On Wed, Oct 3, 2018 at 5:51 AM Jan Kara <jack@xxxxxxx> wrote: > > On Tue 02-10-18 13:18:54, Dan Williams wrote: > > 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. > > So luckily this particular application wasn't widely deployed yet so we > will likely get away with the vendor asking customers to update to a > version not looking into smaps and parsing /proc/mounts instead. > > But I don't find parsing /proc/mounts that beautiful either and I'd prefer > if we had a better interface for applications to query whether they can > avoid page cache for mmaps or not. Yeah, the mount flag is not a good indicator either. I think we need to follow through on the per-inode property of DAX. Darrick and I discussed just allowing the property to be inherited from the parent directory at file creation time. That avoids the dynamic set-up / teardown races that seem intractable at this point. What's wrong with MAP_SYNC as a page-cache detector in the meantime?
