[PATCH] mm/page-writeback: avoid overflow when reporting BDI byte limits

[Hungyu Lin <dennylin0707@gmail.com>]

bdi_get_bytes() multiplies unsigned long operands before assigning the
result to u64. On 32-bit kernels, the intermediate product wraps and
min_bytes and max_bytes report incorrect limits. For example, with
vm.dirty_bytes set to 104857600 and max_ratio set to 100, max_bytes
reports 268 instead of 104857600.

Convert the page count to bytes in u64 and use mul_u64_u32_div() to avoid
overflow in the subsequent multiplication. This also handles large dirty
limits on systems with larger pages. Combining the two divisions keeps
the existing rounding behavior.

Fixes: 00df7d51263b ("mm: add bdi_get_max_bytes() function")
Cc: stable@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
 mm/page-writeback.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 499a35473e4f..ddebec79d8d9 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -665,7 +665,8 @@ static u64 bdi_get_bytes(unsigned int ratio)
 	u64 bytes;
 
 	global_dirty_limits(&background_thresh, &dirty_thresh);
-	bytes = (dirty_thresh * PAGE_SIZE * ratio) / BDI_RATIO_SCALE / 100;
+	bytes = mul_u64_u32_div((u64)dirty_thresh << PAGE_SHIFT, ratio,
+				100 * BDI_RATIO_SCALE);
 
 	return bytes;
 }
-- 
2.43.0