Re: [PATCH RFC v4 2/3] iomap: use BIO_COMPLETE_IN_TASK for dropbehind writeback

[Matthew Wilcox]

On Wed, Apr 08, 2026 at 03:44:37PM -0400, Tal Zussman wrote:
> On 3/25/26 4:21 PM, Matthew Wilcox wrote:
> > On Wed, Mar 25, 2026 at 02:43:01PM -0400, Tal Zussman wrote:
> >> Set BIO_COMPLETE_IN_TASK on iomap writeback bios when
> >> IOMAP_IOEND_DONTCACHE is set. This ensures that bi_end_io runs in task
> >> context, where folio_end_dropbehind() can safely invalidate folios.
> >> 
> >> With the bio layer now handling task-context deferral generically, XFS
> >> no longer needs to route DONTCACHE ioends through its completion
> >> workqueue for page cache invalidation. Remove the DONTCACHE check from
> >> xfs_ioend_needs_wq_completion().
> >> 
> >> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
> >> ---
> >>  fs/iomap/ioend.c  | 2 ++
> >>  fs/xfs/xfs_aops.c | 4 ----
> >>  2 files changed, 2 insertions(+), 4 deletions(-)
> >> 
> >> diff --git a/fs/iomap/ioend.c b/fs/iomap/ioend.c
> >> index e4d57cb969f1..6b8375d11cc0 100644
> >> --- a/fs/iomap/ioend.c
> >> +++ b/fs/iomap/ioend.c
> >> @@ -113,6 +113,8 @@ static struct iomap_ioend *iomap_alloc_ioend(struct iomap_writepage_ctx *wpc,
> >>  			       GFP_NOFS, &iomap_ioend_bioset);
> >>  	bio->bi_iter.bi_sector = iomap_sector(&wpc->iomap, pos);
> >>  	bio->bi_write_hint = wpc->inode->i_write_hint;
> >> +	if (ioend_flags & IOMAP_IOEND_DONTCACHE)
> >> +		bio_set_flag(bio, BIO_COMPLETE_IN_TASK);
> >>  	wbc_init_bio(wpc->wbc, bio);
> >>  	wpc->nr_folios = 0;
> >>  	return iomap_init_ioend(wpc->inode, bio, pos, ioend_flags);
> > 
> > Can't we delete IOMAP_IOEND_DONTCACHE, and just do:
> > 
> > 	if (folio_test_dropbehind(folio))
> > 		bio_set_flag(&ioend->io_bio, BIO_COMPLETE_IN_TASK);
> > 
> > It'd need to move down a few lines in iomap_add_to_ioend() to after
> > bio_add_folio() succeeds.
> > 
> 
> Actually, looking into it more, IOMAP_IOEND_DONTCACHE is used as part of
> IOMAP_IOEND_NOMERGE_FLAGS. I think deleting it while maintaining the
> no-merge behavior would be uglier than leaving it in.

But why was it added to NOMERGE in the first place?  I don't think it's
harmful to merge writeback I/Os which are COMPLETE_IN_TASK and I/Os
which are not, as long as the final I/O is completed in a task.