linux-kernel - Re: [PATCH 0/5]scsi:scsi_debug: Add error injection for single device

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Date:   Thu, 23 Mar 2023 13:24:48 -0400
From:   Douglas Gilbert <dgilbert@...erlog.com>
To:     John Garry <john.g.garry@...cle.com>,
        "haowenchao (C)" <haowenchao2@...wei.com>,
        "James E . J . Bottomley" <jejb@...ux.ibm.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     linfeilong@...wei.com, louhongxiang@...wei.com
Subject: Re: [PATCH 0/5]scsi:scsi_debug: Add error injection for single device

On 2023-03-23 12:25, John Garry wrote:
> On 23/03/2023 13:13, haowenchao (C) wrote:
>> On 2023/3/23 20:40, John Garry wrote:
>>> On 23/03/2023 11:55, Wenchao Hao wrote:
>>>> The original error injection mechanism was based on scsi_host which
>>>> could not inject fault for a single SCSI device.
>>>>
>>>> This patchset provides the ability to inject errors for a single
>>>> SCSI device. Now we supports inject timeout errors, queuecommand
>>>> errors, and hostbyte, driverbyte, statusbyte, and sense data for
>>>> specific SCSI Command.
>>>
>>> There is already a basic mechanism to generate errors - like timeouts - on 
>>> "nth" command. Can you say why you want this new interface? What special 
>>> scenarios are you trying to test/validate (which could not be achieved based 
>>> on the current mechanism)?
>>>
>>
>> I am testing a new error handle policy which is based on single scsi_device
>> without set host to RECOVERY. So I need a method to generate errors for
>> single SCSI devices.
>>
>> While we can not generate errors for single device with current mechanism
>> because it is designed for host-wide error generation.
>>
>>> With this series we would have 2x methods to inject errors, which is less 
>>> than ideal, and they seem to possibly conflict as well, e.g. I set timeout 
>>> for nth command via current interface and then use the new interface to set 
>>> timeout for some other cadence. What behavior to expect ...?
>>
>> I did not take this issue in consideration. I now assume the users would
>> not use these 2 methods at same time.
>>
>> What's more, I don not know where to write the usage of this newly added
>> interface, maybe we can explain these in doc?
> 
> sysfs entries are described in Documentation/ABI, but please don't add elaborate 
> programming interfaces in sysfs files (like in these patches) - a sysfs file 
> should be just for reading or writing a single value

Hi,
Maybe this link might help for scsi_debug documentation:
     https://doug-gilbert.github.io/scsi_debug.html

And rather than sysfs for complicated, per (pseudo_ device
settings, perhaps we could think about a SCSI mechanism like
the "Unit Attention" mode page [0x0] which is vendor specific
and used by Seagate and WDC for this sort of thing.
A framework is already in the scsi_debug driver to change
some mode page settings:

# sdparm /dev/sg0
     /dev/sg0: Linux     scsi_debug        0191
Read write error recovery mode page:
   AWRE          1  [cha: n, def:  1]
   ARRE          1  [cha: n, def:  1]
   PER           0  [cha: n, def:  0]
Caching (SBC) mode page:
   WCE           1  [cha: y, def:  1]
   RCD           0  [cha: n, def:  0]
Control mode page:
   SWP           0  [cha: n, def:  0]
Informational exceptions control mode page:
   EWASC         0  [cha: n, def:  0]
   DEXCPT        1  [cha: n, def:  1]
   MRIE          0  [cha: y, def:  0]

As can be seen WCE and MRIE are changeable, so

# sdparm --clear=WCE /dev/sg0
# sdparm --get=WCE /dev/sg0
     /dev/sg0: Linux     scsi_debug        0191
WCE           0  [cha: y, def:  1]


Doug Gilbert


>>> I'm not saying that I am a huge fan of the current inject mechanism, but at 
>>> the very least you need to provide more justification for this series.
>>>>>
>>>> The first patch add an sysfs interface to add and inquiry single
>>>> device's error injection info; the second patch defined how to remove
>>>> an injection which has been added. The following 3 patches use the
>>>> injection info and generate the related error type.
>>>>
>>>> Wenchao Hao (5):
>>>>    scsi:scsi_debug: Add sysfs interface to manage scsi devices' error
>>>>      injection
>>>>    scsi:scsi_debug: Define grammar to remove added error injection
>>>>    scsi:scsi_debug: timeout command if the error is injected
>>>>    scsi:scsi_debug: Return failed value if the error is injected
>>>>    scsi:scsi_debug: set command's result and sense data if the error is
>>>>      injected
>>>>
>>>>   drivers/scsi/scsi_debug.c | 296 ++++++++++++++++++++++++++++++++++++++
>>>>   1 file changed, 296 insertions(+)
>>>>
>>>
>>>
>>
>