Re: [PATCH v7 3/5] iomap: use BIO_COMPLETE_IN_TASK for dropbehind writeback

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jul 30, 2026 at 02:57:53AM -0400, Tal Zussman wrote:
> Set BIO_COMPLETE_IN_TASK on iomap writeback bios when a dropbehind folio
> is added. 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,
> IOMAP_IOEND_DONTCACHE is no longer needed, as XFS no longer needs to
> route DONTCACHE ioends through its completion workqueue. Remove the flag
> and its NOMERGE entry.
> 
> Without the NOMERGE, regular I/Os that get merged with a dropbehind
> folio will also have their completion deferred to task context.
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>

This seems to be missing the bug fix you identified for the double
deferral?

I although thought a bit how that could be done cleaner, see below:

>  		ioend_flags |= IOMAP_IOEND_SHARED;
> -	if (folio_test_dropbehind(folio))
> -		ioend_flags |= IOMAP_IOEND_DONTCACHE;

I'd keep this flag.

>  	if (pos == wpc->iomap.offset && (wpc->iomap.flags & IOMAP_F_BOUNDARY))
>  		ioend_flags |= IOMAP_IOEND_BOUNDARY;
>  
> @@ -256,6 +254,9 @@ ssize_t iomap_add_to_ioend(struct iomap_writepage_ctx *wpc, struct folio *folio,
>  	if (!bio_add_folio(&ioend->io_bio, folio, map_len, poff))
>  		goto new_ioend;
>  
> +	if (folio_test_dropbehind(folio))
> +		bio_set_flag(&ioend->io_bio, BIO_COMPLETE_IN_TASK);

And move setting the flag to iomap_ioend_writeback_submit, and only
for the case where no io_bio.bi_end_io is already assigned.