linux-kernel - Re: [dm-devel] DM's filesystem lookup in dm_get_dev_t() [was: Re: linux-next: manual merge of the device-mapper tree with Linus' tree]

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Message-ID: <20201222210604.GD29336@agk.fab.redhat.com>
Date:   Tue, 22 Dec 2020 21:06:04 +0000
From:   Alasdair G Kergon <agk@...hat.com>
To:     Hannes Reinecke <hare@...e.de>
Cc:     Mike Snitzer <snitzer@...hat.com>, Christoph Hellwig <hch@....de>,
        linux-block@...r.kernel.org, dm-devel@...hat.com,
        Jens Axboe <axboe@...nel.dk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alasdair G Kergon <agk@...hat.com>
Subject: Re: [dm-devel] DM's filesystem lookup in dm_get_dev_t() [was: Re: linux-next: manual merge of the device-mapper tree with Linus' tree]

On Tue, Dec 22, 2020 at 06:24:09PM +0100, Hannes Reinecke wrote:
> However, lookup_bdev() now always recurses into the filesystem, causing 
> multipath to stall in an all-paths-down scenario.

I have not read the background about whatever the new problem is - I'm
jumping in cold seeing this message - but from the very beginning of
device-mapper we have strongly recommended that userspace supplies the
block device in the form MAJOR:MINOR and all our own tools do that.  We
guarantee not to deadlock in these places when this is done.

We also accept the device in the form of a path name as we know there
are times when this is safe and convenient, but then we offer no
guarantees - we place the responsibility upon userspace only to do this
when it knows it is safe to do so i.e. no race and no deadlock.

Alasdair