Re: [RFC PATCH v2 01/19] fs/locks: Export F_LAYOUT lease to user space

Dave Chinner
2019-08-14T08:06:58Z

On Mon, Aug 12, 2019 at 10:36:26AM -0700, Ira Weiny wrote:
> On Sat, Aug 10, 2019 at 09:52:31AM +1000, Dave Chinner wrote:
> > On Fri, Aug 09, 2019 at 03:58:15PM -0700, ira.weiny@intel.com wrote:
> > > + /*
> > > + * NOTE on F_LAYOUT lease
> > > + *
> > > + * LAYOUT lease types are taken on files which the user knows that
> > > + * they will be pinning in memory for some indeterminate amount of
> > > + * time.
> >
> > Indeed, layout leases have nothing to do with pinning of memory.
>
> Yep, Fair enough. I'll rework the comment.
>
> > That's something an application taht uses layout leases might do,
> > but it largely irrelevant to the functionality layout leases
> > provide. What needs to be done here is explain what the layout lease
> > API actually guarantees w.r.t. the physical file layout, not what
> > some application is going to do with a lease. e.g.
> >
> > The layout lease F_RDLCK guarantees that the holder will be
> > notified that the physical file layout is about to be
> > changed, and that it needs to release any resources it has
> > over the range of this lease, drop the lease and then
> > request it again to wait for the kernel to finish whatever
> > it is doing on that range.
> >
> > The layout lease F_RDLCK also allows the holder to modify
> > the physical layout of the file. If an operation from the
> > lease holder occurs that would modify the layout, that lease
> > holder does not get notification that a change will occur,
> > but it will block until all other F_RDLCK leases have been
> > released by their holders before going ahead.
> >
> > If there is a F_WRLCK lease held on the file, then a F_RDLCK
> > holder will fail any operation that may modify the physical
> > layout of the file. F_WRLCK provides exclusive physical
> > modification access to the holder, guaranteeing nothing else
> > will change the layout of the file while it holds the lease.
> >
> > The F_WRLCK holder can change the physical layout of the
> > file if it so desires, this will block while F_RDLCK holders
> > are notified and release their leases before the
> > modification will take place.
> >
> > We need to define the semantics we expose to userspace first.....
>
> Agreed. I believe I have implemented the semantics you describe above. Do I
> have your permission to use your verbiage as part of reworking the comment and
> commit message?

Of course. :)

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com