Re: [PATCH] ARCH_SLAB_MINALIGN for 2.6.10-rc3

[Paul Mundt <lethal@linux-sh.org>]

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Hi Manfred,

On Sun, Dec 12, 2004 at 11:48:02AM +0100, Manfred Spraul wrote:
> Sorry for the late reply, attached is my proposal:
> I've added the ARCH_SLAB_MINALIGN flag, together with some documentation 
> and a small restructuring.
> What do you think?
> 
Looks fine to me, just tested on sh64 and it works ok.

> #ifndef CONFIG_DEBUG_SLAB
> #define ARCH_SLAB_MINALIGN   8
> #endif
> 
Right now ARCH_KMALLOC_MINALIGN is set unconditionally on sh64, so it
seems that we lose some debug features there. However, even with
ARCH_SLAB_MINALIGN being set to non-zero, redzoning and slab poisoning
still seem to be functional to some extent.

For instance, I've been using the following patch and this did help pin
down a rather irritating bug in the sh64 switch_to().

Is there any reason not to wrap redzoning to ARCH_SLAB_MINALIGN (and set
it to BYTES_PER_WORD by default). It seems at least that the only thing
BYTES_PER_WORD is needed for in the redzoning case is to determine where
to place the second marker. I can see why this would be problematic with
dynamic slab alignment, but when it is fixed at compile time there
shouldn't be anything prohibiting the use of a non-BYTES_PER_WORD value.

We can live with the unaligned accesses in the CONFIG_DEBUG_SLAB case,
but it would still be nice to at least have some partial redzoning and
poisoning with a forced alignment.

--- linux-2.6.10-rc3/mm/slab.c	2004-12-05 19:05:39.000000000 +0200
+++ linux-sh64-2.6.10-rc3/mm/slab.c	2004-12-08 16:56:25.000000000 +0200
@@ -135,6 +135,10 @@
 #define ARCH_KMALLOC_FLAGS SLAB_HWCACHE_ALIGN
 #endif
 
+#ifndef ARCH_SLAB_MINALIGN
+#define ARCH_SLAB_MINALIGN	BYTES_PER_WORD
+#endif
+
 /* Legal flag mask for kmem_cache_create(). */
 #if DEBUG
 # define CREATE_MASK	(SLAB_DEBUG_INITIAL | SLAB_RED_ZONE | \
@@ -404,14 +408,16 @@
 
 /* memory layout of objects:
  * 0		: objp
- * 0 .. cachep->dbghead - BYTES_PER_WORD - 1: padding. This ensures that
- * 		the end of an object is aligned with the end of the real
- * 		allocation. Catches writes behind the end of the allocation.
- * cachep->dbghead - BYTES_PER_WORD .. cachep->dbghead - 1:
- * 		redzone word.
+ * 0 .. cachep->dbghead - ARCH_SLAB_MINALIGN - 1: padding. This ensures that
+ *		the end of an object is aligned with the end of the real
+ *		allocation. Catches writes behind the end of the allocation.
+ * cachep->dbghead - ARCH_SLAB_MINALIGN .. cachep->dbghead - 1:
+ *		redzone word.
  * cachep->dbghead: The real object.
- * cachep->objsize - 2* BYTES_PER_WORD: redzone word [BYTES_PER_WORD long]
- * cachep->objsize - 1* BYTES_PER_WORD: last caller address [BYTES_PER_WORD long]
+ * cachep->objsize - 2* ARCH_SLAB_MINALIGN:
+ *		redzone word [ARCH_SLAB_MINALIGN long]
+ * cachep->objsize - 1* ARCH_SLAB_MINALIGN:
+ *		last caller address [ARCH_SLAB_MINALIGN long]
  */
 static int obj_dbghead(kmem_cache_t *cachep)
 {
@@ -426,21 +432,21 @@
 static unsigned long *dbg_redzone1(kmem_cache_t *cachep, void *objp)
 {
 	BUG_ON(!(cachep->flags & SLAB_RED_ZONE));
-	return (unsigned long*) (objp+obj_dbghead(cachep)-BYTES_PER_WORD);
+	return (unsigned long*) (objp+obj_dbghead(cachep)-ARCH_SLAB_MINALIGN);
 }
 
 static unsigned long *dbg_redzone2(kmem_cache_t *cachep, void *objp)
 {
 	BUG_ON(!(cachep->flags & SLAB_RED_ZONE));
 	if (cachep->flags & SLAB_STORE_USER)
-		return (unsigned long*) (objp+cachep->objsize-2*BYTES_PER_WORD);
-	return (unsigned long*) (objp+cachep->objsize-BYTES_PER_WORD);
+		return (unsigned long*) (objp+cachep->objsize-2*ARCH_SLAB_MINALIGN);
+	return (unsigned long*) (objp+cachep->objsize-ARCH_SLAB_MINALIGN);
 }
 
 static void **dbg_userword(kmem_cache_t *cachep, void *objp)
 {
 	BUG_ON(!(cachep->flags & SLAB_STORE_USER));
-	return (void**)(objp+cachep->objsize-BYTES_PER_WORD);
+	return (void**)(objp+cachep->objsize-ARCH_SLAB_MINALIGN);
 }
 
 #else
@@ -1204,7 +1210,7 @@
 	 * above the next power of two: caches with object sizes just above a
 	 * power of two have a significant amount of internal fragmentation.
 	 */
-	if ((size < 4096 || fls(size-1) == fls(size-1+3*BYTES_PER_WORD)))
+	if ((size < 4096 || fls(size-1) == fls(size-1+3*ARCH_SLAB_MINALIGN)))
 		flags |= SLAB_RED_ZONE|SLAB_STORE_USER;
 	if (!(flags & SLAB_DESTROY_BY_RCU))
 		flags |= SLAB_POISON;
@@ -1237,7 +1243,7 @@
 			while (size <= align/2)
 				align /= 2;
 		} else {
-			align = BYTES_PER_WORD;
+			align = ARCH_SLAB_MINALIGN;
 		}
 	}
 
@@ -1255,25 +1261,25 @@
 		size += (BYTES_PER_WORD-1);
 		size &= ~(BYTES_PER_WORD-1);
 	}
-	
+
 #if DEBUG
 	cachep->reallen = size;
 
 	if (flags & SLAB_RED_ZONE) {
 		/* redzoning only works with word aligned caches */
-		align = BYTES_PER_WORD;
+		align = ARCH_SLAB_MINALIGN;
 
 		/* add space for red zone words */
-		cachep->dbghead += BYTES_PER_WORD;
-		size += 2*BYTES_PER_WORD;
+		cachep->dbghead += ARCH_SLAB_MINALIGN;
+		size += 2*ARCH_SLAB_MINALIGN;
 	}
 	if (flags & SLAB_STORE_USER) {
 		/* user store requires word alignment and
 		 * one word storage behind the end of the real
 		 * object.
 		 */
-		align = BYTES_PER_WORD;
-		size += BYTES_PER_WORD;
+		align = ARCH_SLAB_MINALIGN;
+		size += ARCH_SLAB_MINALIGN;
 	}
 #if FORCED_DEBUG && defined(CONFIG_DEBUG_PAGEALLOC)
 	if (size > 128 && cachep->reallen > cache_line_size() && size < PAGE_SIZE) {
@@ -2292,7 +2298,7 @@
 {
 	unsigned long addr = (unsigned long) ptr;
 	unsigned long min_addr = PAGE_OFFSET;
-	unsigned long align_mask = BYTES_PER_WORD-1;
+	unsigned long align_mask = ARCH_SLAB_MINALIGN-1;
 	unsigned long size = cachep->objsize;
 	struct page *page;
 

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