Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages_remote()

From: Jan Kara <jack@suse.cz>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: "Jan Kara" <jack@suse.cz>, "John Hubbard" <jhubbard@nvidia.com>,
	"Ira Weiny" <ira.weiny@intel.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Christoph Hellwig" <hch@infradead.org>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Dave Chinner" <david@fromorbit.com>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-rdma@vger.kernel.org
Subject: Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages_remote()
Date: Fri, 16 Aug 2019 17:44:04 +0200	[thread overview]
Message-ID: <20190816154404.GF3041@quack2.suse.cz> (raw)
In-Reply-To: <0d6797d8-1e04-1ebe-80a7-3d6895fe71b0@suse.cz>

On Fri 16-08-19 10:47:21, Vlastimil Babka wrote:
> On 8/15/19 3:35 PM, Jan Kara wrote:
> >> 
> >> So when the GUP user uses MMU notifiers to stop writing to pages whenever
> >> they are writeprotected with page_mkclean(), they don't really need page
> >> pin - their access is then fully equivalent to any other mmap userspace
> >> access and filesystem knows how to deal with those. I forgot out this case
> >> when I wrote the above sentence.
> >> 
> >> So to sum up there are three cases:
> >> 1) DIO case - GUP references to pages serving as DIO buffers are needed for
> >>    relatively short time, no special synchronization with page_mkclean() or
> >>    munmap() => needs FOLL_PIN
> >> 2) RDMA case - GUP references to pages serving as DMA buffers needed for a
> >>    long time, no special synchronization with page_mkclean() or munmap()
> >>    => needs FOLL_PIN | FOLL_LONGTERM
> >>    This case has also a special case when the pages are actually DAX. Then
> >>    the caller additionally needs file lease and additional file_pin
> >>    structure is used for tracking this usage.
> >> 3) ODP case - GUP references to pages serving as DMA buffers, MMU notifiers
> >>    used to synchronize with page_mkclean() and munmap() => normal page
> >>    references are fine.
> 
> IMHO the munlock lesson told us about another one, that's in the end equivalent
> to 3)
> 
> 4) pinning for struct page manipulation only => normal page references
> are fine

Right, it's good to have this for clarity.

> > I want to add that I'd like to convert users in cases 1) and 2) from using
> > GUP to using differently named function. Users in case 3) can stay as they
> > are for now although ultimately I'd like to denote such use cases in a
> > special way as well...
> 
> So after 1/2/3 is renamed/specially denoted, only 4) keeps the current
> interface?

Well, munlock() code doesn't even use GUP, just follow_page(). I'd wait to
see what's left after handling cases 1), 2), and 3) to decide about the
interface for the remainder.

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