Re: [PATCH 4/6] writeback: dont redirty tail an inode with dirty pages

[Andrew Morton <akpm@linux-foundation.org>]

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 :(