linux-kernel - [RFC][PATCH] mm: sl[uo]b: fix misleading comments

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Message-Id: <20140128222450.0B32C3FD@viggo.jf.intel.com>
Date:	Tue, 28 Jan 2014 14:24:50 -0800
From:	Dave Hansen <dave@...1.net>
To:	linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org, Dave Hansen <dave@...1.net>,
	cl@...ux-foundation.org, penberg@...nel.org, mpm@...enic.com,
	akpm@...ux-foundation.org
Subject: [RFC][PATCH] mm: sl[uo]b: fix misleading comments


From: Dave Hansen <dave.hansen@...ux.intel.com>

On x86, SLUB creates and handles <=8192-byte allocations internally.
It passes larger ones up to the allocator.  Saying "up to order 2" is,
at best, ambiguous.  Is that order-1?  Or (order-2 bytes)?  Make
it more clear.

SLOB commits a similar sin.  It *handles* page-size requests, but the
comment says that it passes up "all page size and larger requests".

SLOB also swaps around the order of the very-similarly-named
KMALLOC_SHIFT_HIGH and KMALLOC_SHIFT_MAX #defines.  Make it
consistent with the order of the other two allocators.

Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Christoph Lameter <cl@...ux-foundation.org>
Cc: Pekka Enberg <penberg@...nel.org>
Cc: Matt Mackall <mpm@...enic.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>

---

 b/include/linux/slab.h |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff -puN include/linux/slab.h~mm-slub-off-by-one-comment-on-kmalloc-max include/linux/slab.h
--- a/include/linux/slab.h~mm-slub-off-by-one-comment-on-kmalloc-max	2014-01-28 13:27:50.883108273 -0800
+++ b/include/linux/slab.h	2014-01-28 13:27:50.886108408 -0800
@@ -205,8 +205,8 @@ struct kmem_cache {
 
 #ifdef CONFIG_SLUB
 /*
- * SLUB allocates up to order 2 pages directly and otherwise
- * passes the request to the page allocator.
+ * SLUB directly allocates requests fitting in to an order-1 page
+ * (PAGE_SIZE*2).  Larger requests are passed to the page allocator.
  */
 #define KMALLOC_SHIFT_HIGH	(PAGE_SHIFT + 1)
 #define KMALLOC_SHIFT_MAX	(MAX_ORDER + PAGE_SHIFT)
@@ -217,12 +217,12 @@ struct kmem_cache {
 
 #ifdef CONFIG_SLOB
 /*
- * SLOB passes all page size and larger requests to the page allocator.
+ * SLOB passes all requests larger than one page to the page allocator.
  * No kmalloc array is necessary since objects of different sizes can
  * be allocated from the same page.
  */
-#define KMALLOC_SHIFT_MAX	30
 #define KMALLOC_SHIFT_HIGH	PAGE_SHIFT
+#define KMALLOC_SHIFT_MAX	30
 #ifndef KMALLOC_SHIFT_LOW
 #define KMALLOC_SHIFT_LOW	3
 #endif
_
--
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