Re: [PATCH 1/2] block: modify blk_mq_plug() to allow only reads for zoned block devices

[Pankaj Raghav <p.raghav@samsung.com>]

On 2022-09-26 18:32, Jens Axboe wrote:
> On 9/26/22 8:43 AM, Christoph Hellwig wrote:
>> On Mon, Sep 26, 2022 at 08:40:54AM -0600, Jens Axboe wrote:
>>> On 9/26/22 8:37 AM, Christoph Hellwig wrote:
>>>> On Sun, Sep 25, 2022 at 08:53:46PM +0200, Pankaj Raghav wrote:
>>>>> Modify blk_mq_plug() to allow plugging only for read operations in zoned
>>>>> block devices as there are alternative IO paths in the linux block
>>>>> layer which can end up doing a write via driver private requests in
>>>>> sequential write zones.
>>>>
>>>> We should be able to plug for all operations that are not
>>>> REQ_OP_ZONE_APPEND just fine.
>>>
>>> Agree, I think we just want to make this about someone doing a series
>>> of appends. If you mix-and-match with passthrough you will have a bad
>>> time anyway.
>>
>> Err, sorry - what I wrote about is compelte garbage.  I initially
>> wanted to say you can plug for REQ_OP_ZONE_APPEND just fine, and then
>> realized that we also want various other ones that have the write bit
>> set batched.  So I suspect we really want to explicitly check for
>> REQ_OP_WRITE here.
> 
> My memory was a bit hazy, since we have separate ops for the driver
> in/out, I think just checking for REQ_OP_WRITE is indeed the right
> choice. That's the single case we need to care about.
> 
Ah. You are right. I missed it as well. There is even a comment from Christoph:

 *   - if the least significant bit is set transfers are TO the device
 *   - if the least significant bit is not set transfers are FROM the device

I guess the second patch should be enough to apply plugging when applicable
for uring_cmd based nvme passthrough requests.