[Devel] [PATCH RH9 20/23] ms/ipc: remove memcg accounting for sops objects in do_semtimedop()

[Vasily Averin]

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/90e254df-0dfe-f080-011e-b7c53ee7fd20@virtuozzo.com/
Fixes: 18319498fdd4 ("memcg: enable accounting of ipc resources")
Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
Acked-by: Michal Hocko <mhocko at suse.com>
Reviewed-by: Michal Koutný <mkoutny at suse.com>
Acked-by: Shakeel Butt <shakeelb at google.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit 6a4746ba06191e23d30230738e94334b26590a8a)
https://jira.sw.ru/browse/PSBM-133990
Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
---
 ipc/sem.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index 1a8b9f0ac047..4c4fdf010793 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -2005,8 +2005,7 @@ static long do_semtimedop(int semid, struct sembuf __user *tsops,
 	if (nsops > ns->sc_semopm)
 		return -E2BIG;
 	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;
 	}
-- 
2.25.1