Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages_remote()

From: John Hubbard <jhubbard@nvidia.com>
To: Ira Weiny <ira.weiny@intel.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Christoph Hellwig" <hch@infradead.org>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Dave Chinner" <david@fromorbit.com>, "Jan Kara" <jack@suse.cz>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-rdma@vger.kernel.org
Subject: Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages_remote()
Date: Tue, 13 Aug 2019 17:56:31 -0700	[thread overview]
Message-ID: <90e5cd11-fb34-6913-351b-a5cc6e24d85d@nvidia.com> (raw)
In-Reply-To: <a1044a0d-059c-f347-bd68-38be8478bf20@nvidia.com>

On 8/13/19 5:51 PM, John Hubbard wrote:
> On 8/13/19 2:08 PM, Ira Weiny wrote:
>> On Mon, Aug 12, 2019 at 05:07:32PM -0700, John Hubbard wrote:
>>> On 8/12/19 4:49 PM, Ira Weiny wrote:
>>>> On Sun, Aug 11, 2019 at 06:50:44PM -0700, john.hubbard@gmail.com wrote:
>>>>> From: John Hubbard <jhubbard@nvidia.com>
>>> ...
>> Finally, I struggle with converting everyone to a new call.  It is more
>> overhead to use vaddr_pin in the call above because now the GUP code is going
>> to associate a file pin object with that file when in ODP we don't need that
>> because the pages can move around.
> 
> What if the pages in ODP are file-backed? 
> 

oops, strike that, you're right: in that case, even the file system case is covered.
Don't mind me. :)

>>
>> This overhead may be fine, not sure in this case, but I don't see everyone
>> wanting it.

So now I see why you said that, but I will note that ODP hardware is rare,
and will likely remain rare: replayable page faults require really special 
hardware, and after all this time, we still only have CPUs, GPUs, and the
Mellanox cards that do it.

That leaves a lot of other hardware to take care of.

thanks,
-- 
John Hubbard
NVIDIA