Re: [PATCH V2] Re-dirty pages on ENOSPC when converting delayed allocations

[Dave Chinner <david@fromorbit.com>]

On Thu, Oct 09, 2008 at 11:27:26PM +1100, Dave Chinner wrote:
> On Tue, Oct 07, 2008 at 06:15:57PM +1000, Lachlan McIlroy wrote:
> > If we get an error in xfs_page_state_convert() - and it's not EAGAIN - then
> > we throw away the dirty page without converting the delayed allocation.  This
> > leaves delayed allocations that can never be removed and confuses code that
> > expects a flush of the file to clear them.  We need to re-dirty the page on
> > error so we can try again later or report that the flush failed.
> 
> Actually, those delalloc pages can be removed - they just need to
> be handled in ->releasepage. The problem there is that the
> delalloc state is checked by looking at the bufferhead, and by
> the time we get to ->releasepage the buffer heads have already gone
> through discard_buffer() and lost the buffer_delay() flag.
> 
> IIRC I had a patch that did the delalloc conversion correctly in
> ->releasepage by utilising a custom ->invalidatepage callouut, but
> the performance overhead was very bad because it is done a page at a
> time. ISTR even posting it to oss....

FWIW, here's the discussion:

http://oss.sgi.com/archives/xfs/2008-04/msg00542.html

I'm not saying that we ѕhould be fixing this right now, but this
is the underlying problem that is causing the direct I/O read
to trip over stale delalloc extents once they've been invalidated.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com