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Subject: Re: [RFC PATCH v2 02/19] fs/locks: Add Exclusive flag to user Layout
 lease
To:     <ira.weiny@intel.com>, Andrew Morton <akpm@linux-foundation.org>
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References: <20190809225833.6657-1-ira.weiny@intel.com>
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From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <69a7c037-6b4b-dbe3-2b42-77f85043b9eb@nvidia.com>
Date:   Wed, 4 Sep 2019 16:12:35 -0700
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On 8/9/19 3:58 PM, ira.weiny@intel.com wrote:
> From: Ira Weiny <ira.weiny@intel.com>
> 
> Add an exclusive lease flag which indicates that the layout mechanism
> can not be broken.

After studying the rest of these discussions extensively, I think in all
cases FL_EXCLUSIVE is better named "unbreakable", rather than exclusive.

If you read your sentence above, it basically reinforces that idea: "add an
exclusive flag to mean it is unbreakable" is a bit of a disconnect. It 
would be better to say,

Add an "unbreakable" lease flag which indicates that the layout lease
cannot be broken.

Furthermore, while this may or may not be a way forward on the "we cannot
have more than one process take a layout lease on a file/range", it at
least stops making it impossible. In other words, no one is going to
write a patch that allows sharing an exclusive layout lease--but someone
might well update some of these patches here to make it possible to
have multiple processes take unbreakable leases on the same file/range.

I haven't worked through everything there yet, but again:

* FL_UNBREAKABLE is the name you're looking for here, and

* I think we want to allow multiple processes to take FL_UNBREAKABLE
leases on the same file/range, so that we can make RDMA setups
reasonable. By "reasonable" I mean, "no need to have a lead process
that owns all the leases".


thanks,
-- 
John Hubbard
NVIDIA