[PATCH v5 1/3] ipc/sem: account dynamically allocated sops arrays to memcg

[Yingjie Wang <x-professor@outlook.com>]

Commit 6a4746ba0619 ("ipc: remove memcg accounting for sops objects in
do_semtimedop()") removed accounting based on the assumption that sops
arrays would remain small, describing a maximum cost of two pages.

SEMOPM is a UAPI default that can be changed at build time and through
the per-namespace sysctl. Commit 50ec499b9a43 ("sysctl: allow change
system v ipc sysctls inside ipc namespace") made that limit tunable by
IPC namespace owners without capabilities in the initial user namespace.

do_semtimedop() keeps up to SEMOPM_FAST operations on the stack. Charge
every larger, dynamically allocated operation array to the caller's memory
cgroup. This avoids a second size threshold and prevents a configured
SEMOPM limit from expanding an unaccounted heap-allocation range.

In a 4-vCPU KVM guest, with the benchmark pinned to one vCPU in a leaf
memory cgroup, accounting raised the median time per successful semop()
call by 2.3% to 3.2% for 65, 500 and 1000 operations, compared with
unaccounted allocation. An invalid semid, which allocates and copies the
array before returning EINVAL, rose by 5.6% to 11.9%. The 64-operation
stack path changed by at most 0.9%, and successful calls in the root
cgroup by at most 2.3%. Each result used 21 samples from three boots.

Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Link: https://lore.kernel.org/r/20261016152609.2528805-1-x-professor@outlook.com
Fixes: 6a4746ba0619 ("ipc: remove memcg accounting for sops objects in do_semtimedop()")
Cc: stable@vger.kernel.org
Signed-off-by: Yingjie Wang <x-professor@outlook.com>
---
 ipc/sem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index 5ec41de7e85b2..ea95cca5bb0ec 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -2234,7 +2234,7 @@ static long do_semtimedop(int semid, struct sembuf __user *tsops,
 		return -EINVAL;
 
 	if (nsops > SEMOPM_FAST) {
-		sops = kvmalloc_objs(*sops, nsops);
+		sops = kvmalloc_objs(*sops, nsops, GFP_KERNEL_ACCOUNT);
 		if (sops == NULL)
 			return -ENOMEM;
 	}
-- 
2.43.0