Re: [PATCH 1/1] blk/core: Gracefully handle unset make_request_fn

[Stefan Bader]

On 27.01.20 20:32, Mike Snitzer wrote:
> On Thu, Jan 23 2020 at  1:52pm -0500,
> Jens Axboe <axboe@xxxxxxxxx> wrote:
> 
>> On 1/23/20 10:28 AM, Mike Snitzer wrote:
>>> On Thu, Jan 23 2020 at  5:35am -0500,
>>> Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
>>>
>>>> On Thu, Jan 23 2020 at  4:17am -0500,
>>>> Stefan Bader <stefan.bader@xxxxxxxxxxxxx> 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

Thanks Mike,

yeah this looks like it resolves the problem without adding any impact on the
generic I/O path. We certainly had thought about that but felt uncertain whether
it would not open other risks. Like something adding requests just before the
table load. Could this cause some I/O be handled by one function and the rest by
another? And would that really matter?

The other thing that was a bit strange but maybe someone else's problem is that
mount generated I/O requests to start with. The device size should be 0 still.


> 
> From: Mike Snitzer <snitzer@xxxxxxxxxx>
> Date: Mon, 27 Jan 2020 14:07:23 -0500
> Subject: [PATCH] dm: fix potential for q->make_request_fn NULL pointer
> 
> Move blk_queue_make_request() to dm.c:alloc_dev() so that
> q->make_request_fn is never NULL during the lifetime of a DM device
> (even one that is created without a DM table).
> 
> Otherwise generic_make_request() will crash simply by doing:
>   dmsetup create -n test
>   mount /dev/dm-N /mnt
> 
> While at it, move ->congested_data initialization out of
> dm.c:alloc_dev() and into the bio-based specific init method.
> 
> Reported-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
> BugLink: https://bugs.launchpad.net/bugs/1860231
> Fixes: ff36ab34583a ("dm: remove request-based logic from make_request_fn wrapper")
> Depends-on: c12c9a3c3860c ("dm: various cleanups to md->queue initialization code")
> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
> ---
>  drivers/md/dm.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index e8f9661a10a1..b89f07ee2eff 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1859,6 +1859,7 @@ static void dm_init_normal_md_queue(struct mapped_device *md)
>  	/*
>  	 * Initialize aspects of queue that aren't relevant for blk-mq
>  	 */
> +	md->queue->backing_dev_info->congested_data = md;
>  	md->queue->backing_dev_info->congested_fn = dm_any_congested;
>  }
>  
> @@ -1949,7 +1950,12 @@ static struct mapped_device *alloc_dev(int minor)
>  	if (!md->queue)
>  		goto bad;
>  	md->queue->queuedata = md;
> -	md->queue->backing_dev_info->congested_data = md;
> +	/*
> +	 * default to bio-based required ->make_request_fn until DM
> +	 * table is loaded and md->type established. If request-based
> +	 * table is loaded: blk-mq will override accordingly.
> +	 */
> +	blk_queue_make_request(md->queue, dm_make_request);
>  
>  	md->disk = alloc_disk_node(1, md->numa_node_id);
>  	if (!md->disk)
> @@ -2264,7 +2270,6 @@ int dm_setup_md_queue(struct mapped_device *md, struct dm_table *t)
>  	case DM_TYPE_DAX_BIO_BASED:
>  	case DM_TYPE_NVME_BIO_BASED:
>  		dm_init_normal_md_queue(md);
> -		blk_queue_make_request(md->queue, dm_make_request);
>  		break;
>  	case DM_TYPE_NONE:
>  		WARN_ON_ONCE(true);
>

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