Re: [PATCH] blk-mq: setup blk_mq_alloc_data.cmd_flags after submit_bio_checks() is done

[Jens Axboe <axboe@kernel.dk>]

On 11/12/21 1:44 AM, Christoph Hellwig wrote:
> On Fri, Nov 12, 2021 at 04:37:19PM +0800, Ming Lei wrote:
>>> can only be used for reads, and no fua can be set if the preallocating
>>> I/O didn't use fua, etc.
>>>
>>> What are the pitfalls of just chanigng cmd_flags?
>>
>> Then we need to check cmd_flags carefully, such as hctx->type has to
>> be same, flush & passthrough flags has to be same, that said all
>> ->cmd_flags used for allocating rqs have to be same with the following
>> bio->bi_opf.
>>
>> In usual cases, I guess all IOs submitted from same plug batch should be
>> same type. If not, we can switch to change cmd_flags.
> 
> Jens: is this a limit fitting into your use cases?

It's not a big problem for my use case, as that ones all the same type
of IO. And in general I don't expect a lot of mixed uses here, I guess
the main issue might be a plug for writes and an initial meta data read
will "polute" the request cache with requests that we won't be using.
But probably not a big enough thing to worry about in terms of
efficiency.

> I guess as a quick fix this rejecting different flags is probably the
> best we can do for now, but I suspect we'll want to eventually relax
> them.

Agree on both.

-- 
Jens Axboe