+ mm-damon-core-prevent-unnecessary-overflow-in-damos_set_effective_quota.patch added to mm-hotfixes-unstable branch

+ mm-damon-core-prevent-unnecessary-overflow-in-damos_set_effective_quota.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/damon/core: prevent unnecessary overflow in damos_set_effective_quota()
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-damon-core-prevent-unnecessary-overflow-in-damos_set_effective_quota.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-damon-core-prevent-unnecessary-overflow-in-damos_set_effective_quota.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Quanmin Yan <yanquanmin1@huawei.com>
Subject: mm/damon/core: prevent unnecessary overflow in damos_set_effective_quota()
Date: Thu, 21 Aug 2025 20:55:55 +0800

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>
---

 mm/damon/core.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/damon/core.c~mm-damon-core-prevent-unnecessary-overflow-in-damos_set_effective_quota
+++ a/mm/damon/core.c
@@ -2073,8 +2073,8 @@ 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);
_

Patches currently in -mm which might be from yanquanmin1@huawei.com are

mm-damon-core-prevent-unnecessary-overflow-in-damos_set_effective_quota.patch