On Tue, Oct 02, 2018 at 07:45:47AM -0700, Christoph Hellwig wrote:

How does an application "make use of DAX"?  What actual user visible
semantics are associated with a file that has this flag set?

There may not be any user visible semantics of DAX, but there are
promises we gave to application developers praising DAX as _the_
method to map data on persistent memory and get around "the penalty of
the page cache" (however big this is).

As I said in another mail to this thread, applications have started to
poke in procfs to see whether they can use DAX or not.

Party A has promised party B something and they started checking for
it, then commit e1fb4a086495 "dax: remove VM_MIXEDMAP for fsdax and
device dax" removed the way they checked if the kernel can keep up
with this promise.

So technically e1fb4a086495 is a user visible regression and in the
past we have reverted patches introducing these, even if the patch is
generally correct and poking in /proc/self/smaps is a bad idea.

I just wanted to give them a documented way to check for this
promise. Being neutral if this promise is right or wrong, good or bad,
or whatever. That's not my call, but I prefer not having angry users,
yelling at me because of broken applications.

Byte,
	Johannes
-- 
Johannes Thumshirn                                          Storage
jthumshirn@suse.de                                +49 911 74053 689
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nurnberg
GF: Felix Imendorffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nurnberg)
Key fingerprint = EC38 9CAB C2C4 F25D 8600 D0D0 0393 969D 2D76 0850