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

[Chris Mason <chris.mason@oracle.com>]

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

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