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

From: Jason Gunthorpe <jgg@ziepe.ca>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Dave Chinner" <david@fromorbit.com>,
	"Ira Weiny" <ira.weiny@intel.com>, "Jan Kara" <jack@suse.cz>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Theodore Ts'o" <tytso@mit.edu>,
	"Jeff Layton" <jlayton@kernel.org>,
	linux-xfs@vger.kernel.org,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-nvdimm@lists.01.org, linux-ext4@vger.kernel.org,
	linux-mm@kvack.org, linux-rdma@vger.kernel.org
Subject: Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal
Date: Thu, 13 Jun 2019 12:29:53 -0300	[thread overview]
Message-ID: <20190613152953.GD22901@ziepe.ca> (raw)
In-Reply-To: <20190613032320.GG32656@bombadil.infradead.org>

On Wed, Jun 12, 2019 at 08:23:20PM -0700, Matthew Wilcox wrote:
> On Thu, Jun 13, 2019 at 10:25:55AM +1000, Dave Chinner wrote:
> > On Wed, Jun 12, 2019 at 05:37:53AM -0700, Matthew Wilcox wrote:
> > > 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".
> > 
> > Layout leases are not locks, they are a user access policy object.
> > It is the process/fd which holds the lease and it's the process/fd
> > that is granted exclusive access.  This is exactly the same semantic
> > as O_EXCL provides for granting exclusive access to a block device
> > via open(), yes?
> 
> This isn't my understanding of how RDMA wants this to work, so we should
> probably clear that up before we get too far down deciding what name to
> give it.
> 
> For the RDMA usage case, it is entirely possible that both process A
> and process B which don't know about each other want to perform RDMA to
> file F.  So there will be two layout leases active on this file at the
> same time.  It's fine for IOs to simultaneously be active to both leases.
> But if the filesystem wants to move blocks around, it has to break
> both leases.
> 
> If Process C tries to do a write to file F without a lease, there's no
> problem, unless a side-effect of the write would be to change the block
> mapping, in which case either the leases must break first, or the write
> must be denied.
> 
> Jason, please correct me if I've misunderstood the RDMA needs here.

Yes, I think you've captured it

Based on Dave's remarks how frequently a filesystem needs to break the
lease will determine if it is usuable to be combined with RDMA or
not...

Jason