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

[Andrea Righi]

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:

	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>