On Wed 03-10-18 08:13:37, Dan Williams wrote: > On Wed, Oct 3, 2018 at 8:07 AM Jan Kara <jack@xxxxxxx> wrote: > > WRT per-inode DAX property, AFAIU that inode flag is just going to be > > advisory thing - i.e., use DAX if possible. If you mount a filesystem with > > these inode flags set in a configuration which does not allow DAX to be > > used, you will still be able to access such inodes but the access will use > > page cache instead. And querying these flags should better show real > > on-disk status and not just whether DAX is used as that would result in an > > even bigger mess. So this feature seems to be somewhat orthogonal to the > > API I'm looking for. > > True, I imagine once we have that flag we will be able to distinguish > the "saved" property and the "effective / live" property of DAX... > Also it's really not DAX that applications care about as much as "is > there page-cache indirection / overhead for this mapping?". That seems > to be a narrower guarantee that we can make than what "DAX" might > imply. Right. So what do people think about my suggestion earlier in the thread to use madvise(MADV_DIRECT_ACCESS) for this? Currently it would return success when DAX is in use, failure otherwise. Later we could extend it to be also used as a hint for caching policy for the inode... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
