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

Jan Kara
2019-11-13T10:12:28Z

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@suse.com>
SUSE Labs, CR