Re: dm: retain stacked max_sectors when setting queue_limits

[Christoph Hellwig <hch@lst.de>]

On Thu, May 23, 2024 at 10:12:24AM -0400, Mike Snitzer wrote:
> Not sure what is sketchy about the max_sectors == 0 check, the large
> comment block explains that check quite well.  We want to avoid EIO
> for unsupported operations (otherwise we'll get spurious path failures
> in the context of dm-multipath).  Could be we can remove this check
> after an audit of how LLD handle servicing IO for unsupported
> operations -- so best to work through it during a devel cycle.

Think of what happens if you create a dm device, and then reduce
max_sectors using sysfs on the lower device after the dm device
was created: you'll trivially trigger this check.

> Not sure why scsi_debug based testing with mptest isn't triggering it
> for you. Are you seeing these limits for the underlying scsi_debug
> devices?
> 
> ./max_hw_sectors_kb:2147483647
> ./max_sectors_kb:512

root@testvm:~/mptest# cat /sys/block/sdc/queue/max_hw_sectors_kb 
2147483647

root@testvm:~/mptest# cat /sys/block/sdd/queue/max_sectors_kb 
512

root@testvm:~/mptest# cat /sys/block/dm-0/queue/max_hw_sectors_kb 
2147483647

root@testvm:~/mptest# cat /sys/block/dm-0/queue/max_sectors_kb 
1280

so they don't match, but for some reason bigger bios never get built.