linux-kernel - [PATCH 1/3] mm: document zone device struct page reserved fields

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Date:   Fri, 19 Jul 2019 12:06:47 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     <linux-mm@...ck.org>
CC:     <linux-kernel@...r.kernel.org>,
        Ralph Campbell <rcampbell@...dia.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        "Pekka Enberg" <penberg@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Andrey Ryabinin" <aryabinin@...tuozzo.com>,
        Christoph Hellwig <hch@....de>,
        "Jason Gunthorpe" <jgg@...lanox.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 1/3] mm: document zone device struct page reserved fields

Struct page for ZONE_DEVICE private pages uses the reserved fields when
anonymous pages are migrated to device private memory. This is so
the page->mapping and page->index fields are preserved and the page can
be migrated back to system memory.
Document this in comments so it is more clear.

Signed-off-by: Ralph Campbell <rcampbell@...dia.com>
Cc: Matthew Wilcox <mawilcox@...rosoft.com>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Jérôme Glisse <jglisse@...hat.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: Pekka Enberg <penberg@...nel.org>
Cc: Randy Dunlap <rdunlap@...radead.org>
Cc: Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc: Christoph Hellwig <hch@....de>
Cc: Jason Gunthorpe <jgg@...lanox.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
---
 include/linux/mm_types.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3a37a89eb7a7..d6ea74e20306 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -159,7 +159,14 @@ 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 */
+			/*
+			 * The following fields are used to hold the source
+			 * page anonymous mapping information while it is
+			 * migrated to device memory. See migrate_page().
+			 */
+			unsigned long _zd_pad_1;	/* aliases mapping */
+			unsigned long _zd_pad_2;	/* aliases index */
+			unsigned long _zd_pad_3;	/* aliases private */
 		};
 
 		/** @rcu_head: You can use this to free a page by RCU. */
-- 
2.20.1