linux-kernel - Re: [PATCH v2 13/15] slab: use struct page for slab management

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Message-ID: <00000141c7d66282-aa92b1f2-2a69-424b-9498-8e5367304d32-000000@email.amazonses.com>
Date:	Thu, 17 Oct 2013 19:13:38 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
cc:	Pekka Enberg <penberg@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <js1304@...il.com>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 13/15] slab: use struct page for slab management

On Wed, 16 Oct 2013, Joonsoo Kim wrote:

> -					 * see PAGE_MAPPING_ANON below.
> -					 */
> +	union {
> +		struct address_space *mapping;	/* If low bit clear, points to
> +						 * inode address_space, or NULL.
> +						 * If page mapped as anonymous
> +						 * memory, low bit is set, and
> +						 * it points to anon_vma object:
> +						 * see PAGE_MAPPING_ANON below.
> +						 */
> +		void *s_mem;			/* slab first object */
> +	};

The overloading of mapping has caused problems in the past since slab
pages are (or are they no longer?) used for DMA to disk. At that point the
I/O subsystem may be expecting a mapping in the page struct if this field
is not NULL.

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