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

[Hannes Reinecke]

On 12/22/20 3:53 PM, Mike Snitzer wrote:
> [added linux-block and dm-devel, if someone replies to this email to
> continue "proper discussion" _please_ at least drop sfr and linux-next
> from Cc]
>
> On Tue, Dec 22 2020 at  8:15am -0500,
> Christoph Hellwig <hch@xxxxxx> wrote:
>
> > Mike, Hannes,
> >
> > I think this patch is rather harmful.  Why does device mapper even
> > mix file system path with a dev_t and all the other weird forms
> > parsed by name_to_dev_t, which was supposed to be be for the early
> > init code where no file system is available.
>
> OK, I'll need to revisit (unless someone beats me to it) because this
> could've easily been a blind-spot for me when the dm-init code went in.
> Any dm-init specific enabling interface shouldn't be used by more
> traditional DM interfaces.  So Hannes' change might be treating symptom
> rather than the core problem (which would be better treated by factoring
> out dm-init requirements for a name_to_dev_t()-like interface?).
>
> DM has supported passing maj:min and blockdev names on DM table lines
> forever... so we'll need to be very specific about where/why things
> regressed.

Ok. The problem from my perspective is that device-mapper needs to
a) ensure that the arbitrary string passed in with the table definition 
refers to a valid block device
and
b) the block device can be opened with O_EXCL, so that device-mapper can 
then use it.

Originally (ie prior to commit 644bda6f3460) dm_get_device() just 
converted the string to a 'dev_t' representation, and then the block 
device itself was checked and opened in dm_get_table_device().
'lookup_bdev' was just being used to convert the path if the string was 
not in the canonical major:minor format, as then it was assumed that it 
referred to a block device node, and then lookup_bdev kinda makes sense.

However, lookup_bdev() now always recurses into the filesystem, causing 
multipath to stall in an all-paths-down scenario.

So, the real issue is the table definiton; as it also accepts a device 
to be specified by the block device _node_ name, we need to have a way 
of converting that into a dev_t.

If lookup_bdev() is the wrong interface for that, by all means, please, 
do tell me. I'd be happy to draft up a patch.

Alternatively, if Mike says that only major:minor is the valid format 
for a table definition we can kill that code completely. But clearly _I_ 
cannot make the call here.

Cheers,

Hannes
--
Dr. Hannes Reinecke                Kernel Storage Architect
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