On Wed, Oct 3, 2018 at 9:46 AM Jan Kara <jack@xxxxxxx> wrote: > > 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... The only problem is that you can't use it purely as a query. If we ever did plumb it to be a hint you could not read the state without writing the state. mincore(2) seems to be close the intent of discovering whether RAM is being consumed for a given address range, but it currently is implemented to only indicate if *any* mapping is established, not whether RAM is consumed. I can see an argument that a dax mapped file should always report an empty mincore vector.
