Re: Problems with VM_MIXEDMAP removal from /proc//smaps

Jan Kara
2018-10-03T16:44:09Z

On Wed 03-10-18 08:13:37, Dan Williams wrote:
> On Wed, Oct 3, 2018 at 8:07 AM Jan Kara <jack@suse.cz> 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@suse.com>
SUSE Labs, CR