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

From: Jan Kara <jack@suse.cz>
To: John Hubbard <jhubbard@nvidia.com>
Cc: "Daniel Vetter" <daniel@ffwll.ch>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	"Christoph Hellwig" <hch@infradead.org>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Dave Chinner" <david@fromorbit.com>,
	"David Airlie" <airlied@linux.ie>,
	"David S . Miller" <davem@davemloft.net>,
	"Ira Weiny" <ira.weiny@intel.com>, "Jan Kara" <jack@suse.cz>,
	"Jens Axboe" <axboe@kernel.dk>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Magnus Karlsson" <magnus.karlsson@intel.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Michal Hocko" <mhocko@suse.com>,
	"Mike Kravetz" <mike.kravetz@oracle.com>,
	"Paul Mackerras" <paulus@samba.org>,
	"Shuah Khan" <shuah@kernel.org>,
	"Vlastimil Babka" <vbabka@suse.cz>, bpf <bpf@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	kvm@vger.kernel.org, linux-block@vger.kernel.org,
	"Linux Doc Mailing List" <linux-doc@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org,
	"open list:KERNEL SELFTEST FRAMEWORK"
	<linux-kselftest@vger.kernel.org>,
	"open list:DMA BUFFER SHARING FRAMEWORK"
	<linux-media@vger.kernel.org>,
	linux-rdma@vger.kernel.org,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	netdev <netdev@vger.kernel.org>, "Linux MM" <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 00/23] mm/gup: track dma-pinned pages: FOLL_PIN, FOLL_LONGTERM
Date: Wed, 13 Nov 2019 11:12:10 +0100	[thread overview]
Message-ID: <20191113101210.GD6367@quack2.suse.cz> (raw)
In-Reply-To: <7b671bf9-4d94-f2cc-8453-863acd5a1115@nvidia.com>

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