ipc: remove memcg accounting for sops objects in do_semtimedop()

Linus proposes to revert an accounting for sops objects in
do_semtimedop() because it's really just a temporary buffer
for a single semtimedop() system call.

This object can consume up to 2 pages, syscall is sleeping
one, size and duration can be controlled by user, and this
allocation can be repeated by many thread at the same time.

However Shakeel Butt pointed that there are much more popular
objects with the same life time and similar memory
consumption, the accounting of which was decided to be
rejected for performance reasons.

Considering at least 2 pages for task_struct and 2 pages for
the kernel stack, a back of the envelope calculation gives a
footprint amplification of <1.5 so this temporal buffer can be
safely ignored.

The factor would IMO be interesting if it was >> 2 (from the
PoV of excessive (ab)use, fine-grained accounting seems to be
currently unfeasible due to performance impact).

Link: https://lore.kernel.org/lkml/[email protected]/
Fixes: 1831949 ("memcg: enable accounting of ipc resources")
Signed-off-by: Vasily Averin <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Reviewed-by: Michal Koutný <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: vaverin

ipc/sem.c

@@ -2238,7 +2238,7 @@ static long do_semtimedop(int semid, struct sembuf __user *tsops,
 		return -EINVAL;
 
 	if (nsops > SEMOPM_FAST) {
-		sops = kvmalloc_array(nsops, sizeof(*sops), GFP_KERNEL_ACCOUNT);
+		sops = kvmalloc_array(nsops, sizeof(*sops), GFP_KERNEL);
 		if (sops == NULL)
 			return -ENOMEM;
 	}