linux-ext4 - Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

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Date:   Wed, 12 Jun 2019 05:37:53 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Theodore Ts'o <tytso@....edu>,
        Jeff Layton <jlayton@...nel.org>, linux-xfs@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-ext4@...r.kernel.org,
        linux-mm@...ck.org, Jason Gunthorpe <jgg@...pe.ca>,
        linux-rdma@...r.kernel.org
Subject: Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

On Sat, Jun 08, 2019 at 10:10:36AM +1000, Dave Chinner wrote:
> On Fri, Jun 07, 2019 at 11:25:35AM -0700, Ira Weiny wrote:
> > Are you suggesting that we have something like this from user space?
> > 
> > 	fcntl(fd, F_SETLEASE, F_LAYOUT | F_UNBREAKABLE);
> 
> Rather than "unbreakable", perhaps a clearer description of the
> policy it entails is "exclusive"?
> 
> i.e. what we are talking about here is an exclusive lease that
> prevents other processes from changing the layout. i.e. the
> mechanism used to guarantee a lease is exclusive is that the layout
> becomes "unbreakable" at the filesystem level, but the policy we are
> actually presenting to uses is "exclusive access"...

That's rather different from the normal meaning of 'exclusive' in the
context of locks, which is "only one user can have access to this at
a time".  As I understand it, this is rather more like a 'shared' or
'read' lock.  The filesystem would be the one which wants an exclusive
lock, so it can modify the mapping of logical to physical blocks.

The complication being that by default the filesystem has an exclusive
lock on the mapping, and what we're trying to add is the ability for
readers to ask the filesystem to give up its exclusive lock.