Re: [PATCH 4/4] dm: fix REQ_RAHEAD handling

[Mike Snitzer <snitzer@redhat.com>]

On Mon, May 15 2017 at 11:28P -0400,
Christoph Hellwig <hch@lst.de> wrote:

> A few (but not all) dm targets use a special EWOULDBLOCK error code for
> failing REQ_RAHEAD requests that fail due to a lack of available resources.
> But no one else knows about this magic code, and lower level drivers also
> don't generate it when failing read-ahead requests for similar reasons.
> 
> So remove this special casing and ignore all additional error handling for
> REQ_RAHEAD - if this was a real underlying error we'd get a normal read
> once the real read comes in.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  drivers/md/dm-raid1.c  | 4 ++--
>  drivers/md/dm-stripe.c | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/md/dm-raid1.c b/drivers/md/dm-raid1.c
> index a95cbb80fb34..5e30b08b91d9 100644
> --- a/drivers/md/dm-raid1.c
> +++ b/drivers/md/dm-raid1.c
> @@ -1214,7 +1214,7 @@ static int mirror_map(struct dm_target *ti, struct bio *bio)
>  	 */
>  	if (!r || (r == -EWOULDBLOCK)) {
>  		if (bio->bi_opf & REQ_RAHEAD)
> -			return -EWOULDBLOCK;
> +			return -EIO;
>  
>  		queue_bio(ms, bio, rw);
>  		return DM_MAPIO_SUBMITTED;
> @@ -1258,7 +1258,7 @@ static int mirror_end_io(struct dm_target *ti, struct bio *bio, int error)
>  	if (error == -EOPNOTSUPP)
>  		return error;
>  
> -	if ((error == -EWOULDBLOCK) && (bio->bi_opf & REQ_RAHEAD))
> +	if (bio->bi_opf & REQ_RAHEAD)
>  		return error;
>  
>  	if (unlikely(error)) {
> diff --git a/drivers/md/dm-stripe.c b/drivers/md/dm-stripe.c
> index 75152482f3ad..780e95889a7c 100644
> --- a/drivers/md/dm-stripe.c
> +++ b/drivers/md/dm-stripe.c
> @@ -384,7 +384,7 @@ static int stripe_end_io(struct dm_target *ti, struct bio *bio, int error)
>  	if (!error)
>  		return 0; /* I/O complete */
>  
> -	if ((error == -EWOULDBLOCK) && (bio->bi_opf & REQ_RAHEAD))
> +	if (bio->bi_opf & REQ_RAHEAD)
>  		return error;
>  
>  	if (error == -EOPNOTSUPP)
> -- 
> 2.11.0
> 

I'll let this one go for now.. meaning I won't pick it up, nor will I
send it for 4.12.  Please just roll this into your broader block work
that you mentioned.