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From: John Hubbard <jhubbard@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "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>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"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>,
	"Jason Gunthorpe" <jgg@ziepe.ca>, "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@vger.kernel.org, dri-devel@lists.freedesktop.org,
	kvm@vger.kernel.org, linux-block@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kselftest@vger.kernel.org, linux-media@vger.kernel.org,
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	netdev@vger.kernel.org, linux-mm@kvack.org,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 19/19] Documentation/vm: add pin_user_pages.rst
Date: Sun, 3 Nov 2019 11:53:37 -0800	[thread overview]
Message-ID: <58d3ef87-85ef-a69d-5cf7-1719ff356048@nvidia.com> (raw)
In-Reply-To: <20191030224930.3990755-20-jhubbard@nvidia.com>

On 10/30/19 3:49 PM, John Hubbard wrote:
...
> +* struct page may not be increased in size for this, and all fields are already
> +  used.
> +
> +* Given the above, we can overload the page->_refcount field by using, sort of,
> +  the upper bits in that field for a dma-pinned count. "Sort of", means that,
> +  rather than dividing page->_refcount into bit fields, we simple add a medium-
> +  large value (GUP_PIN_COUNTING_BIAS, initially chosen to be 1024: 10 bits) to
> +  page->_refcount. This provides fuzzy behavior: if a page has get_page() called
> +  on it 1024 times, then it will appear to have a single dma-pinned count.
> +  And again, that's acceptable.
> +
> +This also leads to limitations: there are only 32-10==22 bits available for a
> +counter that increments 10 bits at a time.
> +

The above claim is just a "bit" too optimistic, by one bit: page->_refcount, being 
an atomic_t which uses a signed int (and we use the sign bit to check for overflow),
only has 31 total bits available for actual counting, not 32.

I'll adjust the documentation in v2, to account for this.

thanks,

John Hubbard
NVIDIA


      parent reply	other threads:[~2019-11-03 19:53 UTC|newest]

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-30 22:49 [PATCH 00/19] mm/gup: track dma-pinned pages: FOLL_PIN, FOLL_LONGTERM John Hubbard
2019-10-30 22:49 ` [PATCH 01/19] mm/gup: pass flags arg to __gup_device_* functions John Hubbard
2019-10-31 18:18   ` Ira Weiny
2019-10-30 22:49 ` [PATCH 02/19] mm/gup: factor out duplicate code from four routines John Hubbard
2019-10-31 18:35   ` Ira Weiny
2019-10-31 18:43     ` John Hubbard
2019-10-31 21:09       ` Ira Weiny
2019-10-31 22:01         ` John Hubbard
2019-10-30 22:49 ` [PATCH 03/19] goldish_pipe: rename local pin_user_pages() routine John Hubbard
2019-10-31 18:36   ` Ira Weiny
2019-10-30 22:49 ` [PATCH 04/19] media/v4l2-core: set pages dirty upon releasing DMA buffers John Hubbard
2019-10-30 22:49 ` [PATCH 05/19] mm/gup: introduce pin_user_pages*() and FOLL_PIN John Hubbard
2019-10-31 23:15   ` Ira Weiny
2019-10-31 23:43     ` John Hubbard
2019-10-30 22:49 ` [PATCH 06/19] goldish_pipe: convert to pin_user_pages() and put_user_page() John Hubbard
2019-10-30 22:49 ` [PATCH 07/19] infiniband: set FOLL_PIN, FOLL_LONGTERM via pin_longterm_pages*() John Hubbard
2019-10-31 23:25   ` Ira Weiny
2019-10-31 23:49     ` John Hubbard
2019-10-30 22:49 ` [PATCH 08/19] mm/process_vm_access: set FOLL_PIN via pin_user_pages_remote() John Hubbard
2019-10-31 23:35   ` Ira Weiny
2019-10-31 23:46     ` John Hubbard
2019-10-30 22:49 ` [PATCH 09/19] drm/via: set FOLL_PIN via pin_user_pages_fast() John Hubbard
2019-10-31 23:36   ` Ira Weiny
2019-11-04 18:10     ` Daniel Vetter
2019-11-04 19:20       ` John Hubbard
2019-11-05  9:49         ` Daniel Vetter
2019-11-05 18:16           ` John Hubbard
2019-10-30 22:49 ` [PATCH 10/19] fs/io_uring: set FOLL_PIN via pin_user_pages() John Hubbard
2019-10-31 23:37   ` Ira Weiny
2019-11-01 14:49   ` Jens Axboe
2019-10-30 22:49 ` [PATCH 11/19] net/xdp: " John Hubbard
2019-10-31 23:37   ` Ira Weiny
2019-11-02 11:01   ` Björn Töpel
2019-10-30 22:49 ` [PATCH 12/19] mm/gup: track FOLL_PIN pages John Hubbard
2019-10-30 22:49 ` [PATCH 13/19] media/v4l2-core: pin_longterm_pages (FOLL_PIN) and put_user_page() conversion John Hubbard
2019-10-31 23:38   ` Ira Weiny
2019-10-30 22:49 ` [PATCH 14/19] vfio, mm: " John Hubbard
2019-10-30 23:05   ` John Hubbard
2019-10-30 22:49 ` [PATCH 15/19] powerpc: book3s64: convert to pin_longterm_pages() and put_user_page() John Hubbard
2019-10-30 22:49 ` [PATCH 16/19] mm/gup_benchmark: support pin_user_pages() and related calls John Hubbard
2019-10-30 22:49 ` [PATCH 17/19] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage John Hubbard
2019-10-30 22:49 ` [PATCH 18/19] mm/gup: remove support for gup(FOLL_LONGTERM) John Hubbard
2019-10-30 22:49 ` [PATCH 19/19] Documentation/vm: add pin_user_pages.rst John Hubbard
2019-10-31 23:49   ` Ira Weiny
2019-10-31 23:53     ` John Hubbard
2019-11-01  0:48     ` John Hubbard
2019-11-03 19:53   ` John Hubbard [this message]

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