Re: [PATCH 1/2] xfs: cache dax_device lookup result

[Dan Williams]

On Thu, Aug 24, 2017 at 10:32 PM, Darrick J. Wong
<darrick.wong@xxxxxxxxxx> wrote:
> On Thu, Aug 24, 2017 at 05:35:48PM -0700, Dan Williams wrote:
>> The ->iomap_begin() operation is a hot path, so cache the
>> fs_dax_get_by_host() result to avoid the incurring the hash lookup
>> overhead.
>
> Just out of curiosity (and sorry if this has already been discussed to
> death and I'm merely ignorant) but I was wondering why the daxdev isn't
> simply attached to the block_device?  Is it not a 1:1 mapping?

The motivation was to remove all dax details from the block layer.
However, we still have the QUEUE_DAX flag which defeats that goal.
Hmm, a dax_device is similar to a request_queue as a path to access
capacity, so I think it might make sense to just replace the queue
flag with a full ->dax_dev pointer hanging off the gendisk.

This would also fix the bug I just realized was in these patches
around matching dax_get_by_host() with put_dax(). In fact, if it's
acceptable to hang a dax_dev off a gendisk then we don't need
dax_get_by_host() at all.