linux-kernel - Re: [PATCH next/mmotm] slub: partly fix freeze in __slab

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Date:	Tue, 12 Jul 2011 10:55:22 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Pekka Enberg <penberg@...nel.org>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH next/mmotm] slub: partly fix freeze in __slab_free

I wish there would be some easier way to overlay fields. The counters
field exists to allow an easier reference to the bitfield structure in
order to pass it to cmpxchg_double_slab.

In 64 bit the counters field needs to overlay the bitfields and _count
since we can only pass 64 bits integers to cmpxchg_double_slab.

On 32 bit counters field only covers the bitfields.

The "new" page struct in __slab_free is only partially populated and the
overlays are used for conversion between structs and words in order to be
able to pass the struct contents by value.

Signed-off-by: Christoph Lameter <cl@...ux.com>


---
 include/linux/mm_types.h |   22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

Index: linux-2.6/include/linux/mm_types.h
===================================================================
--- linux-2.6.orig/include/linux/mm_types.h	2011-07-12 10:14:08.057706223 -0500
+++ linux-2.6/include/linux/mm_types.h	2011-07-12 10:14:14.117706184 -0500
@@ -56,20 +56,26 @@ struct page {
 		};

 		union {
-			atomic_t _mapcount;	/* Count of ptes mapped in mms,
+			/* Used for cmpxchg_double in slub */
+			unsigned long counters;
+
+			struct {
+
+				union {
+					atomic_t _mapcount;	/* Count of ptes mapped in mms,
 							 * to show when page is mapped
 							 * & limit reverse map searches.
 							 */

-			/* Used for cmpxchg_double in slub */
-			unsigned long counters;
-			struct {
-				unsigned inuse:16;
-				unsigned objects:15;
-				unsigned frozen:1;
+					struct {
+						unsigned inuse:16;
+						unsigned objects:15;
+						unsigned frozen:1;
+					};
+				};
+				atomic_t _count;		/* Usage count, see below. */
 			};
 		};
-		atomic_t _count;		/* Usage count, see below. */
 	};

 	/* Third double word block */
--
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