Re: writeback-highmem

[Andrea Arcangeli <andrea@suse.de>]

On Thu, Jan 20, 2005 at 10:26:30PM -0800, Andrew Morton wrote:
> Andrea Arcangeli <andrea@suse.de> wrote:
> >
> > This needed highmem fix from Rik is still missing too, so please apply
> >  along the other 5 (it's orthogonal so you can apply this one in any
> >  order you want).
> > 
> >  From: Rik van Riel <riel@redhat.com>
> >  Subject: [PATCH][1/2] adjust dirty threshold for lowmem-only mappings
> 
> I've held off on this one because the recent throttling fix should have
> helped this problem.  Has anyone confirmed that this patch still actually
> fixes something?  If so, what was the scenario?

Without this fix write throttling is completely broken for a blkdev and
it won't start _at_all_ and it'll just keep hanging in the allocation
routines. I agree it won't explain oom (with the other fixes the VM
should writeback synchronously instead of running oom) but it may make
the box completely unusable under a cp /dev/zero /dev/somedevice.

There is a reason why we start write throttling before 100% of ram is
being locked by dirty pages in the pagecache path.

The beauty of this fix is that Rik allowed the pagecache not to have the
limit (in 2.4 pagecache had the limit too). Probably async writeback
won't start but at least the write throttling will and that's all we
need to keep the box running other apps at the same time of the write.

If the system goes unresponsive for 10 minutes and swaps during backups
or workloads working on the blkdev, they'll file bugreports and they'd
be correct.

In short I agree this shouldn't be applied for oom, but it's still
definitely a correct and needed fix (and I rate it a bit more than just
an optimization).