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Subject[PATCH] bitmap: fix memset optimization on big-endian systems
Date
From: Omar Sandoval <osandov@fb.com>

Commit 2a98dc028f91 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: stable@kernel.org
Reported-by: "Erhard F." <erhard_f@mailbox.org>
Signed-off-by: Omar Sandoval <osandov@fb.com>
---
include/linux/bitmap.h | 22 +++++++++++++++++-----
1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 5f11fbdc27f8..1ee46f492267 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -302,12 +302,20 @@ static inline void bitmap_complement(unsigned long *dst, const unsigned long *sr
__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);
}
@@ -358,8 +366,10 @@ static __always_inline void bitmap_set(unsigned long *map, unsigned int start,
{
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);
@@ -370,8 +380,10 @@ static __always_inline void bitmap_clear(unsigned long *map, unsigned int start,
{
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);
--
2.16.3
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