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

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Date:   Wed, 14 Aug 2019 20:01:07 -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 8/14/19 5:02 PM, John Hubbard wrote:
> On 8/14/19 4:50 PM, Ira Weiny wrote:
>> On Tue, Aug 13, 2019 at 05:56:31PM -0700, John Hubbard wrote:
>>> 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. :)
>>
>> Ok so are we agreed we will drop the patch to the ODP code?  I'm going to keep
>> the FOLL_PIN flag and addition in the vaddr_pin_pages.
>>
> 
> Yes. I hope I'm not overlooking anything, but it all seems to make sense to
> let ODP just rely on the MMU notifiers.
> 

Hold on, I *was* forgetting something: this was a two part thing, and you're
conflating the two points, but they need to remain separate and distinct. There
were:

1. FOLL_PIN is necessary because the caller is clearly in the use case that
requires it--however briefly they might be there. As Jan described it,

"Anything that gets page reference and then touches page data (e.g. direct IO)
needs the new kind of tracking so that filesystem knows someone is messing with
the page data." [1]

2. Releasing the pin: for ODP, we can use MMU notifiers instead of requiring a
lease.

This second point does not invalidate the first point. Therefore, I still see the
need for the call within ODP, to something that sets FOLL_PIN. And that means
either vaddr_pin_[user?]_pages_remote, or some other wrapper of your choice. :)

I guess shows that the API might need to be refined. We're trying to solve
two closely related issues, but they're not identical.

thanks,
-- 
John Hubbard
NVIDIA