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Patch "bitmap: fix memset optimization on big-endian systems" has been added to the 4.15-stable tree

PATCH [email protected] 1 participant 1 message Apr 4, 2018

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This is a note to let you know that I've just added the patch titled

bitmap: fix memset optimization on big-endian systems

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
bitmap-fix-memset-optimization-on-big-endian-systems.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

Quoted 1 line

From 21035965f60b0502fc6537b232839389bb4ce664 Mon Sep 17 00:00:00 2001

From: Omar Sandoval <[email protected]>
Date: Mon, 2 Apr 2018 15:58:31 -0700
Subject: bitmap: fix memset optimization on big-endian systems

From: Omar Sandoval <[email protected]>

commit 21035965f60b0502fc6537b232839389bb4ce664 upstream.

Commit 2a98dc028f91 ("include/linux/bitmap.h: turn bitmap_set and bitmap_clear into memset when possible") introduced an optimization to bitmap_{set,clear}() which uses memset() when the start and length are constants aligned to a byte.

This is wrong on big-endian systems; our bitmaps are arrays of unsigned long, so bit n is not at byte n / 8 in memory. This was caught by the Btrfs selftests, but the bitmap selftests also fail when run on a big-endian machine.

We can still use memset if the start and length are aligned to an unsigned long, so do that on big-endian. The same problem applies to the memcmp in bitmap_equal(), so fix it there, too.

Fixes 2a98dc028f91 ("include/linux/bitmap.h: turn bitmap_set and bitmap_clear into memset when possible")
Fixes 2c6deb01525a ("bitmap: use memcmp optimisation in more situations")
Cc [email protected]
Reported-by "Erhard F." <[email protected]>
Cc Matthew Wilcox <[email protected]>
Cc Rasmus Villemoes <[email protected]>
Cc Andrew Morton <[email protected]>
Cc Arnd Bergmann <[email protected]>
Signed-off-by Omar Sandoval <[email protected]>
Signed-off-by Linus Torvalds <[email protected]>
Signed-off-by Greg Kroah-Hartman <[email protected]>
---
 include/linux/bitmap.h |   22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)
include/linux/bitmap.h +17 -5
@@ -271,12 +271,20 @@ static inline void bitmap_complement(uns
 		__bitmap_complement(dst, src, nbits);
 }
 
+#ifdef __LITTLE_ENDIAN
+#define BITMAP_MEM_ALIGNMENT 8
+#else
+#define BITMAP_MEM_ALIGNMENT (8 * sizeof(unsigned long))
+#endif
+#define BITMAP_MEM_MASK (BITMAP_MEM_ALIGNMENT - 1)
+
 static inline int bitmap_equal(const unsigned long *src1,
 			const unsigned long *src2, unsigned int nbits)
 {
 	if (small_const_nbits(nbits))
 		return !((*src1 ^ *src2) & BITMAP_LAST_WORD_MASK(nbits));
-	if (__builtin_constant_p(nbits & 7) && IS_ALIGNED(nbits, 8))
+	if (__builtin_constant_p(nbits & BITMAP_MEM_MASK) &&
+	    IS_ALIGNED(nbits, BITMAP_MEM_ALIGNMENT))
 		return !memcmp(src1, src2, nbits / 8);
 	return __bitmap_equal(src1, src2, nbits);
 }

@@ -327,8 +335,10 @@ static __always_inline void bitmap_set(u
 {
 	if (__builtin_constant_p(nbits) && nbits == 1)
 		__set_bit(start, map);
-	else if (__builtin_constant_p(start & 7) && IS_ALIGNED(start, 8) &&
-		 __builtin_constant_p(nbits & 7) && IS_ALIGNED(nbits, 8))
+	else if (__builtin_constant_p(start & BITMAP_MEM_MASK) &&
+		 IS_ALIGNED(start, BITMAP_MEM_ALIGNMENT) &&
+		 __builtin_constant_p(nbits & BITMAP_MEM_MASK) &&
+		 IS_ALIGNED(nbits, BITMAP_MEM_ALIGNMENT))
 		memset((char *)map + start / 8, 0xff, nbits / 8);
 	else
 		__bitmap_set(map, start, nbits);

@@ -339,8 +349,10 @@ static __always_inline void bitmap_clear
 {
 	if (__builtin_constant_p(nbits) && nbits == 1)
 		__clear_bit(start, map);
-	else if (__builtin_constant_p(start & 7) && IS_ALIGNED(start, 8) &&
-		 __builtin_constant_p(nbits & 7) && IS_ALIGNED(nbits, 8))
+	else if (__builtin_constant_p(start & BITMAP_MEM_MASK) &&
+		 IS_ALIGNED(start, BITMAP_MEM_ALIGNMENT) &&
+		 __builtin_constant_p(nbits & BITMAP_MEM_MASK) &&
+		 IS_ALIGNED(nbits, BITMAP_MEM_ALIGNMENT))
 		memset((char *)map + start / 8, 0, nbits / 8);
 	else
 		__bitmap_clear(map, start, nbits);

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/bitmap-fix-memset-optimization-on-big-endian-systems.patch