Re: mmap over nfs leads to excessive system load

[Andrew Morton <akpm@osdl.org>]

Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
>
> 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()"?
> 

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).

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...
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