Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary

Eric W. Biederman
2020-08-21T04:43:20Z

Michal Hocko <mhocko@suse.com> writes:

> On Thu 20-08-20 08:56:53, Suren Baghdasaryan wrote:
> [...]
>> Catching up on the discussion which was going on while I was asleep...
>> So it sounds like there is a consensus that oom_adj should be moved to
>> mm_struct rather than trying to synchronize it among tasks sharing mm.
>> That sounds reasonable to me too. Michal answered all the earlier
>> questions about this patch, so I won't be reiterating them, thanks
>> Michal. If any questions are still lingering about the original patch
>> I'll be glad to answer them.
>
> I think it still makes some sense to go with a simpler (aka less tricky)
> solution which would be your original patch with an incremental fix for
> vfork and the proper ordering (http://lkml.kernel.org/r/20200820124109.GI5033@dhcp22.suse.cz)
> and then make a more complex shift to mm struct on top of that. The
> former will be less tricky to backport to stable IMHO.

So I am confused.

I don't know how a subtle dependency on something in clone
is better than something flat footed in exec.

That said if we are going for a small change why not:

/*
* Make sure we will check other processes sharing the mm if this is
* not vfrok which wants its own oom_score_adj.
* pin the mm so it doesn't go away and get reused after task_unlock
*/
if (!task->vfork_done) {
struct task_struct *p = find_lock_task_mm(task);

if (p) {
- if (atomic_read(&p->mm->mm_users) > 1) {
+ if (atomic_read(&p->mm->mm_users) > p->signal->nr_threads) {
mm = p->mm;
mmgrab(mm);
}
task_unlock(p);
}
}

That would seem to be the minimal change to make this happen. That has
the advantage that if a processes does vfork it won't always have to
take the slow path.

Moving to the mm_struct is much less racy but this is simple.

Eric