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Subject: Re: [PATCH 1/1] blk/core: Gracefully handle unset make_request_fn
To:     Mike Snitzer <snitzer@redhat.com>, Jens Axboe <axboe@kernel.dk>
Cc:     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:   Stefan Bader <stefan.bader@canonical.com>
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Message-ID: <e0475dae-a55f-f30e-a82f-ee35cdb171c4@canonical.com>
Date:   Tue, 28 Jan 2020 15:32:14 +0100
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On 27.01.20 20:32, Mike Snitzer wrote:
> On Thu, Jan 23 2020 at  1:52pm -0500,
> Jens Axboe <axboe@kernel.dk> wrote:
>=20
>> 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 regressio=
n
>>>> since an additional pointer comparison is performed on each block la=
yer
>>>> request but this is unlikely to be noticeable."
>>>>
>>>> This captures my immediate concern: slowing down everyone for this D=
M
>>>> edge-case isn't desirable.
>>>
>>> SO I had a look and there isn't anything easier than adding the propo=
sed
>>> 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 wi=
th
>>> it: cool.  If not, I'm open to suggestions for how to proceed. =20
>>>
>>
>> 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.
>=20
> I just staged the following DM fix:
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.=
git/commit/?h=3Ddm-5.6&id=3D28a101d6b344f5a38d482a686d18b1205bc92333

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 i=
s that
mount generated I/O requests to start with. The device size should be 0 s=
till.


>=20
> From: Mike Snitzer <snitzer@redhat.com>
> Date: Mon, 27 Jan 2020 14:07:23 -0500
> Subject: [PATCH] dm: fix potential for q->make_request_fn NULL pointer
>=20
> 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).
>=20
> Otherwise generic_make_request() will crash simply by doing:
>   dmsetup create -n test
>   mount /dev/dm-N /mnt
>=20
> While at it, move ->congested_data initialization out of
> dm.c:alloc_dev() and into the bio-based specific init method.
>=20
> Reported-by: Stefan Bader <stefan.bader@canonical.com>
> 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 initializ=
ation code")
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> ---
>  drivers/md/dm.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>=20
> 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 =3D md;
>  	md->queue->backing_dev_info->congested_fn =3D dm_any_congested;
>  }
> =20
> @@ -1949,7 +1950,12 @@ static struct mapped_device *alloc_dev(int minor=
)
>  	if (!md->queue)
>  		goto bad;
>  	md->queue->queuedata =3D md;
> -	md->queue->backing_dev_info->congested_data =3D 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);
> =20
>  	md->disk =3D alloc_disk_node(1, md->numa_node_id);
>  	if (!md->disk)
> @@ -2264,7 +2270,6 @@ int dm_setup_md_queue(struct mapped_device *md, s=
truct 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);
>=20


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