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

[Matthew Wilcox <willy@infradead.org>]

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@columbia.edu>
> ---
>  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.