[dm-devel] block: Fix a WRITE SAME BUG_ON

[Martin K. Petersen]

John,

>> So disallowing WRITE SAME unless all component devices have the same LBS
>> is the correct fix.
>
> Alternately, could possibly WRITE_SAME bios be accepted with the
> minimum sector size of the stack rather than the max, e.g. 512 in this
> example rather than 4k? They'd need to have a granularity of the
> larger sector size, though, presumabily necessitating new queue limits
> write_same_{granularity,block_size}, which might be too much work.

I don't have a problem restricting the buffer contents to be consistent
within a page. Or even change the upper layer semantics to specify the
buffer contents using a single byte (0x00..0xff).

But the issue of head and tail remains if there is a block size mismatch
so it's important that we keep scaling the logical block size up when
stacking and reject any bio that can't be honored on a 4Kn device.

> (I use WRITE_SAME to fill devices with a particular pattern in order
> to catch failures to initialize disk structures appropriately,
> personally, but it's just for convenience/speed.)

The intent was for stuff like MD to use it to initialize parity disks,
etc. But adoption has been pretty slow.

I don't have any problems keeping WRITE_SAME around if people are
actually using it. It just seemed like most active users only cared
about writing zeroes.

-- 
Martin K. Petersen	Oracle Linux Engineering