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Date:   Thu, 13 Jun 2019 14:13:21 -0700
From:   Ira Weiny <ira.weiny@intel.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Dave Chinner <david@fromorbit.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>,
        =?iso-8859-1?B?Suly9G1l?= 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, Jason Gunthorpe <jgg@ziepe.ca>,
        linux-rdma@vger.kernel.org
Subject: Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal
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On Thu, Jun 13, 2019 at 08:27:55AM -0700, Matthew Wilcox wrote:
> On Thu, Jun 13, 2019 at 10:25:55AM +1000, Dave Chinner wrote:
> > e.g. Process A has an exclusive layout lease on file F. It does an
> > IO to file F. The filesystem IO path checks that Process A owns the
> > lease on the file and so skips straight through layout breaking
> > because it owns the lease and is allowed to modify the layout. It
> > then takes the inode metadata locks to allocate new space and write
> > new data.
> > 
> > Process B now tries to write to file F. The FS checks whether
> > Process B owns a layout lease on file F. It doesn't, so then it
> > tries to break the layout lease so the IO can proceed. The layout
> > breaking code sees that process A has an exclusive layout lease
> > granted, and so returns -ETXTBSY to process B - it is not allowed to
> > break the lease and so the IO fails with -ETXTBSY.
> 
> This description doesn't match the behaviour that RDMA wants either.
> Even if Process A has a lease on the file, an IO from Process A which
> results in blocks being freed from the file is going to result in the
> RDMA device being able to write to blocks which are now freed (and
> potentially reallocated to another file).

I don't understand why this would not work for RDMA?  As long as the layout
does not change the page pins can remain in place.

Ira