Re: [PATCH v2] iomap: don't lose folio dropbehind state for overwrites

[Dave Chinner <david@fromorbit.com>]

On Tue, May 27, 2025 at 05:01:31PM -0600, Jens Axboe wrote:
> DONTCACHE I/O must have the completion punted to a workqueue, just like
> what is done for unwritten extents, as the completion needs task context
> to perform the invalidation of the folio(s). However, if writeback is
> started off filemap_fdatawrite_range() off generic_sync() and it's an
> overwrite, then the DONTCACHE marking gets lost as iomap_add_to_ioend()
> don't look at the folio being added and no further state is passed down
> to help it know that this is a dropbehind/DONTCACHE write.
> 
> Check if the folio being added is marked as dropbehind, and set
> IOMAP_IOEND_DONTCACHE if that is the case. Then XFS can factor this into
> the decision making of completion context in xfs_submit_ioend().
> Additionally include this ioend flag in the NOMERGE flags, to avoid
> mixing it with unrelated IO.
> 
> Since this is the 3rd flag that will cause XFS to punt the completion to
> a workqueue, add a helper so that each one of them can get appropriately
> commented.
> 
> This fixes extra page cache being instantiated when the write performed
> is an overwrite, rather than newly instantiated blocks.
> 
> Fixes: b2cd5ae693a3 ("iomap: make buffered writes work with RWF_DONTCACHE")
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

LGTM.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
-- 
Dave Chinner
david@fromorbit.com