[PATCH v2] mm/damon/stat: avoid overflow in estimated_memory_bandwidth calculation

[Wang Guangke <gkwang@linx-info.com>]

estimated_memory_bandwidth is calculated by access_bytes *
USEC_PER_MSEC * MSEC_PER_SEC (= USEC_PER_SEC) / aggr_interval,
and users read it via
/sys/module/damon_stat/parameters/estimated_memory_bandwidth.

On 32-bit systems, access_bytes * USEC_PER_SEC overflows unsigned
long once access_bytes exceeds about 4294 (U32_MAX / USEC_PER_SEC),
so users get a wrong value.

Fix it by doing the calculation in 64-bit and splitting the division
into quotient and remainder.

The issue was reproduced on a 32-bit system (qemu-system-i386): the
parameter estimated_memory_bandwidth reported 362 instead of the
expected 1926150; with the fix, the value matches the expectation.

Fixes: fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth")
Cc: <stable@vger.kernel.org>
Signed-off-by: Wang Guangke <gkwang@linx-info.com>
---
Changes in v2:
- v1: https://lore.kernel.org/all/20261101180001.167198-1-gkwang@linx-info.com
- Clarify the issue and its impact on users.
- Add Cc: stable@.
- Rename variable bytes_per_interval to bytes_per_us.
- Use USEC_PER_SEC instead of 'USEC_PER_MSEC * MSEC_PER_SEC'.
---
 mm/damon/stat.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index b05b68f73e10..e44f5bf3645c 100644
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -32,8 +32,8 @@ static bool enabled __read_mostly = IS_ENABLED(
 module_param_cb(enabled, &enabled_param_ops, NULL, 0600);
 MODULE_PARM_DESC(enabled, "Enable of disable DAMON_STAT");
 
-static unsigned long estimated_memory_bandwidth __read_mostly;
-module_param(estimated_memory_bandwidth, ulong, 0400);
+static u64 estimated_memory_bandwidth __read_mostly;
+module_param(estimated_memory_bandwidth, ullong, 0400);
 MODULE_PARM_DESC(estimated_memory_bandwidth,
 		"Estimated memory bandwidth usage in bytes per second");
 
@@ -55,15 +55,18 @@ static void damon_stat_set_estimated_memory_bandwidth(struct damon_ctx *c)
 {
 	struct damon_target *t;
 	struct damon_region *r;
-	unsigned long access_bytes = 0;
+	u64 access_bytes = 0;
+	u64 remainder, bytes_per_us;
 
 	damon_for_each_target(t, c) {
 		damon_for_each_region(r, t)
-			access_bytes += (r->ar.end - r->ar.start) *
+			access_bytes += (u64)(r->ar.end - r->ar.start) *
 				r->nr_accesses;
 	}
-	estimated_memory_bandwidth = access_bytes * USEC_PER_MSEC *
-		MSEC_PER_SEC / c->attrs.aggr_interval;
+	bytes_per_us = div64_u64_rem(access_bytes,
+			c->attrs.aggr_interval, &remainder);
+	estimated_memory_bandwidth = bytes_per_us * USEC_PER_SEC +
+		div64_u64(remainder * USEC_PER_SEC, c->attrs.aggr_interval);
 }
 
 static int damon_stat_idletime(const struct damon_region *r)
-- 
2.47.3