mm/damon/core: prevent unnecessary overflow in damos_set_effective_quota()

On 32-bit systems, the throughput calculation in
damos_set_effective_quota() is prone to unnecessary multiplication
overflow.  Using mult_frac() to fix it.

Andrew Paniakin also recently found and privately reported this issue, on
64 bit systems.  This can also happen on 64-bit systems, once the charged
size exceeds ~17 TiB.  On systems running for long time in production,
this issue can actually happen.

More specifically, when a DAMOS scheme having the time quota run for
longtime, throughput calculation can overflow and set esz too small.  As a
result, speed of the scheme get unexpectedly slow.

Link: https://lkml.kernel.org/r/20250821125555.3020951-1-yanquanmin1@huawei.com
Fixes: 1cd243030059 ("mm/damon/schemes: implement time quota")
Signed-off-by: Quanmin Yan <yanquanmin1@huawei.com>
Reported-by: Andrew Paniakin <apanyaki@amazon.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: ze zuo <zuoze1@huawei.com>
Cc: <stable@vger.kernel.org>	[5.16+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 70eff5cbe6ee11a234c97ab68cc54e2d0cdce198..106ee8b0f2d5f15a18d1fb9c0f462d9b498f99b7 100644 (file)
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2073,8 +2073,8 @@ static void damos_set_effective_quota(struct damos_quota *quota)
 
        if (quota->ms) {
                if (quota->total_charged_ns)
-                       throughput = quota->total_charged_sz * 1000000 /
-                               quota->total_charged_ns;
+                       throughput = mult_frac(quota->total_charged_sz, 1000000,
+                                                       quota->total_charged_ns);
                else
                        throughput = PAGE_SIZE * 1024;
                esz = min(throughput * quota->ms, esz);