Re: [PATCH 1/3] mm: document zone device struct page reserved fields

[John Hubbard]

On 7/16/19 5:14 PM, Ralph Campbell wrote:
> 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@xxxxxxxxxx>
> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> ---
>  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 */

Actually, I do think this helps. It's hard to document these fields, and
the ZONE_DEVICE pages have a really complicated situation during migration
to a device. 

Additionally, I'm not sure, but should we go even further, and do this on the 
other side of the alias:

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index d6ea74e20306..c5ce5989d8a8 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -83,7 +83,12 @@ struct page {
                         * by the page owner.
                         */
                        struct list_head lru;
-                       /* See page-flags.h for PAGE_MAPPING_FLAGS */
+                       /*
+                        * See page-flags.h for PAGE_MAPPING_FLAGS.
+                        *
+                        * Also: the next three fields (mapping, index and
+                        * private) are all used by ZONE_DEVICE pages.
+                        */
                        struct address_space *mapping;
                        pgoff_t index;          /* Our offset within mapping. */
                        /**

?

Either way, you can add:

    Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>


thanks,
-- 
John Hubbard
NVIDIA