linux-kernel - [PATCH v3 1/3] mm: document zone device struct page field usage

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Date:   Wed, 24 Jul 2019 16:26:58 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     <linux-mm@...ck.org>
CC:     <linux-kernel@...r.kernel.org>,
        Ralph Campbell <rcampbell@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        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 v3 1/3] mm: document zone device struct page field usage

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 <rcampbell@...dia.com>
Reviewed-by: John Hubbard <jhubbard@...dia.com>
Cc: Matthew Wilcox <willy@...radead.org>
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 | 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