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Date: Fri, 23 Aug 2019 09:04:29 -0300
From: Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)
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To: Dave Chinner <david@fromorbit.com>
Cc: Ira Weiny <ira.weiny@intel.com>, Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>, Dan Williams <dan.j.williams@intel.com>, Matthew Wilcox <willy@infradead.org>, Theodore Ts'o <tytso@mit.edu>, John Hubbard <jhubbard@nvidia.com>, Michal Hocko <mhocko@suse.com>, linux-xfs@vger.kernel.org, linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-nvdimm@lists.01.org, linux-ext4@vger.kernel.org, linux-mm@kvack.org
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On Fri, Aug 23, 2019 at 01:23:45PM +1000, Dave Chinner wrote:

> > But the fact that RDMA, and potentially others, can "pass the
> > pins" to other processes is something I spent a lot of time trying to work out.
> 
> There's nothing in file layout lease architecture that says you
> can't "pass the pins" to another process.  All the file layout lease
> requirements say is that if you are going to pass a resource for
> which the layout lease guarantees access for to another process,
> then the destination process already have a valid, active layout
> lease that covers the range of the pins being passed to it via the
> RDMA handle.

How would the kernel detect and enforce this? There are many ways to
pass a FD.

IMHO it is wrong to try and create a model where the file lease exists
independently from the kernel object relying on it. In other words the
IB MR object itself should hold a reference to the lease it relies
upon to function properly.

Then we don't have to wreck the unix FD model to fit this in.

Jason