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

From: John Hubbard <jhubbard@nvidia.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Michal Hocko <mhocko@suse.com>, Jan Kara <jack@suse.cz>,
	linux-nvdimm@lists.01.org, linux-rdma@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Matthew Wilcox <willy@infradead.org>,
	linux-xfs@vger.kernel.org, Jason Gunthorpe <jgg@ziepe.ca>,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	Theodore Ts'o <tytso@mit.edu>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-ext4@vger.kernel.org
Subject: Re: [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ; -)
Date: Mon, 19 Aug 2019 20:09:33 -0700	[thread overview]
Message-ID: <84318b51-bd07-1d9b-d842-e65cac2ff484@nvidia.com> (raw)
In-Reply-To: <20190820012021.GQ7777@dread.disaster.area>

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:
>> ...
>>
>> Any thoughts about sockets? I'm looking at net/xdp/xdp_umem.c which pins
>> memory with FOLL_LONGTERM, and wondering how to make that work here.
> 
> I'm not sure how this interacts with file mappings? I mean, this
> is just pinning anonymous pages for direct data placement into
> userspace, right?
> 
> Are you asking "what if this pinned memory was a file mapping?",
> or something else?

Yes, mainly that one. Especially since the FOLL_LONGTERM flag is
already there in xdp_umem_pin_pages(), unconditionally. So the
simple rules about struct *vaddr_pin usage (set it to NULL if FOLL_LONGTERM is
not set) are not going to work here.

> 
>> These are close to files, in how they're handled, but just different
>> enough that it's not clear to me how to make work with this system.
> 
> I'm guessing that if they are pinning a file backed mapping, they
> are trying to dma direct to the file (zero copy into page cache?)
> and so they'll need to either play by ODP rules or take layout
> leases, too....
> 

OK. I was just wondering if there was some simple way to dig up a
struct file associated with a socket (I don't think so), but it sounds
like this is an exercise that's potentially different for each subsystem.

thanks,
-- 
John Hubbard
NVIDIA
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