couple of years back that there are
> really subtle races with doing iput() on the vmscan->writepage() path. 
> Cannot remember what they were though...

Possibly to do with block filesystems that may trigger ->writepage()
while inside iput_final()? NFS can't do that.

Cheers,
  Trond

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