mm: document zone device struct page field usage

Patch series "mm/hmm: fixes for device private page migration", v3.

Testing the latest linux git tree turned up a few bugs with page
migration to and from ZONE_DEVICE private and anonymous pages.
Hopefully it clarifies how ZONE_DEVICE private struct page uses the same
mapping and index fields from the source anonymous page mapping.

This patch (of 3):

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.

Add comments to struct page and remove the unused "_zd_pad_1" field to
make this more clear.

Link: http://lkml.kernel.org/r/20190724232700.23327-2-rcampbell@nvidia.comSigned-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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm_types.h

@@ -159,7 +159,16 @@ 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 */
+			/*
+			 * ZONE_DEVICE private pages are counted as being
+			 * mapped so the next 3 words hold the mapping, index,
+			 * and private fields from the source anonymous or
+			 * page cache page while the page is migrated to device
+			 * private memory.
+			 * ZONE_DEVICE MEMORY_DEVICE_FS_DAX pages also
+			 * use the mapping, index, and private fields when
+			 * pmem backed DAX files are mapped.
+			 */
 		};
 
 		/** @rcu_head: You can use this to free a page by RCU. */