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From: Christopher Lameter <cl@linux.com>
To: Jan Kara <jack@suse.cz>
Cc: John Hubbard <jhubbard@nvidia.com>,
	john.hubbard@gmail.com, Matthew Wilcox <willy@infradead.org>,
	Michal Hocko <mhocko@kernel.org>, Jason Gunthorpe <jgg@ziepe.ca>,
	Dan Williams <dan.j.williams@intel.com>,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
	linux-rdma <linux-rdma@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 5/6] mm: track gup pages with page->dma_pinned_* fields
Date: Thu, 5 Jul 2018 14:17:19 +0000	[thread overview]
Message-ID: <010001646acdf1d8-0460be04-cc74-4a2d-be89-a337461bd485-000000@email.amazonses.com> (raw)
In-Reply-To: <20180704104318.f5pnqtnn3unkwauw@quack2.suse.cz>

On Wed, 4 Jul 2018, Jan Kara wrote:

> > So this seems unsolvable without having the caller specify that it knows the
> > page type, and that it is therefore safe to decrement page->dma_pinned_count.
> > I was hoping I'd found a way, but clearly I haven't. :)
>
> Well, I think the misconception is that "pinned" is a fundamental property
> of a page. It is not. "pinned" is a property of a page reference (i.e., a
> kind of reference that can be used for DMA access) and page gets into
> "pinned" state if it has any reference of "pinned" type. And when you
> realize this, it is obvious that you just have to have a special api for
> getting and dropping references of this "pinned" type. For getting we
> already have get_user_pages(), for putting we have to create the api...

Maybe we can do something by creating a special "pinned" bit in the pte?
If it is a RDMA reference then set that pinned bit there.

Thus any of the references could cause a pin. Since the page struct does
not contain that information we therefore have to scan through the ptes to
figure out if a page is pinned?

If so then we would not need a special function for dropping the
reference.

References to a page can also be created from devices mmu. Maybe we could
at some point start to manage them in a similar way to the page tables of
the processor? The mmu notifiers are a bit awkward if we need closer mm
integration.

  reply	other threads:[~2018-07-05 14:17 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-02  0:56 [PATCH v2 0/6] mm/fs: gup: don't unmap or drop filesystem buffers john.hubbard
2018-07-02  0:56 ` [PATCH v2 1/6] mm: get_user_pages: consolidate error handling john.hubbard
2018-07-02 10:17   ` Jan Kara
2018-07-02 21:34     ` John Hubbard
2018-07-02 21:34       ` John Hubbard
2018-07-02  0:56 ` [PATCH v2 2/6] mm: introduce page->dma_pinned_flags, _count john.hubbard
2018-07-02  0:56 ` [PATCH v2 3/6] mm: introduce zone_gup_lock, for dma-pinned pages john.hubbard
2018-07-02  0:56 ` [PATCH v2 4/6] mm/fs: add a sync_mode param for clear_page_dirty_for_io() john.hubbard
2018-07-02  2:11   ` kbuild test robot
2018-07-02  4:40     ` John Hubbard
2018-07-02  4:40       ` John Hubbard
2018-07-02  2:47   ` kbuild test robot
2018-07-02  4:40     ` John Hubbard
2018-07-02  4:40       ` John Hubbard
2018-07-02  0:56 ` [PATCH v2 5/6] mm: track gup pages with page->dma_pinned_* fields john.hubbard
2018-07-02  2:11   ` kbuild test robot
2018-07-02  2:58   ` kbuild test robot
2018-07-02  5:05     ` John Hubbard
2018-07-02  5:05       ` John Hubbard
2018-07-02  9:53   ` Jan Kara
2018-07-02 20:43     ` John Hubbard
2018-07-02 20:43       ` John Hubbard
2018-07-03  0:08       ` Christopher Lameter
2018-07-03  4:30         ` John Hubbard
2018-07-03  4:30           ` John Hubbard
2018-07-03 17:08           ` Christopher Lameter
2018-07-03 17:36             ` John Hubbard
2018-07-03 17:36               ` John Hubbard
2018-07-03 17:48               ` Christopher Lameter
2018-07-03 18:48                 ` John Hubbard
2018-07-03 18:48                   ` John Hubbard
2018-07-04 10:43               ` Jan Kara
2018-07-05 14:17                 ` Christopher Lameter [this message]
2018-07-09 13:49                   ` Jan Kara
2018-07-02  0:56 ` [PATCH v2 6/6] mm: page_mkclean, ttu: handle pinned pages john.hubbard
2018-07-02 10:15   ` Jan Kara
2018-07-02 21:07     ` John Hubbard
2018-07-02 21:07       ` John Hubbard
2018-07-02  5:54 ` [PATCH v2 0/6] mm/fs: gup: don't unmap or drop filesystem buffers John Hubbard
2018-07-02  5:54   ` John Hubbard

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