Re: [PATCH] nvme-core: fix io interrupt when work with dm-multipah

[Chao Leng <lengchao@huawei.com>]

On 2020/7/29 13:59, Christoph Hellwig wrote:
> On Wed, Jul 29, 2020 at 10:54:29AM +0800, Chao Leng wrote:
>>
>>
>> On 2020/7/28 19:19, Christoph Hellwig wrote:
>>> On Mon, Jul 27, 2020 at 01:58:18PM +0800, Chao Leng wrote:
>>>> The protocol NVM-Express-1.4 define:
>>>> Command Interrupted: Command processing was interrupted and the
>>>> controller is unable to successfully complete the command. The host
>>>> should retry the command. If this status code is returned, then
>>>> the controller shall clear the Do Not Retry bit to ‘0’ in the Status
>>>> field of the CQE (refer to Figure 124). The controller shall not return
>>>> this status code unless the host has set the Advanced Command Retry
>>>> Enable (ACRE) field to 1h in the Host Behavior Support feature(refer to
>>>> section 5.21.1.22).
>>>>
>>>> According the protocol define, NVME_SC_CMD_INTERRUPTED need retry.
>>>> The error code NVME_SC_CMD_INTERRUPTED should not translate to
>>>> BLK_STS_TARGET, because if the error code translate to BLK_STS_TARGET,
>>>> dm-multipah will return error to application. So if target return error
>>>> code NVME_SC_CMD_INTERRUPTED, io will interrupt. NVME_SC_CMD_INTERRUPTED
>>>> should translate to BLK_STS_IOERR by default, dm-multipath will fail
>>>> over to other path retry the io.
>>>
>>> IOERR still seems wrong, though.
>>> .
>>
>> BLK_STS_TARGET means target has critical error. NVME_SC_CMD_INTERRUPTED
>> just means target need retry io. It is not suitable to translate
>> NVME_SC_CMD_INTERRUPTED to BLK_STS_TARGET. Maybe translate to
>> BLK_STS_IOERR is also not suitable, we should translate
>> NVME_SC_CMD_INTERRUPTED to BLK_STS_AGAIN.
>> We can do like this:
> 
> BLK_STS_AGAIN is a bad choice as we use it for calls that block when
> the callers asked for non-blocking submission.  I'm really not sure
> we want to change anything here - the error definition clearly states
> it is not a failure but a request to retry later.
> .
> 
BLK_STS_AGAIN is not a good choice, but BLK_STS_TARGET is also not
a good choice. I find the patch that translate NVME_SC_CMD_INTERRUPTED
to BLK_STS_TARGET.

commit:35038bffa87da282010b91108cadd13238bb5bbd
nvme: Translate more status codes to blk_status_t
Decode interrupted command and not ready namespace nvme status codes to
BLK_STS_TARGET. These are not generic IO errors and should use a non-path
specific error so that it can use the non-failover retry path.
Reported-by: John Meneghini <John.Meneghini@netapp.com>

In the old version, need translate NVME_SC_CMD_INTERRUPTED to
BLK_STS_TARGET, because nvme multipath check the blk_path_error,
we expect retry IO in the current path, so have to translate
NVME_SC_CMD_INTERRUPTED to BLK_STS_TARGET. Although converting to
BLK_STS_TARGET is not a good choice, there seems to be no better choice
for the old version code. But now we do not need translate
NVME_SC_CMD_INTERRUPTED to BLK_STS_TARGET, nvme multipath already
improve and does not depend on blk_path_error.

According to the description of BLK_STS_TARGET:
[BLK_STS_TARGET]	= { -EREMOTEIO,	"critical target" }
BLK_STS_TARGET may easily mistaken as a fatal error on the storage.
I already check all the error code of BLK_STS_xx, there is no good
choice for NVME_SC_CMD_INTERRUPTED now. In the absence of a suitable
error code, it is a good choice to retain the default. So I strongly
suggest delete translate NVME_SC_CMD_INTERRUPTED to BLK_STS_TARGET.

John, what do you think?

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