linux-kernel - Re: [PATCH 1/2] xfs: cache dax

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Date:   Thu, 24 Aug 2017 23:24:10 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/2] xfs: cache dax_device lookup result

On Thu, Aug 24, 2017 at 10:32 PM, Darrick J. Wong
<darrick.wong@...cle.com> 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.