Re: [PATCH] ipc: remove memcg accounting for sops objects in do_semtimedop()

[Michal Koutný]

On Tue, Sep 14, 2021 at 09:13:48AM +0200, Michal Hocko <mhocko@xxxxxxxx> wrote:
> "
> 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.
> "
> 
> It sounds like a problem, except it is not because? A worst case
> scenario evaluation would be beneficial for example

AFAICS, the offending allocation is in place only during the duration of
the syscall. So it's basically O(#tasks).
Considering at least 2 pages for task_struct + 2 pages for kernel stack,
back of the envelope calculation gives me the footprint amplification is
<1.5.
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).

The commit message can be more explicit about this but to the patch
Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx>