[PATCH v2] mm: cma: fix CMA aligned offset calculation

[Danesh Petigara]

The CMA aligned offset calculation is incorrect for
non-zero order_per_bit values.

For example, if cma->order_per_bit=1, cma->base_pfn=
0x2f800000 and align_order=12, the function returns
a value of 0x17c00 instead of 0x400.

This patch fixes the CMA aligned offset calculation.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Danesh Petigara <dpetigara@xxxxxxxxxxxx>
Reviewed-by: Gregory Fong <gregory.0xf0@xxxxxxxxx>
Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>
---
Changes since v1:
	- moved comment out of function
	- removed unused 'alignment' variable

v1: https://lkml.org/lkml/2015/2/24/598

 mm/cma.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/cma.c b/mm/cma.c
index 75016fd..68ecb7a 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -64,15 +64,17 @@ static unsigned long cma_bitmap_aligned_mask(struct cma *cma, int align_order)
 	return (1UL << (align_order - cma->order_per_bit)) - 1;
 }
 
+/*
+ * Find a PFN aligned to the specified order and return an offset represented in
+ * order_per_bits.
+ */
 static unsigned long cma_bitmap_aligned_offset(struct cma *cma, int align_order)
 {
-	unsigned int alignment;
-
 	if (align_order <= cma->order_per_bit)
 		return 0;
-	alignment = 1UL << (align_order - cma->order_per_bit);
-	return ALIGN(cma->base_pfn, alignment) -
-		(cma->base_pfn >> cma->order_per_bit);
+
+	return (ALIGN(cma->base_pfn, (1UL << align_order))
+		- cma->base_pfn) >> cma->order_per_bit;
 }
 
 static unsigned long cma_bitmap_maxno(struct cma *cma)
-- 
1.9.1

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