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

[Christoph Hellwig <hch@infradead.org>]

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?