From b0eaacb0b9b0412916e3ca0c769f8aab34d82725 Mon Sep 17 00:00:00 2001 From: Quanmin Yan Date: Thu, 21 Aug 2025 20:55:55 +0800 Subject: 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 Reported-by: Andrew Paniakin Reviewed-by: SeongJae Park Cc: Kefeng Wang Cc: ze zuo Cc: [5.16+] Signed-off-by: Andrew Morton --- mm/damon/core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2026,8 +2026,8 @@ static void damos_set_effective_quota(st 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);