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    SubjectRe: [PATCH 4/6] writeback: dont redirty tail an inode with dirty pages
    On Mon, 12 Jul 2010 23:31:27 +0800
    Wu Fengguang <fengguang.wu@intel.com> wrote:

    > > > + } else if (inode->i_state & I_DIRTY) {
    > > > + /*
    > > > + * At least XFS will redirty the inode during the
    > > > + * writeback (delalloc) and on io completion (isize).
    > > > + */
    > > > + redirty_tail(inode);
    > >
    > > I'd drop the mention of XFS here - any filesystem that does delayed
    > > allocation or unwritten extent conversion after Io completion will
    > > cause this. Perhaps make the comment:
    > >
    > > /*
    > > * Filesystems can dirty the inode during writeback
    > > * operations, such as delayed allocation during submission
    > > * or metadata updates after data IO completion.
    > > */
    >
    > Thanks, comments updated accordingly.
    >
    > ---
    > writeback: don't redirty tail an inode with dirty pages
    >
    > This avoids delaying writeback for an expired (XFS) inode with lots of
    > dirty pages, but no active dirtier at the moment. Previously we only do
    > that for the kupdate case.
    >

    You didn't actually explain the _reason_ for making this change.
    Please always do that.

    The patch is... surprisingly complicated, although the end result
    looks OK. This is not aided by the partial duplication between
    mapping_tagged(PAGECACHE_TAG_DIRTY) and I_DIRTY_PAGES. I don't think
    we can easily remove I_DIRTY_PAGES because it's used for the
    did-someone-just-dirty-a-page test here.

    This code is way too complex and fragile and I fear that anything we do
    to it will break something :(



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