From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
The "third double word block" isn't on 32-bit systems. The layout
looks like this:
unsigned long flags;
struct address_space *mapping
pgoff_t index;
atomic_t _mapcount;
atomic_t _refcount;
which is 32 bytes on 64-bit, but 20 bytes on 32-bit. Nobody is trying
to use the fact that it's double-word aligned today, so just remove the
misleading claims.
Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
---
include/linux/mm_types.h | 13 +++++--------
1 file changed, 5 insertions(+), 8 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 27973166af28..c2294e6204e8 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -33,11 +33,11 @@ struct hmm;
* a page, though if it is a pagecache page, rmap structures can tell us
* who is mapping it.
*
- * The objects in struct page are organized in double word blocks in
- * order to allows us to use atomic double word operations on portions
- * of struct page. That is currently only used by slub but the arrangement
- * allows the use of atomic double word operations on the flags/mapping
- * and lru list pointers also.
+ * SLUB uses cmpxchg_double() to atomically update its freelist and
+ * counters. That requires that freelist & counters be adjacent and
+ * double-word aligned. We align all struct pages to double-word
+ * boundaries, and ensure that 'freelist' is aligned within the
+ * struct.
*/
#ifdef CONFIG_HAVE_ALIGNED_STRUCT_PAGE
#define _struct_page_alignment __aligned(2 * sizeof(unsigned long))
@@ -113,8 +113,6 @@ struct page {
};
/*
- * Third double word block
- *
* WARNING: bit 0 of the first word encode PageTail(). That means
* the rest users of the storage space MUST NOT use the bit to
* avoid collision and false-positive PageTail().
@@ -175,7 +173,6 @@ struct page {
#endif
};
- /* Remainder is not double word aligned */
union {
unsigned long private; /* Mapping-private opaque data:
* usually used for buffer_heads
--
2.15.1
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