linux-ext4 - Re: [RFC PATCH v2 02/19] fs/locks: Add Exclusive flag to user Layout lease

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Date:   Wed, 4 Sep 2019 16:12:35 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     <ira.weiny@...el.com>, Andrew Morton <akpm@...ux-foundation.org>
CC:     Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Theodore Ts'o <tytso@....edu>, Michal Hocko <mhocko@...e.com>,
        Dave Chinner <david@...morbit.com>,
        <linux-xfs@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-nvdimm@...ts.01.org>, <linux-ext4@...r.kernel.org>,
        <linux-mm@...ck.org>
Subject: Re: [RFC PATCH v2 02/19] fs/locks: Add Exclusive flag to user Layout
 lease

On 8/9/19 3:58 PM, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.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