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From: Vlastimil Babka <vbabka@suse.cz>
To: Ralph Campbell <rcampbell@nvidia.com>,
	Alex Sierra <alex.sierra@amd.com>,
	akpm@linux-foundation.org, Felix.Kuehling@amd.com,
	linux-mm@kvack.org, linux-ext4@vger.kernel.org,
	linux-xfs@vger.kernel.org
Cc: amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
	hch@lst.de, jgg@nvidia.com, jglisse@redhat.com
Subject: Re: [PATCH v1 02/14] mm: remove extra ZONE_DEVICE struct page refcount
Date: Fri, 27 Aug 2021 13:26:48 +0200	[thread overview]
Message-ID: <6a2513ac-1ea1-ba54-9d57-487504c6c86d@suse.cz> (raw)
In-Reply-To: <505e4fe4-241a-8205-d2ef-f603e9cc46c6@nvidia.com>

On 8/25/21 19:49, Ralph Campbell wrote:
> 
> On 8/25/21 4:15 AM, Vlastimil Babka wrote:
>> On 8/25/21 05:48, Alex Sierra wrote:
>>> From: Ralph Campbell <rcampbell@nvidia.com>
>>>
>>> ZONE_DEVICE struct pages have an extra reference count that complicates the
>>> code for put_page() and several places in the kernel that need to check the
>>> reference count to see that a page is not being used (gup, compaction,
>>> migration, etc.). Clean up the code so the reference count doesn't need to
>>> be treated specially for ZONE_DEVICE.
>> That's certainly welcome. I just wonder what was the reason to use 1 in the
>> first place and why it's no longer necessary?
> 
> I'm sure this is a long story that I don't know most of the history.
> I'm guessing that ZONE_DEVICE started out with a reference count of
> one since that is what most "normal" struct page pages start with.
> Then put_page() is used to free newly initialized struct pages to the
> slab/slob/slub page allocator.
> This made it easy to call get_page()/put_page() on ZONE_DEVICE pages
> since get_page() asserts that the caller has a reference.
> However, most drivers that create ZONE_DEVICE struct pages just insert
> a PTE into the user page tables and don't increment/decrement the
> reference count. MEMORY_DEVICE_FS_DAX used the >1 to 1 reference count
> transition to signal that a page was idle so that made put_page() a
> bit more complex. Then MEMORY_DEVICE_PRIVATE pages were added and this
> special casing of what "idle" meant got more complicated and more parts
> of mm had to check for is_device_private_page().
> My goal was to make ZONE_DEVICE struct pages reference counts be zero
> based and allocated/freed similar to the page allocator so that more
> of the mm code could be used, like THP ZONE_DEVICE pages, without special
> casing ZONE_DEVICE.

Thanks for the explanation. I was afraid there was something more fundamental
that required to catch the 2->1 refcount transition, seems like it's not. I
agree with the simplification!



  reply	other threads:[~2021-08-27 11:26 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-25  3:48 [PATCH v1 00/14] Add MEMORY_DEVICE_PUBLIC for CPU-accessible coherent device memory Alex Sierra
2021-08-25  3:48 ` [PATCH v1 01/14] ext4/xfs: add page refcount helper Alex Sierra
2021-08-25  7:35   ` Christoph Hellwig
2021-08-25 15:09   ` Theodore Ts'o
2021-08-25 15:49   ` Darrick J. Wong
2021-08-25  3:48 ` [PATCH v1 02/14] mm: remove extra ZONE_DEVICE struct page refcount Alex Sierra
2021-08-25  7:35   ` Christoph Hellwig
2021-08-25 11:15   ` Vlastimil Babka
2021-08-25 17:49     ` Ralph Campbell
2021-08-27 11:26       ` Vlastimil Babka [this message]
2021-08-25  3:48 ` [PATCH v1 03/14] mm: add iomem vma selection for memory migration Alex Sierra
2021-08-25  7:40   ` Christoph Hellwig
2021-08-25  7:46   ` Christoph Hellwig
2021-08-25 18:24     ` Sierra Guiza, Alejandro (Alex)
2021-08-26 22:27       ` Felix Kuehling
2021-08-30  8:28         ` Christoph Hellwig
2021-08-30 17:04           ` Felix Kuehling
2021-09-01  8:29             ` Christoph Hellwig
2021-09-01 15:40               ` Felix Kuehling
2021-09-01 22:03                 ` Dave Chinner
2021-09-01 23:07                   ` Felix Kuehling
2021-09-02  1:14                     ` Dave Chinner
2021-09-09  4:55                       ` Felix Kuehling
2021-09-02  8:18                 ` Christoph Hellwig
2021-09-02 18:07                   ` Dan Williams
2021-09-09  4:02                   ` Felix Kuehling
2021-08-25 14:24   ` Felix Kuehling
2021-08-25  3:48 ` [PATCH v1 04/14] mm: add zone device public type memory support Alex Sierra
2021-08-25  3:48 ` [PATCH v1 05/14] drm/amdkfd: ref count init for device pages Alex Sierra
2021-08-25 14:34   ` Felix Kuehling
2021-08-25  3:48 ` [PATCH v1 06/14] drm/amdkfd: add SPM support for SVM Alex Sierra
2021-08-25 14:45   ` Felix Kuehling
2021-08-25  3:48 ` [PATCH v1 07/14] drm/amdkfd: public type as sys mem on migration to ram Alex Sierra
2021-08-25  3:48 ` [PATCH v1 08/14] mm: add public type support to migrate_vma helpers Alex Sierra
2021-08-25  7:47   ` Christoph Hellwig
2021-08-25  3:48 ` [PATCH v1 09/14] mm: call pgmap->ops->page_free for DEVICE_PUBLIC pages Alex Sierra
2021-08-25  7:46   ` Christoph Hellwig
2021-08-25  3:48 ` [PATCH v1 10/14] lib: test_hmm add ioctl to get zone device type Alex Sierra
2021-08-25  3:48 ` [PATCH v1 11/14] lib: test_hmm add module param for " Alex Sierra
2021-08-25  3:48 ` [PATCH v1 12/14] lib: add support for device public type in test_hmm Alex Sierra
2021-08-25  3:48 ` [PATCH v1 13/14] tools: update hmm-test to support device public type Alex Sierra
2021-08-25  3:48 ` [PATCH v1 14/14] tools: update test_hmm script to support SP config Alex Sierra

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