From: Jan Kara <jack@suse.cz>
To: Jerome Glisse <jglisse@redhat.com>
Cc: Jan Kara <jack@suse.cz>, Vlastimil Babka <vbabka@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>,
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 18:13:55 +0200 [thread overview]
Message-ID: <20190816161355.GL3041@quack2.suse.cz> (raw)
In-Reply-To: <20190816155220.GC3149@redhat.com>
On Fri 16-08-19 11:52:20, Jerome Glisse wrote:
> On Fri, Aug 16, 2019 at 05:44:04PM +0200, Jan Kara wrote:
> > 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.
> >
>
> For 3 we do not need to take a reference at all :) So just forget about 3
> it does not exist. For 3 the reference is the reference the CPU page table
> has on the page and that's it. GUP is no longer involve in ODP or anything
> like that.
Yes, I understand. But the fact is that GUP calls are currently still there
e.g. in ODP code. If you can make the code work without taking a page
reference at all, I'm only happy :)
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2019-08-16 16:14 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-12 1:50 [RFC PATCH 0/2] mm/gup: introduce vaddr_pin_pages_remote(), FOLL_PIN john.hubbard
2019-08-12 1:50 ` [RFC PATCH 1/2] mm/gup: introduce FOLL_PIN flag for get_user_pages() john.hubbard
2019-08-12 1:50 ` [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages_remote() john.hubbard
2019-08-12 22:03 ` Ira Weiny
2019-08-12 22:21 ` John Hubbard
2019-08-12 23:49 ` Ira Weiny
2019-08-13 0:07 ` John Hubbard
2019-08-13 21:08 ` Ira Weiny
2019-08-14 0:51 ` John Hubbard
2019-08-14 0:56 ` John Hubbard
2019-08-14 23:50 ` Ira Weiny
2019-08-15 0:02 ` John Hubbard
2019-08-15 3:01 ` John Hubbard
2019-08-15 13:26 ` Jan Kara
2019-08-15 13:35 ` Jan Kara
2019-08-15 14:51 ` Jason Gunthorpe
2019-08-15 17:32 ` Ira Weiny
2019-08-15 17:41 ` John Hubbard
2019-08-16 2:14 ` John Hubbard
2019-08-16 15:41 ` Jan Kara
2019-08-16 18:33 ` Ira Weiny
2019-08-16 18:50 ` John Hubbard
2019-08-16 21:59 ` Ira Weiny
2019-08-16 22:36 ` John Hubbard
2019-08-16 8:47 ` Vlastimil Babka
2019-08-16 15:44 ` Jan Kara
2019-08-16 15:52 ` Jerome Glisse
2019-08-16 16:13 ` Jan Kara [this message]
2019-08-16 16:31 ` Jason Gunthorpe
2019-08-16 16:54 ` Jerome Glisse
2019-08-16 17:04 ` Jason Gunthorpe
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