Re: [RFC PATCH 0/6] Do not call ->writepage[s] from direct reclaim and use a_ops->writepages() where possible

Chris Mason
2010-06-11T19:14:29Z

On Fri, Jun 11, 2010 at 12:29:12PM -0400, Christoph Hellwig wrote:
> On Tue, Jun 08, 2010 at 10:28:14AM +0100, Mel Gorman wrote:
> > > - we also need to care about ->releasepage. At least for XFS it
> > > can end up in the same deep allocator chain as ->writepage because
> > > it does all the extent state conversions, even if it doesn't
> > > start I/O.
> >
> > Dang.
> >
> > > I haven't managed yet to decode the ext4/btrfs codepaths
> > > for ->releasepage yet to figure out how they release a page that
> > > covers a delayed allocated or unwritten range.
> > >
> >
> > If ext4/btrfs are also very deep call-chains and this series is going more
> > or less the right direction, then avoiding calling ->releasepage from direct
> > reclaim is one, somewhat unfortunate, option. The second is to avoid it on
> > a per-filesystem basis for direct reclaim using PF_MEMALLOC to detect
> > reclaimers and PF_KSWAPD to tell the difference between direct
> > reclaimers and kswapd.
>
> I went throught this a bit more and I can't actually hit that code in
> XFS ->releasepage anymore. I've also audited the caller and can't see
> how we could theoretically hit it anymore. Do the VM gurus know a case
> where we would call ->releasepage on a page that's actually dirty and
> hasn't been through block_invalidatepage before?

Which part of xfs releasepage are you trying to avoid?

dirty = xfs_page_state_convert(inode, page, &wbc, 0, 0);
if (dirty == 0 && !unwritten)
goto free_buffers;

I'd expect the above was fixed by page_mkwrite, which should be dealing
with all the funny corners that we used to have to mess with in
releasepage.

btrfs_release_page does no allocations, it only checks to see if the
page is busy somehow (dirty/writeback etc).

-chris