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SubjectRe: [PATCH 1/3] mm: document zone device struct page reserved fields
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On 7/16/19 5:14 PM, Ralph Campbell wrote:
> Struct page for ZONE_DEVICE private pages uses the reserved fields when
> anonymous pages are migrated to device private memory. This is so
> the page->mapping and page->index fields are preserved and the page can
> be migrated back to system memory.
> Document this in comments so it is more clear.
>
> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
> Cc: Matthew Wilcox <mawilcox@microsoft.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Jérôme Glisse <jglisse@redhat.com>
> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Jason Gunthorpe <jgg@mellanox.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> ---
> include/linux/mm_types.h | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 3a37a89eb7a7..d6ea74e20306 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -159,7 +159,14 @@ struct page {
> /** @pgmap: Points to the hosting device page map. */
> struct dev_pagemap *pgmap;
> void *zone_device_data;
> - unsigned long _zd_pad_1; /* uses mapping */
> + /*
> + * The following fields are used to hold the source
> + * page anonymous mapping information while it is
> + * migrated to device memory. See migrate_page().
> + */
> + unsigned long _zd_pad_1; /* aliases mapping */
> + unsigned long _zd_pad_2; /* aliases index */
> + unsigned long _zd_pad_3; /* aliases private */

Actually, I do think this helps. It's hard to document these fields, and
the ZONE_DEVICE pages have a really complicated situation during migration
to a device.

Additionally, I'm not sure, but should we go even further, and do this on the
other side of the alias:

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index d6ea74e20306..c5ce5989d8a8 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -83,7 +83,12 @@ struct page {
* by the page owner.
*/
struct list_head lru;
- /* See page-flags.h for PAGE_MAPPING_FLAGS */
+ /*
+ * See page-flags.h for PAGE_MAPPING_FLAGS.
+ *
+ * Also: the next three fields (mapping, index and
+ * private) are all used by ZONE_DEVICE pages.
+ */
struct address_space *mapping;
pgoff_t index; /* Our offset within mapping. */
/**
?

Either way, you can add:

Reviewed-by: John Hubbard <jhubbard@nvidia.com>


thanks,
--
John Hubbard
NVIDIA

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