mm/page_counter: avoid integer overflow in effective_protection()

Patch series "mm/mglru: fix ineffective memory protection for non-kswapd
reclaim", v4.

For MGLRU, memory.min/low is not honored during non-kswapd global reclaim
(global direct reclaim and root-level memory.reclaim), because these paths
shrink memcgs using stale protection (emin/elow).

Patch 2 is the actual fix.  Patch 1 is a prerequisite: an integer overflow
in effective_protection(), spotted by the sashiko review tool, which patch
2's new caller would also be exposed to.


This patch (of 2):

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].

Link: https://lore.kernel.org/20260907025445.1836238-1-ridong.chen@linux.dev
Link: https://lore.kernel.org/20260907025445.1836238-2-ridong.chen@linux.dev
Link: https://sashiko.dev/#/patchset/20260826133054.88529-1-ridong.chen@linux.dev?part=1 [1]
Fixes: bc50bcc6e00b ("mm: memcontrol: clean up and document effective low/min calculations")
Fixes: 8a931f801340 ("mm: memcontrol: recursive memory.low protection")
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Assisted-by: Claude:claude-opus-4-8
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Chris Down <chris@chrisdown.name>
Cc: David Hildenbrand <david@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: <stable@vger.kernel.org>

0 files changed, 0 insertions, 0 deletions