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Subject: Re: [PATCH 1/1] blk/core: Gracefully handle unset make_request_fn
To:     Mike Snitzer <snitzer@redhat.com>
Cc:     Stefan Bader <stefan.bader@canonical.com>,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
        dm-devel@redhat.com, Tyler Hicks <tyler.hicks@canonical.com>,
        Alasdair Kergon <agk@redhat.com>
References: <20200123091713.12623-1-stefan.bader@canonical.com>
 <20200123091713.12623-2-stefan.bader@canonical.com>
 <20200123103541.GA28102@redhat.com> <20200123172816.GA31063@redhat.com>
 <81055166-37fb-ad65-6a53-11c22c626ab1@kernel.dk>
 <20200127193225.GA5065@redhat.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <988c930f-b4ee-6387-e1b4-6bfe7af9bcaf@kernel.dk>
Date:   Mon, 27 Jan 2020 12:39:04 -0700
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On 1/27/20 12:32 PM, Mike Snitzer wrote:
> On Thu, Jan 23 2020 at  1:52pm -0500,
> Jens Axboe <axboe@kernel.dk> wrote:
> 
>> On 1/23/20 10:28 AM, Mike Snitzer wrote:
>>> On Thu, Jan 23 2020 at  5:35am -0500,
>>> Mike Snitzer <snitzer@redhat.com> wrote:
>>>
>>>> On Thu, Jan 23 2020 at  4:17am -0500,
>>>> Stefan Bader <stefan.bader@canonical.com> wrote:
>>>>
>>>>> When device-mapper adapted for multi-queue functionality, they
>>>>> also re-organized the way the make-request function was set.
>>>>> Before, this happened when the device-mapper logical device was
>>>>> created. Now it is done once the mapping table gets loaded the
>>>>> first time (this also decides whether the block device is request
>>>>> or bio based).
>>>>>
>>>>> However in generic_make_request(), the request function gets used
>>>>> without further checks and this happens if one tries to mount such
>>>>> a partially set up device.
>>>>>
>>>>> This can easily be reproduced with the following steps:
>>>>>  - dmsetup create -n test
>>>>>  - mount /dev/dm-<#> /mnt
>>>>>
>>>>> This maybe is something which also should be fixed up in device-
>>>>> mapper.
>>>>
>>>> I'll look closer at other options.
>>>>
>>>>> But given there is already a check for an unset queue
>>>>> pointer and potentially there could be other drivers which do or
>>>>> might do the same, it sounds like a good move to add another check
>>>>> to generic_make_request_checks() and to bail out if the request
>>>>> function has not been set, yet.
>>>>>
>>>>> BugLink: https://bugs.launchpad.net/bugs/1860231
>>>>
>>>> >From that bug;
>>>> "The currently proposed fix introduces no chance of stability
>>>> regressions. There is a chance of a very small performance regression
>>>> since an additional pointer comparison is performed on each block layer
>>>> request but this is unlikely to be noticeable."
>>>>
>>>> This captures my immediate concern: slowing down everyone for this DM
>>>> edge-case isn't desirable.
>>>
>>> SO I had a look and there isn't anything easier than adding the proposed
>>> NULL check in generic_make_request_checks().  Given the many
>>> conditionals in that  function.. what's one more? ;)
>>>
>>> I looked at marking the queue frozen to prevent IO via
>>> blk_queue_enter()'s existing cheeck -- but that quickly felt like an
>>> abuse, especially in that there isn't a queue unfreeze for bio-based.
>>>
>>> Jens, I'll defer to you to judge this patch further.  If you're OK with
>>> it: cool.  If not, I'm open to suggestions for how to proceed.  
>>>
>>
>> It does kinda suck... The generic_make_request_checks() is a mess, and
>> this doesn't make it any better. Any reason why we can't solve this
>> two step setup in a clean fashion instead of patching around it like
>> this? Feels like a pretty bad hack, tbh.
> 
> I just staged the following DM fix:
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.6&id=28a101d6b344f5a38d482a686d18b1205bc92333

I like that a lot more than the NULL check in the core.

-- 
Jens Axboe