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Subject: Re: [PATCH v2 1/3] mm: document zone device struct page field usage
To:     Ralph Campbell <rcampbell@nvidia.com>, <linux-mm@kvack.org>
CC:     <linux-kernel@vger.kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Christoph Lameter <cl@linux.com>,
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        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
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        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Christoph Hellwig <hch@lst.de>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
References: <20190719192955.30462-1-rcampbell@nvidia.com>
 <20190719192955.30462-2-rcampbell@nvidia.com>
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From:   John Hubbard <jhubbard@nvidia.com>
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On 7/19/19 12:29 PM, Ralph Campbell wrote:
> 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.
>=20
> Restructure struct page and add comments to make this more clear.
>=20
> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
> Reviewed-by: John Hubbard <jhubbard@nvidia.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: J=C3=A9r=C3=B4me Glisse <jglisse@redhat.com>
> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Jason Gunthorpe <jgg@mellanox.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> ---
>  include/linux/mm_types.h | 42 +++++++++++++++++++++++++++-------------
>  1 file changed, 29 insertions(+), 13 deletions(-)
>=20
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 3a37a89eb7a7..f6c52e44d40c 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -76,13 +76,35 @@ struct page {
>  	 * avoid collision and false-positive PageTail().
>  	 */
>  	union {
> -		struct {	/* Page cache and anonymous pages */
> -			/**
> -			 * @lru: Pageout list, eg. active_list protected by
> -			 * pgdat->lru_lock.  Sometimes used as a generic list
> -			 * by the page owner.
> -			 */
> -			struct list_head lru;
> +		struct {	/* Page cache, anonymous, ZONE_DEVICE pages */
> +			union {
> +				/**
> +				 * @lru: Pageout list, e.g., active_list
> +				 * protected by pgdat->lru_lock. Sometimes
> +				 * used as a generic list by the page owner.
> +				 */
> +				struct list_head lru;
> +				/**
> +				 * ZONE_DEVICE pages are never on the lru
> +				 * list so they reuse the list space.
> +				 * ZONE_DEVICE private pages are counted as
> +				 * being mapped so the @mapping and @index
> +				 * fields are used while the page is migrated
> +				 * to device private memory.
> +				 * ZONE_DEVICE MEMORY_DEVICE_FS_DAX pages also
> +				 * use the @mapping and @index fields when pmem
> +				 * backed DAX files are mapped.
> +				 */
> +				struct {
> +					/**
> +					 * @pgmap: Points to the hosting
> +					 * device page map.
> +					 */
> +					struct dev_pagemap *pgmap;
> +					/** @zone_device_data: opaque data. */

This is nice, and I think it's a solid step forward in documentation via
clearer code. I recall a number of email, and even face to face discussions=
,
in which the statement kept coming up: "remember, the ZONE_DEVICE pages use
mapping and index, too. Actually, that reminds me: page->private is often
in use in that case, too, so ZONE_DEVICE couldn't use that, either. I don't
think we need to explicitly say that, though, with this new layout.

nit: the above comment can be deleted, because it merely echoes the actual
code that directly follows it.

Either way, you can add:

	Reviewed-by: John Hubbard <jhubbard@nvidia.com>


thanks,
--=20
John Hubbard
NVIDIA

> +					void *zone_device_data;
> +				};
> +			};
>  			/* See page-flags.h for PAGE_MAPPING_FLAGS */
>  			struct address_space *mapping;
>  			pgoff_t index;		/* Our offset within mapping. */
> @@ -155,12 +177,6 @@ struct page {
>  			spinlock_t ptl;
>  #endif
>  		};
> -		struct {	/* ZONE_DEVICE pages */
> -			/** @pgmap: Points to the hosting device page map. */
> -			struct dev_pagemap *pgmap;
> -			void *zone_device_data;
> -			unsigned long _zd_pad_1;	/* uses mapping */
> -		};
> =20
>  		/** @rcu_head: You can use this to free a page by RCU. */
>  		struct rcu_head rcu_head;
>=20