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Date:   Wed, 14 Aug 2019 18:05:47 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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>,
        John Hubbard <jhubbard@...dia.com>,
        Michal Hocko <mhocko@...e.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 01/19] fs/locks: Export F_LAYOUT lease to user
 space

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@...el.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@...morbit.com

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