Re: [PATCH v2] dm: respect REQ_NOWAIT flag in bios issued to DM

[Ming Lei <ming.lei@redhat.com>]

On Wed, Oct 25, 2023 at 03:34:40PM -0400, Mike Snitzer wrote:
> From: Mikulas Patocka <mpatocka@redhat.com>
> 
> Update DM core's IO submission to allocate required memory using
> GFP_NOWAIT if REQ_NOWAIT is set.  Lone exception is in the error path
> where DM's requeue support needs to allocate a clone bio in
> dm_io_rewind() to allow the IO to be resubmitted: GFP_NOWAIT is used
> first but if it fails GFP_NOIO is used as a last resort.
> 
> Tested with simple test provided in commit a9ce385344f916 ("dm: don't
> attempt to queue IO under RCU protection") that was enhanced to check
> error codes.  Also tested using fio's pvsync2 with nowait=1.
> 
> But testing with induced GFP_NOWAIT allocation failures wasn't
> performed.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> Co-developed-by: Mike Snitzer <snitzer@kernel.org>
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> ---
>  drivers/md/dm-io-rewind.c | 13 ++++++--
>  drivers/md/dm.c           | 66 ++++++++++++++++++++++++++++++++-------
>  2 files changed, 65 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/md/dm-io-rewind.c b/drivers/md/dm-io-rewind.c
> index 6155b0117c9d..bde5a53e2d88 100644
> --- a/drivers/md/dm-io-rewind.c
> +++ b/drivers/md/dm-io-rewind.c
> @@ -143,8 +143,17 @@ static void dm_bio_rewind(struct bio *bio, unsigned int bytes)
>  void dm_io_rewind(struct dm_io *io, struct bio_set *bs)
>  {
>  	struct bio *orig = io->orig_bio;
> -	struct bio *new_orig = bio_alloc_clone(orig->bi_bdev, orig,
> -					       GFP_NOIO, bs);
> +	struct bio *new_orig;
> +
> +	new_orig = bio_alloc_clone(orig->bi_bdev, orig, GFP_NOWAIT, bs);
> +	if (unlikely(!new_orig)) {
> +		/*
> +		 * Returning early and failing rewind isn't an option, even
> +		 * if orig has REQ_NOWAIT set, so retry alloc with GFP_NOIO.
> +		 */
> +		new_orig = bio_alloc_clone(orig->bi_bdev, orig, GFP_NOIO, bs);
> +	}

dm_bio_rewind() is only called in requeue work fn, so there shouldn't
be issue with NOWAIT, cause upper IO submission code path won't be blocked
by requeue.


Thanks, 
Ming