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Date:   Thu, 23 Jan 2020 12:28:16 -0500
From:   Mike Snitzer <snitzer@redhat.com>
To:     Stefan Bader <stefan.bader@canonical.com>
Cc:     Jens Axboe <axboe@kernel.dk>, 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>
Subject: Re: [PATCH 1/1] blk/core: Gracefully handle unset make_request_fn
Message-ID: <20200123172816.GA31063@redhat.com>
References: <20200123091713.12623-1-stefan.bader@canonical.com>
 <20200123091713.12623-2-stefan.bader@canonical.com>
 <20200123103541.GA28102@redhat.com>
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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.