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. Signed-off-by: Ralph Campbell <[email protected]> Reviewed-by: John Hubbard <[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 | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 3a37a89eb7a7..6a7a1083b6fb 100644 --- a/include/linux/mm_types.h +++ b/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. */ -- 2.20.1
