Re: [PATCH v2 2/4] dm: Always split write BIOs to zoned device limits

[Damien Le Moal <dlemoal@kernel.org>]

On 6/25/25 3:15 PM, Christoph Hellwig wrote:
> On Wed, Jun 25, 2025 at 02:59:06PM +0900, Damien Le Moal wrote:
>> Any zoned DM target that requires zone append emulation will use the
>> block layer zone write plugging. In such case, DM target drivers must
>> not split BIOs using dm_accept_partial_bio() as doing so can potentially
>> lead to deadlocks with queue freeze operations. Regular write operations
>> used to emulate zone append operations also cannot be split by the
>> target driver as that would result in an invalid writen sector value
>> return using the BIO sector.
>>
>> In order for zoned DM target drivers to avoid such incorrect BIO
>> splitting, we must ensure that large BIOs are split before being passed
>> to the map() function of the target, thus guaranteeing that the
>> limits for the mapped device are not exceeded.
>>
>> dm-crypt and dm-flakey are the only target drivers supporting zoned
>> devices and using dm_accept_partial_bio().
> 
> Is there any good way to catch usage dm_accept_partial_bio on zone
> devices so that issues like this don't get reintroduced later?

patch 4 does that. Though with the heavy BUG_ON() hammer. That can be cleaned
up later though.


-- 
Damien Le Moal
Western Digital Research