[PATCH] cma: fix calculation of aligned offset

[Doug Berger <opendmb@gmail.com>]

The align_offset parameter is used by bitmap_find_next_zero_area_off()
to represent the offset of map's base from the previous alignment
boundary; the function ensures that the returned index, plus the
align_offset, honors the specified align_mask.

The logic introduced by commit b5be83e308f7 ("mm: cma: align to
physical address, not CMA region position") has the cma driver
calculate the offset to the *next* alignment boundary.  In most cases,
the base alignment is greater than that specified when making
allocations, resulting in a zero offset whether we align up or down.
In the example given with the commit, the base alignment (8MB) was
half the requested alignment (16MB) so the math also happened to work
since the offset is 8MB in both directions.  However, when requesting
allocations with an alignment greater than twice that of the base,
the returned index would not be correctly aligned.

Also, the align_order arguments of cma_bitmap_aligned_mask() and
cma_bitmap_aligned_offset() should not be negative so the argument
type was made unsigned.

Fixes: b5be83e308f7 ("mm: cma: align to physical address, not CMA region position")
Signed-off-by: Angus Clark <angus@angusclark.org>
Signed-off-by: Doug Berger <opendmb@gmail.com>
---
 mm/cma.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/mm/cma.c b/mm/cma.c
index 978b4a1441ef..56a388eb0242 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -59,7 +59,7 @@ const char *cma_get_name(const struct cma *cma)
 }
 
 static unsigned long cma_bitmap_aligned_mask(const struct cma *cma,
-					     int align_order)
+					     unsigned int align_order)
 {
 	if (align_order <= cma->order_per_bit)
 		return 0;
@@ -67,17 +67,14 @@ static unsigned long cma_bitmap_aligned_mask(const struct cma *cma,
 }
 
 /*
- * Find a PFN aligned to the specified order and return an offset represented in
- * order_per_bits.
+ * Find the offset of the base PFN from the specified align_order.
+ * The value returned is represented in order_per_bits.
  */
 static unsigned long cma_bitmap_aligned_offset(const struct cma *cma,
-					       int align_order)
+					       unsigned int align_order)
 {
-	if (align_order <= cma->order_per_bit)
-		return 0;
-
-	return (ALIGN(cma->base_pfn, (1UL << align_order))
-		- cma->base_pfn) >> cma->order_per_bit;
+	return (cma->base_pfn & ((1UL << align_order) - 1))
+		>> cma->order_per_bit;
 }
 
 static unsigned long cma_bitmap_pages_to_bits(const struct cma *cma,
-- 
2.13.0

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