Re: mmap over nfs leads to excessive system load

[Trond Myklebust]

On Wed, 2005-11-16 at 17:23 -0500, Trond Myklebust wrote:
> 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.

IMHO, the correct way to distinguish between the two would be to use the
wbc->nr_to_write field. If all the instances of writepage() were to set
that field to '1', then the filesystems could do the right thing.

As it is, you have shrink_list() that sets it to the value
"SWAP_CLUSTER_MAX" for no apparent reason...

Cheers,
  Trond

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