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

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Date:   Wed, 13 Nov 2019 11:12:10 +0100
From:   Jan Kara <jack@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Daniel Vetter <daniel@...ll.ch>, Jason Gunthorpe <jgg@...pe.ca>,
        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>,
        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>,
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        Shuah Khan <shuah@...nel.org>,
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Subject: Re: [PATCH v3 00/23] mm/gup: track dma-pinned pages: FOLL_PIN,
 FOLL_LONGTERM

On Wed 13-11-19 01:02:02, John Hubbard wrote:
> On 11/13/19 12:22 AM, Daniel Vetter wrote:
> ...
> > > > 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().
> > 
> > Can't we call this unpin_user_page then, for some symmetry? Or is that
> > even more churn?
> > 
> > Looking from afar the naming here seems really confusing.
> 
> 
> That look from afar is valuable, because I'm too close to the problem to see
> how the naming looks. :)
> 
> unpin_user_page() sounds symmetrical. It's true that it would cause more
> churn (which is why I started off with a proposal that avoids changing the
> names of put_user_page*() APIs). But OTOH, the amount of churn is proportional
> to the change in direction here, and it's really only 10 or 20 lines changed,
> in the end.
> 
> So I'm open to changing to that naming. It would be nice to hear what others
> prefer, too...

FWIW I'd find unpin_user_page() also better than put_user_page() as a
counterpart to pin_user_pages().

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR