Re: [PATCH v3 1/2] mm/page_counter: avoid integer overflow in effective_protection()

[David Laight <david.laight.linux@gmail.com>]

On Thu,  3 Sep 2026 11:19:51 +0800
Ridong Chen <ridong.chen@linux.dev> wrote:

> From: Ridong Chen <chenridong@xiaomi.com>
> 
> effective_protection() scales a parent's protection by a ratio of page
> counts, e.g. for recursive protection:
> 
> 	(parent_effective - siblings_protected) * (usage - protected)
> 		/ (parent_usage - siblings_protected)
> 
> The multiply is done at unsigned long width before dividing. On systems
> with >= 16TB RAM the product can exceed 2^64 and wrap, giving a bogus
> protection value and silently breaking memory.min/low enforcement.
> 
> Use mul_u64_u64_div_u64() to multiply in a 128-bit intermediate. Because
> usage and parent_usage are not read atomically (a child is charged
> before its parent), usage - protected can briefly exceed the divisor,
> making the quotient overflow 64 bits and trap (#DE on x86). Cap it so
> the ratio stays <= 1.
> 
> Reported by the sashiko review tool [1].
> 
> [1] https://sashiko.dev/#/patchset/20260826133054.88529-1-ridong.chen@linux.dev?part=1
> 
> Fixes: bc50bcc6e00b ("mm: memcontrol: clean up and document effective low/min calculations")
> Fixes: 8a931f801340 ("mm: memcontrol: recursive memory.low protection")
> Cc: stable@vger.kernel.org
> Assisted-by: Claude:claude-opus-4-8
> Reviewed-by: Barry Song <baohua@kernel.org>
> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
> ---
>  mm/page_counter.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/page_counter.c b/mm/page_counter.c
> index 661e0f2a5127..e8bd512069c5 100644
> --- a/mm/page_counter.c
> +++ b/mm/page_counter.c
> @@ -8,6 +8,7 @@
>  #include <linux/page_counter.h>
>  #include <linux/atomic.h>
>  #include <linux/kernel.h>
> +#include <linux/math64.h>
>  #include <linux/string.h>
>  #include <linux/sched.h>
>  #include <linux/bug.h>
> @@ -356,7 +357,8 @@ static unsigned long effective_protection(unsigned long usage,
>  	 * otherwise get a smaller chunk than what they claimed.
>  	 */
>  	if (siblings_protected > parent_effective)
> -		return protected * parent_effective / siblings_protected;
> +		return mul_u64_u64_div_u64(protected, parent_effective,
> +					   siblings_protected);

On 32bit it is only necessary to use a 64bit intermediary.
mul_u64_u64_div_u64() will drop back to the (probably faster) 64 by 64
divide (and then maybe to a 64 by 32 one).
But there is a lot of extra code before that happens.

>  
>  	/*
>  	 * Ok, utilized protection of all children is within what the
> @@ -397,13 +399,18 @@ static unsigned long effective_protection(unsigned long usage,
>  	if (parent_effective > siblings_protected &&
>  	    parent_usage > siblings_protected &&
>  	    usage > protected) {
> -		unsigned long unclaimed;
> +		unsigned long unclaimed = parent_effective - siblings_protected;
> +		unsigned long unprotected = usage - protected;
> +		unsigned long parent_unprotected = parent_usage - siblings_protected;
>  
> -		unclaimed = parent_effective - siblings_protected;
> -		unclaimed *= usage - protected;
> -		unclaimed /= parent_usage - siblings_protected;
> +		/*
> +		 * The usages aren't read atomically, so a child can transiently
> +		 * appear to use more than its parent, making the ratio exceed 1
> +		 * and the quotient overflow 64 bits (#DE on x86).  Cap it.
> +		 */
> +		unprotected = min(unprotected, parent_unprotected);
>  
> -		ep += unclaimed;
> +		ep += mul_u64_u64_div_u64(unclaimed, unprotected, parent_unprotected);

If the ratio is forced to 1 there is no point doing the scaling.
So maybe:
		if (likely(parent_unprotected > unprotected))
			unclaimed = mul_u64_u64_div_u64(unclaimed, unprotected,
							parent_unprotected);
		ep += unclaimed;
OTOH if the min() generates a cmov rather than a conditional branch
then you don't get a statically mispredicted branch in the normal case
(which is very likely with the empty 'else' branch).

David

>  	}
>  
>  	return ep;