On Mon, Nov 16, 2015 at 6:41 AM, Jan Kara [off-list ref] wrote:

On Fri 13-11-15 17:06:39, Ross Zwisler wrote:

quoted

This patch series adds support for fsync/msync to DAX.

Patches 1 through 7 add various utilities that the DAX code will eventually
need, and the DAX code itself is added by patch 8.  Patches 9-11 update the
three filesystems that currently support DAX, ext2, ext4 and XFS, to use
the new DAX fsync/msync code.

These patches build on the recent DAX locking changes from Dave Chinner,
Jan Kara and myself.  Dave's changes for XFS and my changes for ext2 have
been merged in the v4.4 window, but Jan's are still unmerged.  You can grab
them here:

http://www.spinics.net/lists/linux-ext4/msg49951.html

I had a quick look and the patches look sane to me. I'll try to give them
more detailed look later this week. When thinking about the general design
I was wondering: When we have this infrastructure to track data potentially
lingering in CPU caches, would not it be a performance win to use standard
cached stores in dax_io() and mark corresponding pages as dirty in page
cache the same way as this patch set does it for mmaped writes? I have no
idea how costly are non-temporal stores compared to cached ones and how
would this compare to the cost of dirty tracking so this may be just
completely bogus...

Keep in mind that this approach will flush every virtual address that
may be dirty.  For example, if you touch 1byte in a 2MB page we'll end
up looping through the entire 2MB range.  At some point the dirty size
becomes large enough that is cheaper to flush the entire cache, we
have not measured where that crossover point is.

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