Re: [RFC PATCH v2 02/19] fs/locks: Add Exclusive flag to user Layout lease

[Jeff Layton]

On Thu, 2019-08-29 at 16:34 -0700, Ira Weiny wrote:
> Missed this.  sorry.
> 
> On Mon, Aug 26, 2019 at 06:41:07AM -0400, Jeff Layton wrote:
> > On Thu, 2019-08-15 at 07:56 +1000, Dave Chinner wrote:
> > > On Wed, Aug 14, 2019 at 10:15:06AM -0400, Jeff Layton wrote:
> > > > On Fri, 2019-08-09 at 15:58 -0700, ira.weiny@xxxxxxxxx wrote:
> > > > > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > > > > 
> > > > > Add an exclusive lease flag which indicates that the layout mechanism
> > > > > can not be broken.
> > > > > 
> > > > > Exclusive layout leases allow the file system to know that pages may be
> > > > > GUP pined and that attempts to change the layout, ie truncate, should be
> > > > > failed.
> > > > > 
> > > > > A process which attempts to break it's own exclusive lease gets an
> > > > > EDEADLOCK return to help determine that this is likely a programming bug
> > > > > vs someone else holding a resource.
> > > .....
> > > > > diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
> > > > > index baddd54f3031..88b175ceccbc 100644
> > > > > --- a/include/uapi/asm-generic/fcntl.h
> > > > > +++ b/include/uapi/asm-generic/fcntl.h
> > > > > @@ -176,6 +176,8 @@ struct f_owner_ex {
> > > > >  
> > > > >  #define F_LAYOUT	16      /* layout lease to allow longterm pins such as
> > > > >  				   RDMA */
> > > > > +#define F_EXCLUSIVE	32      /* layout lease is exclusive */
> > > > > +				/* FIXME or shoudl this be F_EXLCK??? */
> > > > >  
> > > > >  /* operations for bsd flock(), also used by the kernel implementation */
> > > > >  #define LOCK_SH		1	/* shared lock */
> > > > 
> > > > This interface just seems weird to me. The existing F_*LCK values aren't
> > > > really set up to be flags, but are enumerated values (even if there are
> > > > some gaps on some arches). For instance, on parisc and sparc:
> > > 
> > > I don't think we need to worry about this - the F_WRLCK version of
> > > the layout lease should have these exclusive access semantics (i.e
> > > other ops fail rather than block waiting for lease recall) and hence
> > > the API shouldn't need a new flag to specify them.
> > > 
> > > i.e. the primary difference between F_RDLCK and F_WRLCK layout
> > > leases is that the F_RDLCK is a shared, co-operative lease model
> > > where only delays in operations will be seen, while F_WRLCK is a
> > > "guarantee exclusive access and I don't care what it breaks"
> > > model... :)
> > > 
> > 
> > Not exactly...
> > 
> > F_WRLCK and F_RDLCK leases can both be broken, and will eventually time
> > out if there is conflicting access. The F_EXCLUSIVE flag on the other
> > hand is there to prevent any sort of lease break from 
> 
> Right EXCLUSIVE will not break for any reason.  It will fail truncate and hole
> punch as we discussed back in June.  This is for the use case where the user
> has handed this file/pages off to some hardware for which removing the lease
> would be impossible.  _And_ we don't anticipate any valid use case that someone
> will need to truncate short of killing the process to free up file system
> space.
> 
> > I'm guessing what Ira really wants with the F_EXCLUSIVE flag is
> > something akin to what happens when we set fl_break_time to 0 in the
> > nfsd code. nfsd never wants the locks code to time out a lease of any
> > sort, since it handles that timeout itself.
> > 
> > If you're going to add this functionality, it'd be good to also convert
> > knfsd to use it as well, so we don't end up with multiple ways to deal
> > with that situation.
> 
> Could you point me at the source for knfsd?  I looked in 
> 
> git://git.linux-nfs.org/projects/steved/nfs-utils.git
> 
> but I don't see anywhere leases are used in that source?
> 

Ahh sorry that wasn't clear. It's the fs/nfsd directory in the Linux
kernel sources. See nfsd4_layout_lm_break and nfsd_break_deleg_cb in
particular.

-- 
Jeff Layton <jlayton@xxxxxxxxxx>