linux-ext4 - Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)

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Date:   Wed, 21 Aug 2019 12:09:04 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Theodore Ts'o <tytso@....edu>, 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 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-)

On Wed, Aug 21, 2019 at 11:43:30AM -0700, John Hubbard wrote:
> On 8/19/19 8:36 PM, Dave Chinner wrote:
> > On Mon, Aug 19, 2019 at 08:09:33PM -0700, John Hubbard wrote:
> > > On 8/19/19 6:20 PM, Dave Chinner wrote:
> > > > On Mon, Aug 19, 2019 at 05:05:53PM -0700, John Hubbard wrote:
> > > > > On 8/19/19 2:24 AM, Dave Chinner wrote:
> > > > > > On Mon, Aug 19, 2019 at 08:34:12AM +0200, Jan Kara wrote:
> > > > > > > On Sat 17-08-19 12:26:03, Dave Chinner wrote:
> > > > > > > > On Fri, Aug 16, 2019 at 12:05:28PM -0700, Ira Weiny wrote:
> > > > > > > > > On Thu, Aug 15, 2019 at 03:05:58PM +0200, Jan Kara wrote:
> > > > > > > > > > On Wed 14-08-19 11:08:49, Ira Weiny wrote:
> > > > > > > > > > > On Wed, Aug 14, 2019 at 12:17:14PM +0200, Jan Kara wrote:
> > > > > ...
> > AFAIA, there is no struct file here - the memory that has been pinned
> > is just something mapped into the application's address space.
> > 
> > It seems to me that the socket here is equivalent of the RDMA handle
> > that that owns the hardware that pins the pages. Again, that RDMA
> > handle is not aware of waht the mapping represents, hence need to
> > hold a layout lease if it's a file mapping.
> > 
> > SO from the filesystem persepctive, there's no difference between
> > XDP or RDMA - if it's a FSDAX mapping then it is DMAing directly
> > into the filesystem's backing store and that will require use of
> > layout leases to perform safely.
> > 
> 
> OK, got it! Makes perfect sense.

Just to chime in here... Yea from the FS perspective it is the same.

But on the driver side it is more complicated because of how the references to
the pins can be shared among other processes.

See the other branch of this thread

https://lkml.org/lkml/2019/8/21/828

Ira

> 
> thanks,
> -- 
> John Hubbard
> NVIDIA