Re: [PATCH] dm: retain stacked max_sectors when setting queue_limits

[Christoph Hellwig]

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.

> 
> 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?

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;