linux-kernel - Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL

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Message-ID: <9ac948a4-59bf-2427-2007-e460aad2848a@nvidia.com>
Date:   Tue, 5 Nov 2019 11:00:06 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Mike Rapoport <rppt@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <bpf@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <kvm@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <linux-media@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <netdev@...r.kernel.org>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL_PIN

On 11/5/19 5:10 AM, Mike Rapoport wrote:
...
>> ---
>>  Documentation/vm/index.rst          |   1 +
>>  Documentation/vm/pin_user_pages.rst | 212 ++++++++++++++++++++++
> 
> I think it belongs to Documentation/core-api.

Done:

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index ab0eae1c153a..413f7d7c8642 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -31,6 +31,7 @@ Core utilities
    generic-radix-tree
    memory-allocation
    mm-api
+   pin_user_pages
    gfp_mask-from-fs-io
    timekeeping
    boot-time-mm


...
>> diff --git a/Documentation/vm/pin_user_pages.rst b/Documentation/vm/pin_user_pages.rst
>> new file mode 100644
>> index 000000000000..3910f49ca98c
>> --- /dev/null
>> +++ b/Documentation/vm/pin_user_pages.rst
>> @@ -0,0 +1,212 @@
>> +.. SPDX-License-Identifier: GPL-2.0
>> +
>> +====================================================
>> +pin_user_pages() and related calls
>> +====================================================
> 
> I know this is too much to ask, but having pin_user_pages() a part of more
> general GUP description would be really great :)
> 

Yes, definitely. But until I saw the reaction to the pin_user_pages() API
family, I didn't want to write too much--it could have all been tossed out
in favor of a whole different API. But now that we've had some initial
reviews, I'm much more confident in being able to write about the larger 
API set.

So yes, I'll put that on my pending list.


...
>> +This document describes the following functions: ::
>> +
>> + pin_user_pages
>> + pin_user_pages_fast
>> + pin_user_pages_remote
>> +
>> + pin_longterm_pages
>> + pin_longterm_pages_fast
>> + pin_longterm_pages_remote
>> +
>> +Basic description of FOLL_PIN
>> +=============================
>> +
>> +A new flag for get_user_pages ("gup") has been added: FOLL_PIN. FOLL_PIN has
> 
> Consider reading this after, say, half a year ;-)
> 

OK, OK. I knew when I wrote that that it was not going to stay new forever, but
somehow failed to write the right thing anyway. :) 

Here's a revised set of paragraphs:

Basic description of FOLL_PIN
=============================

FOLL_PIN and FOLL_LONGTERM are flags that can be passed to the get_user_pages*()
("gup") family of functions. FOLL_PIN has significant interactions and
interdependencies with FOLL_LONGTERM, so both are covered here.

Both FOLL_PIN and FOLL_LONGTERM are internal to gup, meaning that neither
FOLL_PIN nor FOLL_LONGTERM should not appear at the gup call sites. This allows
the associated wrapper functions  (pin_user_pages() and others) to set the
correct combination of these flags, and to check for problems as well.


thanks,

John Hubbard
NVIDIA