On Tue, Jan 17, 2017 at 10:07 PM,  [off-list ref] wrote:

On Tue, Jan 17, 2017 at 10:01:30PM -0800, Dan Williams wrote:

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- 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).

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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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