linux-kernel - Re: [PATCH v4 09/23] mm/gup: introduce pin_user_pages*() and FOLL

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Message-ID: <7c590a1a-25c6-a8e7-d471-8855ceea8606@nvidia.com>
Date:   Wed, 13 Nov 2019 22:08:12 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jan Kara <jack@...e.cz>
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>,
        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>,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH v4 09/23] mm/gup: introduce pin_user_pages*() and FOLL_PIN

On 11/13/19 3:22 PM, John Hubbard wrote:
> On 11/13/19 2:43 AM, Jan Kara wrote:
> ...
>> How does FOLL_PIN result in grabbing (at least normal, for now) page reference?
>> I didn't find that anywhere in this patch but it is a prerequisite to
>> converting any user to pin_user_pages() interface, right?
> 
> 
> ohhh, I messed up on this intermediate patch: it doesn't quite stand alone as
> it should, as you noticed. To correct this, I can do one of the following:
> 
> a) move the new pin*() routines into the later patch 16 ("mm/gup:
> track FOLL_PIN pages"), or
> 
> b) do a temporary thing here, such as setting FOLL_GET and adding a TODO,
> within the pin*() implementations. And this switching it over to FOLL_PIN
> in patch 16.
> 
> I'm thinking (a) is less error-prone, so I'm going with that unless someone
> points out that that is stupid. :)
> 

OK, just to save anyone from wasting time reading the above: (a) is, in fact,
stupid, after all. ha. That is because pin_user_pages() is called in the 
intervening patches.
 
So anyway, I'll work out an ordering to fix it up, it's not complicated.


thanks,
-- 
John Hubbard
NVIDIA