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