linux-kernel - Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages

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Message-ID: <90e5cd11-fb34-6913-351b-a5cc6e24d85d@nvidia.com>
Date:   Tue, 13 Aug 2019 17:56:31 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Ira Weiny <ira.weiny@...el.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jérôme Glisse <jglisse@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-rdma@...r.kernel.org>
Subject: Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages_remote()

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@...il.com wrote:
>>>>> From: John Hubbard <jhubbard@...dia.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