netdev - Re: [PATCH v3 00/23] mm/gup: track dma-pinned pages: FOLL_PIN, FOLL

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Date:   Tue, 12 Nov 2019 13:10:47 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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>,
        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 v3 00/23] mm/gup: track dma-pinned pages: FOLL_PIN,
 FOLL_LONGTERM

On 11/12/19 12:38 PM, Jason Gunthorpe wrote:
> On Mon, Nov 11, 2019 at 04:06:37PM -0800, John Hubbard wrote:
>> Hi,
>>
>> The cover letter is long, so the more important stuff is first:
>>
>> * Jason, if you or someone could look at the the VFIO cleanup (patch 8)
>>   and conversion to FOLL_PIN (patch 18), to make sure it's use of
>>   remote and longterm gup matches what we discussed during the review
>>   of v2, I'd appreciate it.
>>
>> * Also for Jason and IB: as noted below, in patch 11, I am (too?) boldly
>>   converting from put_user_pages() to release_pages().
> 
> Why are we doing this? I think things got confused here someplace, as


Because:

a) These need put_page() calls,  and

b) there is no put_pages() call, but there is a release_pages() call that
is, arguably, what put_pages() would be.


> the comment still says:
> 
> /**
>  * put_user_page() - release a gup-pinned page
>  * @page:            pointer to page to be released
>  *
>  * Pages that were pinned via get_user_pages*() must be released via
>  * either put_user_page(), or one of the put_user_pages*() routines
>  * below.


Ohhh, I missed those comments. They need to all be changed over to
say "pages that were pinned via pin_user_pages*() or 
pin_longterm_pages*() must be released via put_user_page*()."

The get_user_pages*() pages must still be released via put_page.

The churn is due to a fairly significant change in strategy, whis
is: instead of changing all get_user_pages*() sites to call 
put_user_page(), change selected sites to call pin_user_pages*() or 
pin_longterm_pages*(), plus put_user_page().

That allows incrementally converting the kernel over to using the
new pin APIs, without taking on the huge risk of a big one-shot
conversion. 

So, I've ended up with one place that actually needs to get reverted
back to get_user_pages(), and that's the IB ODP code.

> 
> I feel like if put_user_pages() is not the correct way to undo
> get_user_pages() then it needs to be deleted.
> 

Yes, you're right. I'll fix the put_user_page comments() as described.


thanks,

John Hubbard
NVIDIA