Re: [PATCH] libblkid: add dax capability detection in topology probing

[Anthony Iliopoulos <ailiop@suse.com>]

On Wed, May 06, 2020 at 03:08:47PM +0200, Karel Zak wrote:
> On Tue, May 05, 2020 at 04:31:45PM +0200, Anthony Iliopoulos wrote:
> > The dax (direct access) blockdev capability is exposed via sysfs, add it
> > to the list of topology values to be obtained while probing.
> > 
> > Expose blkid_topology_get_dax() symbol that programs can link against
> > for querying the capability.
> 
> Do we have any use-case for this change? 
> 
> We maintain blkid_topology_* mostly for mkfs-like programs portability 
> (years ago we had only ioctls, etc..). You can see that libblkid export
> only small subset topology stuff, so why we need DAX there? ;-)

The use-case is indeed mkfs. I am planning to submit a patch to xfsprogs
that warns if the blockdev is dax-capable but the default or specified
fs blocksize isn't matching the platform page size (in which case the fs
cannot be used/mounted with dax). This comes up with archs like ppc64
where the page size is larger than the default (usually 4K) fs blocksize.

I wanted to avoid parsing sysfs from xfsprogs, especially given that
libblkid is already leveraged there to obtain the topology, and I assume
it may be handy for e2fsprogs too, as ext4 (and any other fs that may
support dax in the future) is bound to the same restriction.