Re: [PATCH] iomap: add a separate bio_set for iomap_split_ioend

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jun 29, 2026 at 02:52:29PM +0200, Christoph Hellwig wrote:
> iomap_split_ioend can split bios that already come from
> iomap_ioend_bioset and thus deadlock when the bioset is exhausted.
> 
> Add a separate bio_set to avoid this deadlock.

What happens if &iomap_ioend_split_bioset is exhausted?  Won't that also
stall the ioend submission?

<confused>

--D

> Fixes: 5fcbd555d483 ("iomap: split bios to zone append limits in the submission handlers")
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/iomap/ioend.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/iomap/ioend.c b/fs/iomap/ioend.c
> index 01287f1ae021..2fbcb06f6938 100644
> --- a/fs/iomap/ioend.c
> +++ b/fs/iomap/ioend.c
> @@ -13,6 +13,7 @@
>  
>  struct bio_set iomap_ioend_bioset;
>  EXPORT_SYMBOL_GPL(iomap_ioend_bioset);
> +struct bio_set iomap_ioend_split_bioset;
>  
>  struct iomap_ioend *iomap_init_ioend(struct inode *inode,
>  		struct bio *bio, loff_t file_offset, u16 ioend_flags)
> @@ -483,7 +484,8 @@ struct iomap_ioend *iomap_split_ioend(struct iomap_ioend *ioend,
>  	sector_offset = ALIGN_DOWN(sector_offset << SECTOR_SHIFT,
>  			i_blocksize(ioend->io_inode)) >> SECTOR_SHIFT;
>  
> -	split = bio_split(bio, sector_offset, GFP_NOFS, &iomap_ioend_bioset);
> +	split = bio_split(bio, sector_offset, GFP_NOFS,
> +			&iomap_ioend_split_bioset);
>  	if (IS_ERR(split))
>  		return ERR_CAST(split);
>  	split->bi_private = bio->bi_private;
> @@ -506,8 +508,23 @@ EXPORT_SYMBOL_GPL(iomap_split_ioend);
>  
>  static int __init iomap_ioend_init(void)
>  {
> -	return bioset_init(&iomap_ioend_bioset, 4 * (PAGE_SIZE / SECTOR_SIZE),
> +	const unsigned int nr_mempool_entries = 4 * (PAGE_SIZE / SECTOR_SIZE);
> +	int error;
> +
> +	error = bioset_init(&iomap_ioend_bioset, nr_mempool_entries,
>  			   offsetof(struct iomap_ioend, io_bio),
>  			   BIOSET_NEED_BVECS);
> +	if (error)
> +		return error;
> +	error = bioset_init(&iomap_ioend_split_bioset, nr_mempool_entries,
> +			   offsetof(struct iomap_ioend, io_bio),
> +			   BIOSET_NEED_BVECS);
> +	if (error)
> +		goto out_exit_ioend_bioset;
> +	return 0;
> +
> +out_exit_ioend_bioset:
> +	bioset_exit(&iomap_ioend_bioset);
> +	return error;
>  }
>  fs_initcall(iomap_ioend_init);
> -- 
> 2.53.0
>