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

[Ridong Chen <ridong.chen@linux.dev>]

On 9/4/2026 4:37 PM, David Laight wrote:
> 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.
> 

Hi David,

Thank you for your review.

Do you want me to split this for 32-bit, or is keeping the single
mul_u64_u64_div_u64() call for readability fine with you?

>>   
>>   	/*
>>   	 * 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).
> 

I'd rather keep it as-is and avoid the extra branch. Does that work for you?

> 
>>   	}
>>   
>>   	return ep;
> 

-- 
Best regards
Ridong