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