Re: [PATCH 2/2] dax: move writeback calls into the filesystems

[Ross Zwisler]

On Tue, Feb 09, 2016 at 05:01:34PM +0100, Jan Kara wrote:
> On Tue 09-02-16 10:43:53, Jan Kara wrote:
> > On Mon 08-02-16 12:55:24, Dan Williams wrote:
> > > On Mon, Feb 8, 2016 at 12:18 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > > [..]
> > > >> Setting aside the current block zeroing problem you seem to assuming
> > > >> that DAX will always be faster and that may not be true at a media
> > > >> level.  Waiting years for some applications to determine if DAX makes
> > > >> sense for their use case seems completely reasonable.  In the meantime
> > > >> the apps that are already making these changes want to know that a DAX
> > > >> mapping request has not silently dropped backed to page cache.  They
> > > >> also want to know if they successfully jumped through all the hoops to
> > > >> get a larger than pte mapping.
> > > >>
> > > >> I agree it is useful to be able to force DAX on an unmodified
> > > >> application to see what happens, and it follows that if those
> > > >> applications want to run in that mode they will need functional
> > > >> fsync()...
> > > >>
> > > >> I would feel better if we were talking about specific applications and
> > > >> performance numbers to know if forcing DAX on application is a debug
> > > >> facility or a production level capability.  You seem to have already
> > > >> made that determination and I'm curious what I'm missing.
> > > >
> > > > I'm not setting any policy here at all.  This whole argument is
> > > > based around the DAX mount option doing "global fs enable or
> > > > silently turning it off" and the application not knowing about that.
> > > >
> > > > The whole point of having a persistent per-inode DAX flags is that
> > > > it is a policy mechanism, not a policy.  The application can, if it
> > > > is DAX aware, directly control whether DAX is used on a file or not.
> > > > The application can even query and clear that persistent inode flag
> > > > if it is configured not to (or cannot) use DAX.
> > > >
> > > > If the filesystem cannot support DAX, then we can error out attempts
> > > > to set the DAX flag and then the app knows DAX is not available.
> > > > i.e. the attempt to set policy failed. If the flag is set, then the
> > > > inode will *always* use DAX - there is no "fall back to page cache"
> > > > when DAX is enabled.
> > > >
> > > > If the applicaiton is not DAX aware, then the admin can control the
> > > > DAX policy by manipulating these flags themselves, and hence control
> > > > whether DAX is used by the application or not.
> > > >
> > > > If you think I'm dictating policy for DAX users and application,
> > > > then you haven't understood anything I've previously said about why
> > > > the DAX mount option needs to die before any of this is considered
> > > > production ready. DAX is not an opaque "all or nothing" option. XFS
> > > > will provide apps and admins with fine-grained, persistent,
> > > > discoverable policy flags to allow admins and applications to set
> > > > DAX policies however they see fit. This simply cannot be done if the
> > > > only knob you have is a mount option that may or may not stick.
> > > 
> > > I agree the mount option needs to die, and I fully grok the reasoning.
> > >   What I'm concerned with is that a system using fully-DAX-aware
> > > applications is forced to incur the overhead of maintaining *sync
> > > semantics, periodic sync(2) in particular,  even if it is not relying
> > > on those semantics.
> > 
> > Let me somewhat correct this: IMO hard requirement is maintaining sync(2)
> > semantics. Periodic writeback does not have any hard durability guarantees
> > and we are free to ignore such requests in ->writepages() (that function
> > has enough information in the writeback_control structure to differentiate
> > between periodic writeback and data integrity sync) if we decide it is
> > useful. Actually, we could do that even for 4.5.
> 
> Attached is a version of Ross' patch that will work for sync(2) and
> fsync(2) and we won't flush caches during periodic writeback. The patch is
> only compile-tested. Ross?

This looks great.  I'll send out a v2 with this and with the
dax_clear_sectors() changes after I'm done testing.