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 <[email protected]> Cc: Matthew Wilcox <[email protected]> Cc: Vlastimil Babka <[email protected]> Cc: Christoph Lameter <[email protected]> Cc: Dave Hansen <[email protected]> Cc: Jérôme Glisse <[email protected]> Cc: "Kirill A . Shutemov" <[email protected]> Cc: Lai Jiangshan <[email protected]> Cc: Martin Schwidefsky <[email protected]> Cc: Pekka Enberg <[email protected]> Cc: Randy Dunlap <[email protected]> Cc: Andrey Ryabinin <[email protected]> Cc: Christoph Hellwig <[email protected]> Cc: Jason Gunthorpe <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Linus Torvalds <[email protected]> --- 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 */ }; /** @rcu_head: You can use this to free a page by RCU. */ -- 2.20.1
