linux-kernel - [PATCH] mm/damon/core: prevent unnecessary overflow in damos_set_effective

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250821125555.3020951-1-yanquanmin1@huawei.com>
Date: Thu, 21 Aug 2025 20:55:55 +0800
From: Quanmin Yan <yanquanmin1@...wei.com>
To: <sj@...nel.org>
CC: <akpm@...ux-foundation.org>, <damon@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<yanquanmin1@...wei.com>, <wangkefeng.wang@...wei.com>, <zuoze1@...wei.com>,
	<apanyaki@...zon.com>
Subject: [PATCH] mm/damon/core: prevent unnecessary overflow in damos_set_effective_quota()

On 32-bit systems, the throughput calculation in function
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.

Fixes: 1cd243030059 ("mm/damon/schemes: implement time quota")
Cc: <stable@...r.kernel.org> # 5.16.x
Signed-off-by: Quanmin Yan <yanquanmin1@...wei.com>
Reported-by: Andrew Paniakin <apanyaki@...zon.com>
Closes: N/A # privately reported
---
 mm/damon/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 831a9afc6cf6..fe1c19307ddd 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2103,8 +2103,8 @@ 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);
-- 
2.43.0