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@xxxxxxx +49 911 74053 689 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: Felix Imendörffer, Jane Smithard, Graham Norton HRB 21284 (AG Nürnberg) Key fingerprint = EC38 9CAB C2C4 F25D 8600 D0D0 0393 969D 2D76 0850
