ACK/Cmnt: [PATCH F/Unstable 1/1] UBUNTU: SAUCE: blk/core: Gracefully handle unset make_request_fn

[Stefan Bader]

On 23.01.20 11:13, Andrea Righi wrote:
> On Thu, Jan 23, 2020 at 10:33:03AM +0200, Stefan Bader wrote:
>> BugLink: https://bugs.launchpad.net/bugs/1860231
>>
>> 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. 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.
>>
>> Fixes: ff36ab34583a ("dm: remove request-based logic from make_request_fn wrapper")
>> Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
>> ---
>>  block/blk-core.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 1075aaff606d..adcd042edd2d 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -884,6 +884,13 @@ generic_make_request_checks(struct bio *bio)
>>  			bio_devname(bio, b), (long long)bio->bi_iter.bi_sector);
>>  		goto end_io;
>>  	}
>> +	if (unlikely(!q->make_request_fn)) {
>> +		printk(KERN_ERR
>> +		       "generic_make_request: Trying to access "
>> +		       "block-device without request function: %s\n",
>> +		       bio_devname(bio, b));
>> +		goto end_io;
>> +	}
> 
> Instead of returning an I/O error, I was wondering if it would be better
> to give a chance to the caller to repeat the I/O request gracefully,
> returning -EAGAIN. Something like this:

This potentially can take some time (potentially).Like in the example given you
create a device without that mapping function until you load the table. That
might be much later and maybe by that time there are already a lot of I/O
requests in flight. Plus I basically repeated the handling which is done when
you submit I/O to a device which has no request queue set up at all.

> 
> 	if (unlikely(!q->make_request_fn)) {
> 		printk(KERN_WARNING
> 		       "generic_make_request: Trying to access "
> 		       "block-device without request function: %s\n",
> 		       bio_devname(bio, b));
> 		status = BLK_STS_AGAIN;
> 		goto end_io;
> 	}
> 
>>  
>>  	/*
>>  	 * Non-mq queues do not honor REQ_NOWAIT, so complete a bio
>> -- 
>> 2.17.1
> 
> However, this change also looks safe as it is (definitely better than
> crashing the kernel when q->make_request_fn is NULL), therefore:
> 
> Acked-by: Andrea Righi <andrea.righi at canonical.com>
> 


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