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 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.

yes, except ->writepages is supposed to decrement nr_to_write as it proceeds,
so it'll end up at `1' by accident on the last go around the loop.

I think a separate boolean is better - it's just a single bit.

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

How weird.  That's presumably wrong, but I'd need to check the changelogs
to doublecheck.  ugh, 264 of them.