Re: dm: retain stacked max_sectors when setting queue_limits

[Mike Snitzer <snitzer@kernel.org>]

On Thu, May 23, 2024 at 05:50:09PM +0200, Christoph Hellwig wrote:
> On Thu, May 23, 2024 at 11:44:05AM -0400, Mike Snitzer wrote:
> > a difference on larger IOs being formed (given it is virtual
> > scsi_debug devices).
> > 
> > In any case, we know I can reproduce with this scsi_debug-based mptest
> > test and Marco has verified my fix resolves the issue on his FC
> > multipath testbed.
> > 
> > But I've just floated a patch to elevate the fix to block core (based
> > on Ming's suggestion):
> > https://patchwork.kernel.org/project/dm-devel/patch/Zk9i7V2GRoHxBPRu@kernel.org/
> 
> I still think that is wrong.  Unfortunately I can't actually reproduce
> the issue locally, but I think we want sd to set the user_max_sectors
> and stack if you want to see the limits propagated, i.e. the combined
> patch below.   In the longer run I need to get SCSI out of messing
> with max_sectors directly, and the blk-mq stacking to stop looking
> at it vs just the hardware limits (or just drop the check).

This "works" but it doesn't safeguard blk_stack_limits() and
blk_validate_limits() from other drivers that weren't trained to
(ab)use max_user_sectors to get blk_validate_limits() to preserve the
underlying device's max_sectors.

But I suppose we can worry about any other similar issues if/when
reported.

Please send a proper patch to Jens so we can get this fixed for
6.10-rc1. Thanks.

Acked-by: Mike Snitzer <snitzer@kernel.org>

> 
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index a7fe8e90240a6e..7a672021daee6a 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -611,6 +611,8 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
>  	unsigned int top, bottom, alignment, ret = 0;
>  
>  	t->max_sectors = min_not_zero(t->max_sectors, b->max_sectors);
> +	t->max_user_sectors = min_not_zero(t->max_user_sectors,
> +			b->max_user_sectors);
>  	t->max_hw_sectors = min_not_zero(t->max_hw_sectors, b->max_hw_sectors);
>  	t->max_dev_sectors = min_not_zero(t->max_dev_sectors, b->max_dev_sectors);
>  	t->max_write_zeroes_sectors = min(t->max_write_zeroes_sectors,
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index 332eb9dac22d91..f6c822c9cbd2d3 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -3700,8 +3700,10 @@ static int sd_revalidate_disk(struct gendisk *disk)
>  	 */
>  	if (sdkp->first_scan ||
>  	    q->limits.max_sectors > q->limits.max_dev_sectors ||
> -	    q->limits.max_sectors > q->limits.max_hw_sectors)
> +	    q->limits.max_sectors > q->limits.max_hw_sectors) {
>  		q->limits.max_sectors = rw_max;
> +		q->limits.max_user_sectors = rw_max;
> +	}
>  
>  	sdkp->first_scan = 0;
>  
> 
>