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

[Mike Snitzer <snitzer@redhat.com>]

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

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

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@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 initialization code")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 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);
-- 
2.21.GIT