From: Ridong Chen 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 Signed-off-by: Ridong Chen --- 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 #include #include +#include #include #include #include @@ -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); /* * 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); } return ep; -- 2.34.1