netdev - Re: [PATCH 2/3] mm/gup: Introduce get_user_pages_fast

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Message-ID: <fc9c880b-24f8-7063-6094-00175bc27f7d@nvidia.com>
Date:   Mon, 11 Feb 2019 13:39:12 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Ira Weiny <ira.weiny@...el.com>
CC:     Jason Gunthorpe <jgg@...pe.ca>, <linux-rdma@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Davidlohr Bueso <dave@...olabs.net>, <netdev@...r.kernel.org>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 2/3] mm/gup: Introduce get_user_pages_fast_longterm()

On 2/11/19 1:26 PM, Ira Weiny wrote:
> On Mon, Feb 11, 2019 at 01:13:56PM -0800, John Hubbard wrote:
>> On 2/11/19 12:39 PM, Jason Gunthorpe wrote:
>>> On Mon, Feb 11, 2019 at 12:16:42PM -0800, ira.weiny@...el.com wrote:
>>>> From: Ira Weiny <ira.weiny@...el.com>
>> [...]
>> It seems to me that the longterm vs. short-term is of questionable value.
> 
> This is exactly why I did not post this before.  I've been waiting our other
> discussions on how GUP pins are going to be handled to play out.  But with the
> netdev thread today[1] it seems like we need to make sure we have a "safe" fast
> variant for a while.  Introducing FOLL_LONGTERM seemed like the cleanest way to
> do that even if we will not need the distinction in the future...  :-(

Yes, I agree. Below...

> [...]
> This is also why I did not change the get_user_pages_longterm because we could
> be ripping this all out by the end of the year...  (I hope. :-)
> 
> So while this does "pollute" the GUP family of calls I'm hoping it is not
> forever.
> 
> Ira
> 
> [1] https://lkml.org/lkml/2019/2/11/1789
> 

Yes, and to be clear, I think your patchset here is fine. It is easy to find
the FOLL_LONGTERM callers if and when we want to change anything. I just think
also it's appopriate to go a bit further, and use FOLL_LONGTERM all by itself.

That's because in either design outcome, it's better that way:

-- If we keep the concept of "I'm a long-term gup call site", then FOLL_LONGTERM
is just right. The gup API already has _fast and non-fast variants, and once
you get past a couple, you end up with a multiplication of names that really
work better as flags. We're there.

-- If we drop the concept, then you've already done part of the work, by removing
the _longterm API variants.



thanks,
-- 
John Hubbard
NVIDIA