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

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Date:   Wed, 12 Jun 2019 20:23:20 -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 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.