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From: Dan Williams <dan.j.williams@intel.com>
Date: Tue, 17 Jan 2017 22:25:42 -0800
Message-ID: <CAPcyv4gvMzPbqpotKTRYhuiRgKRUz-Xrz81MFjpBmh3i=8euFQ@mail.gmail.com>
Subject: Re: [LSF/MM TOPIC] Future direction of DAX
To: willy@bombadil.infradead.org
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>, lsf-pc@lists.linux-foundation.org,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>, linux-block@vger.kernel.org,
	Linux MM <linux-mm@kvack.org>
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On Tue, Jan 17, 2017 at 10:07 PM,  <willy@bombadil.infradead.org> wrote:
> On Tue, Jan 17, 2017 at 10:01:30PM -0800, Dan Williams wrote:
>> >> - Jan suggested [2] that we could use the radix tree as a cache to service DAX
>> >>   faults without needing to call into the filesystem.  Are there any issues
>> >>   with this approach, and should we move forward with it as an optimization?
>> >
>> > Ahem.  I believe I proposed this at last year's LSFMM.  And I sent
>> > patches to start that work.  And Dan blocked it.  So I'm not terribly
>> > amused to see somebody else given credit for the idea.
>>
>> I "blocked" moving the phys to virt translation out of the driver
>> since that mapping lifetime is device specific.
>
> The problem is that DAX currently assumes that there *is* a block driver,
> and it might be a char device or no device at all (the two examples I
> gave earlier).
>
>> However, I think caching the file offset to physical sector/address
>> result is a great idea.
>
> OK, great.  The lifetime problem I think you care about (hotplug) can be
> handled by removing all the cached entries for every file on every file
> on that block device ... I know there were prototype patches for that;
> did they ever get merged?

No, they didn't.. The last review comment was from Al. He wanted the
mechanism converted from explicit calls at del_gendisk() time into a
notifier chain since it's not just filesystems that may want to
register for a block-device end-of-life event.

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