[PATCH v2 1/3] mm: document zone device struct page field usage

[Ralph Campbell <rcampbell@nvidia.com>]

Struct page for ZONE_DEVICE private pages uses the page->mapping and
and page->index fields while the source anonymous pages are migrated to
device private memory. This is so rmap_walk() can find the page when
migrating the ZONE_DEVICE private page back to system memory.
ZONE_DEVICE pmem backed fsdax pages also use the page->mapping and
page->index fields when files are mapped into a process address space.

Restructure struct page and add comments to make this more clear.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
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 | 42 +++++++++++++++++++++++++++-------------
 1 file changed, 29 insertions(+), 13 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3a37a89eb7a7..f6c52e44d40c 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -76,13 +76,35 @@ struct page {
 	 * avoid collision and false-positive PageTail().
 	 */
 	union {
-		struct {	/* Page cache and anonymous pages */
-			/**
-			 * @lru: Pageout list, eg. active_list protected by
-			 * pgdat->lru_lock.  Sometimes used as a generic list
-			 * by the page owner.
-			 */
-			struct list_head lru;
+		struct {	/* Page cache, anonymous, ZONE_DEVICE pages */
+			union {
+				/**
+				 * @lru: Pageout list, e.g., active_list
+				 * protected by pgdat->lru_lock. Sometimes
+				 * used as a generic list by the page owner.
+				 */
+				struct list_head lru;
+				/**
+				 * ZONE_DEVICE pages are never on the lru
+				 * list so they reuse the list space.
+				 * ZONE_DEVICE private pages are counted as
+				 * being mapped so the @mapping and @index
+				 * fields are used while the page is migrated
+				 * to device private memory.
+				 * ZONE_DEVICE MEMORY_DEVICE_FS_DAX pages also
+				 * use the @mapping and @index fields when pmem
+				 * backed DAX files are mapped.
+				 */
+				struct {
+					/**
+					 * @pgmap: Points to the hosting
+					 * device page map.
+					 */
+					struct dev_pagemap *pgmap;
+					/** @zone_device_data: opaque data. */
+					void *zone_device_data;
+				};
+			};
 			/* See page-flags.h for PAGE_MAPPING_FLAGS */
 			struct address_space *mapping;
 			pgoff_t index;		/* Our offset within mapping. */
@@ -155,12 +177,6 @@ struct page {
 			spinlock_t ptl;
 #endif
 		};
-		struct {	/* ZONE_DEVICE pages */
-			/** @pgmap: Points to the hosting device page map. */
-			struct dev_pagemap *pgmap;
-			void *zone_device_data;
-			unsigned long _zd_pad_1;	/* uses mapping */
-		};
 
 		/** @rcu_head: You can use this to free a page by RCU. */
 		struct rcu_head rcu_head;
-- 
2.20.1