[PATCH 1/3] mm: document zone device struct page reserved fields

[Ralph Campbell]

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@xxxxxxxxxx>
Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
 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