Re: Lease semantic proposal

From: "J. Bruce Fields" <bfields@fieldses.org>
To: Jeff Layton <jlayton@kernel.org>
Cc: Ira Weiny <ira.weiny@intel.com>,
	linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-ext4@vger.kernel.org, linux-rdma@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org,
	linux-mm@kvack.org, Dave Chinner <david@fromorbit.com>,
	Jan Kara <jack@suse.cz>, Theodore Ts'o <tytso@mit.edu>,
	John Hubbard <jhubbard@nvidia.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: Lease semantic proposal
Date: Thu, 3 Oct 2019 11:37:43 -0400	[thread overview]
Message-ID: <20191003153743.GA24657@fieldses.org> (raw)
In-Reply-To: <df9022f0f5d18d71f37ed494a05eaa4509cf0a68.camel@kernel.org>

On Wed, Oct 02, 2019 at 04:35:55PM -0400, Jeff Layton wrote:
> On Wed, 2019-10-02 at 15:27 -0400, J. Bruce Fields wrote:
> > On Wed, Oct 02, 2019 at 08:28:40AM -0400, Jeff Layton wrote:
> > > For the byte ranges, the catch there is that extending the userland
> > > interface for that later will be difficult.
> > 
> > Why would it be difficult?
> 
> Legacy userland code that wanted to use byte range enabled layouts would
> have to be rebuilt to take advantage of them. If we require a range from
> the get-go, then they will get the benefit of them once they're
> available.

I can't see writing byte-range code for a kernel that doesn't support
that yet.  How would I test it?

> > > What I'd probably suggest
> > > (and what would jive with the way pNFS works) would be to go ahead and
> > > add an offset and length to the arguments and result (maybe also
> > > whence?).
> > 
> > Why not add new commands with range arguments later if it turns out to
> > be necessary?
> 
> We could do that. It'd be a little ugly, IMO, simply because then we'd
> end up with two interfaces that do almost the exact same thing.
> 
> Should byte-range layouts at that point conflict with non-byte range
> layouts, or should they be in different "spaces" (a'la POSIX and flock
> locks)? When it's all one interface, those sorts of questions sort of
> answer themselves. When they aren't we'll have to document them clearly
> and I think the result will be more confusing for userland programmers.

I was hoping they'd be in the same space, with the old interface just
defined to deal in locks with range [0,∞).

I'm just worried about getting the interface wrong if it's specified
without being implemented.  Maybe this is straightforward enough that
there's not a risk, I don't know.

--b.