Re: dm: retain stacked max_sectors when setting queue_limits

[Mike Snitzer <snitzer@kernel.org>]

On Wed, May 22, 2024 at 04:24:58PM +0200, Christoph Hellwig wrote:
> On Tue, May 21, 2024 at 10:51:17PM -0400, Mike Snitzer wrote:
> > Otherwise, blk_validate_limits() will throw-away the max_sectors that
> > was stacked from underlying device(s). In doing so it can set a
> > max_sectors limit that violates underlying device limits.
> 
> Hmm, yes it sort of is "throwing the limit away", but it really
> recalculates it from max_hw_sectors, max_dev_sectors and user_max_sectors.

Yes, but it needs to do that recalculation at each level of a stacked
device.  And then we need to combine them via blk_stack_limits() -- as
is done with the various limits stacking loops in
drivers/md/dm-table.c:dm_calculate_queue_limits

> > This caused dm-multipath IO failures like the following because the
> > underlying devices' max_sectors were stacked up to be 1024, yet
> > blk_validate_limits() defaulted max_sectors to BLK_DEF_MAX_SECTORS_CAP
> > (2560):
> 
> I suspect the problem is that SCSI messed directly with max_sectors instead
> and ignores max_user_sectors (and really shouldn't touch either, but that's
> a separate discussion).  Can you try the patch below and maybe also provide
> the sysfs output for max_sectors_kb and max_hw_sectors_kb for all involved
> devices?

FYI, you can easily reproduce with:

git clone https://github.com/snitm/mptest.git
cd mptest
<edit so it uses: export MULTIPATH_BACKEND_MODULE="scsidebug">
./runtest ./tests/test_00_no_failure

Also, best to change this line:
./lib/mpath_generic:        local _feature="4 queue_if_no_path retain_attached_hw_handler queue_mode $MULTIPATH_QUEUE_MODE"
to:
./lib/mpath_generic:        local _feature="3 retain_attached_hw_handler queue_mode $MULTIPATH_QUEUE_MODE"
Otherwise the test will hang due to queue_if_no_path.

all underlying scsi-debug scsi devices:

./max_hw_sectors_kb:2147483647
./max_sectors_kb:512

multipath device:

before my fix:
./max_hw_sectors_kb:2147483647
./max_sectors_kb:1280

after my fix:
./max_hw_sectors_kb:2147483647
./max_sectors_kb:512
 
> 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;
>  
> 

Driver shouldn't be changing max_user_sectors..

But it also didn't fix it (mpath still gets ./max_sectors_kb:1280):

[   74.872485] blk_insert_cloned_request: over max size limit. (2048 > 1024)
[   74.872505] device-mapper: multipath: 254:3: Failing path 8:16.
[   74.872620] blk_insert_cloned_request: over max size limit. (2048 > 1024)
[   74.872641] device-mapper: multipath: 254:3: Failing path 8:32.
[   74.872712] blk_insert_cloned_request: over max size limit. (2048 > 1024)
[   74.872732] device-mapper: multipath: 254:3: Failing path 8:48.
[   74.872788] blk_insert_cloned_request: over max size limit. (2048 > 1024)
[   74.872808] device-mapper: multipath: 254:3: Failing path 8:64.

Simply setting max_user_sectors won't help with stacked devices
because blk_stack_limits() doesn't stack max_user_sectors.  It'll
inform the underlying device's blk_validate_limits() calculation which
will result in max_sectors having the desired value (which it already
did, as I showed above).  But when stacking limits from underlying
devices up to the higher-level dm-mpath queue_limits we still have
information loss.

Mike