Re: dax: Arrange for dax_supported check to span multiple devices

[Mike Snitzer <snitzer@redhat.com>]

On Tue, May 14 2019 at 11:48pm -0400,
Dan Williams <dan.j.williams@intel.com> wrote:

> Pankaj reports that starting with commit ad428cdb525a "dax: Check the
> end of the block-device capacity with dax_direct_access()" device-mapper
> no longer allows dax operation. This results from the stricter checks in
> __bdev_dax_supported() that validate that the start and end of a
> block-device map to the same 'pagemap' instance.
> 
> Teach the dax-core and device-mapper to validate the 'pagemap' on a
> per-target basis. This is accomplished by refactoring the
> bdev_dax_supported() internals into generic_fsdax_supported() which
> takes a sector range to validate. Consequently generic_fsdax_supported()
> is suitable to be used in a device-mapper ->iterate_devices() callback.
> A new ->dax_supported() operation is added to allow composite devices to
> split and route upper-level bdev_dax_supported() requests.
> 
> Fixes: ad428cdb525a ("dax: Check the end of the block-device...")
> Cc: <stable@vger.kernel.org>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: Dave Jiang <dave.jiang@intel.com>
> Cc: Mike Snitzer <snitzer@redhat.com>
> Cc: Keith Busch <keith.busch@intel.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Vishal Verma <vishal.l.verma@intel.com>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Reported-by: Pankaj Gupta <pagupta@redhat.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
> Hi Mike,
> 
> Another day another new dax operation to allow device-mapper to better
> scope dax operations.
> 
> Let me know if the device-mapper changes look sane. This passes a new
> unit test that indeed fails on current mainline.
> 
> https://github.com/pmem/ndctl/blob/device-mapper-pending/test/dm.sh
> 
>  drivers/dax/super.c          |   88 +++++++++++++++++++++++++++---------------
>  drivers/md/dm-table.c        |   17 +++++---
>  drivers/md/dm.c              |   20 ++++++++++
>  drivers/md/dm.h              |    1 
>  drivers/nvdimm/pmem.c        |    1 
>  drivers/s390/block/dcssblk.c |    1 
>  include/linux/dax.h          |   19 +++++++++
>  7 files changed, 110 insertions(+), 37 deletions(-)
> 

...

> diff --git a/drivers/md/dm.h b/drivers/md/dm.h
> index 2d539b82ec08..e5e240bfa2d0 100644
> --- a/drivers/md/dm.h
> +++ b/drivers/md/dm.h
> @@ -78,6 +78,7 @@ void dm_unlock_md_type(struct mapped_device *md);
>  void dm_set_md_type(struct mapped_device *md, enum dm_queue_mode type);
>  enum dm_queue_mode dm_get_md_type(struct mapped_device *md);
>  struct target_type *dm_get_immutable_target_type(struct mapped_device *md);
> +bool dm_table_supports_dax(struct dm_table *t, int blocksize);
>  
>  int dm_setup_md_queue(struct mapped_device *md, struct dm_table *t);
>  

I'd prefer to have dm_table_supports_dax come just after
dm_table_get_md_mempools in the preceding dm_table section of dm.h (just
above this mapped_device section you extended).

But other than that nit, patch looks great on a DM level:

Reviewed-by: Mike Snitzer <snitzer@redhat.com>