From 76470ccd62f18bfa0954bec10f2329339f793914 Mon Sep 17 00:00:00 2001 From: Ralph Campbell Date: Tue, 13 Aug 2019 15:37:04 -0700 Subject: [PATCH] mm: document zone device struct page field usage MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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.com Signed-off-by: Ralph Campbell Reviewed-by: John Hubbard Cc: Matthew Wilcox Cc: Vlastimil Babka Cc: Christoph Lameter Cc: Dave Hansen Cc: Jérôme Glisse Cc: "Kirill A . Shutemov" Cc: Lai Jiangshan Cc: Martin Schwidefsky Cc: Pekka Enberg Cc: Randy Dunlap Cc: Andrey Ryabinin Cc: Christoph Hellwig Cc: Jason Gunthorpe Cc: Andrew Morton Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- 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. */ -- 2.39.5