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From: Matthew Wilcox <willy@infradead.org>
To: linux-mm@kvack.org
Cc: akpm@linuxfoundation.org, Matthew Wilcox <mawilcox@microsoft.com>
Subject: [PATCH v2 3/8] mm: Remove misleading alignment claims
Date: Wed, 20 Dec 2017 07:55:47 -0800	[thread overview]
Message-ID: <20171220155552.15884-4-willy@infradead.org> (raw)
In-Reply-To: <20171220155552.15884-1-willy@infradead.org>

From: Matthew Wilcox <mawilcox@microsoft.com>

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@microsoft.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Christoph Lameter <cl@linux.com>
---
 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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  parent reply	other threads:[~2017-12-20 15:56 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-12-20 15:55 [PATCH v2 0/8] Restructure struct page Matthew Wilcox
2017-12-20 15:55 ` [PATCH v2 1/8] mm: Align struct page more aesthetically Matthew Wilcox
2017-12-20 15:55 ` [PATCH v2 2/8] mm: De-indent struct page Matthew Wilcox
2017-12-20 15:55 ` Matthew Wilcox [this message]
2017-12-20 15:55 ` [PATCH v2 4/8] mm: Improve comment on page->mapping Matthew Wilcox
2017-12-20 15:55 ` [PATCH v2 5/8] mm: Introduce _slub_counter_t Matthew Wilcox
2017-12-20 15:55 ` [PATCH v2 6/8] mm: Store compound_dtor / compound_order as bytes Matthew Wilcox
2017-12-20 23:39   ` Andrew Morton
2017-12-21  0:01     ` Matthew Wilcox
2017-12-20 15:55 ` [PATCH v2 7/8] mm: Document how to use struct page Matthew Wilcox
2017-12-20 15:55 ` [PATCH v2 8/8] mm: Remove reference to PG_buddy Matthew Wilcox

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