On Tue, Oct 2, 2018 at 8:32 AM Jan Kara <jack@xxxxxxx> wrote: > > On Tue 02-10-18 07:52:06, Christoph Hellwig wrote: > > On Tue, Oct 02, 2018 at 04:44:13PM +0200, Johannes Thumshirn wrote: > > > On Tue, Oct 02, 2018 at 07:37:13AM -0700, Christoph Hellwig wrote: > > > > No, it should not. DAX is an implementation detail thay may change > > > > or go away at any time. > > > > > > Well we had an issue with an application checking for dax, this is how > > > we landed here in the first place. > > > > So what exacty is that "DAX" they are querying about (and no, I'm not > > joking, nor being philosophical). > > I believe the application we are speaking about is mostly concerned about > the memory overhead of the page cache. Think of a machine that has ~ 1TB of > DRAM, the database running on it is about that size as well and they want > database state stored somewhere persistently - which they may want to do by > modifying mmaped database files if they do small updates... So they really > want to be able to use close to all DRAM for the DB and not leave slack > space for the kernel page cache to cache 1TB of database files. VM_MIXEDMAP was never a reliable indication of DAX because it could be set for random other device-drivers that use vm_insert_mixed(). The MAP_SYNC flag positively indicates that page cache is disabled for a given mapping, although whether that property is due to "dax" or some other kernel mechanics is purely an internal detail. I'm not opposed to faking out VM_MIXEDMAP if this broken check has made it into production, but again, it's unreliable.
