Re: [-mm][PATCH 3/4] Add rlimit controller accounting and control

[Pavel Emelyanov <xemul@openvz.org>]

Paul Menage wrote:
> On Sat, May 3, 2008 at 2:38 PM, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>>  +
>>  +int rlimit_cgroup_charge_as(struct mm_struct *mm, unsigned long nr_pages)
>>  +{
>>  +       int ret;
>>  +       struct rlimit_cgroup *rcg;
>>  +
>>  +       rcu_read_lock();
>>  +       rcg = rlimit_cgroup_from_task(rcu_dereference(mm->owner));
>>  +       css_get(&rcg->css);
>>  +       rcu_read_unlock();
>>  +
>>  +       ret = res_counter_charge(&rcg->as_res, (nr_pages << PAGE_SHIFT));
>>  +       css_put(&rcg->css);
>>  +       return ret;
>>  +}
> 
> You need to synchronize against mm->owner changing, or
> mm->owner->cgroups changing. How about:
> 
> int rlimit_cgroup_charge_as(struct mm_struct *mm, unsigned long nr_pages)
> {
>   int ret;
>   struct rlimit_cgroup *rcg;
>   struct task_struct *owner;
> 
>   rcu_read_lock();
>  again:
> 
>   /* Find and lock the owner of the mm */
>   owner = rcu_dereference(mm->owner);
>   task_lock(owner);
>   if (mm->owner != owner) {
>     task_unlock(owner);
>     goto again;
>   }
> 
>   /* Charge the owner's cgroup with the new memory */
>   rcg = rlimit_cgroup_from_task(owner);
>   ret = res_counter_charge(&rcg->as_res, (nr_pages << PAGE_SHIFT));
>   task_unlock(owner);
>   rcu_read_unlock();
>   return ret;
> }
> 
>>  +
>>  +void rlimit_cgroup_uncharge_as(struct mm_struct *mm, unsigned long nr_pages)
>>  +{
>>  +       struct rlimit_cgroup *rcg;
>>  +
>>  +       rcu_read_lock();
>>  +       rcg = rlimit_cgroup_from_task(rcu_dereference(mm->owner));
>>  +       css_get(&rcg->css);
>>  +       rcu_read_unlock();
>>  +
>>  +       res_counter_uncharge(&rcg->as_res, (nr_pages << PAGE_SHIFT));
>>  +       css_put(&rcg->css);
>>  +}
> 
> Can't this be implemented as just a call to charge() with a negative
> value? (Possibly fixing res_counter_charge() to handle negative values
> if necessary) Seems simpler.

No, I'd keep two calls - charge and uncharge. This makes you sure that
the code xxx_charge(value) is a charge, regardless of what the "value"
is. Besides, xxx_charge returns an error code, you need to check (BTW, I
think we should add a __must_check attribute there), while uncharge does
not.

>>  +/*
>>  + * TODO: get the attach callbacks to fail and disallow task movement.
>>  + */
> 
> You mean disallow all movement within a hierarchy that has this cgroup
> mounted? Doesn't that make it rather hard to use?
> 
>>  +static void rlimit_cgroup_move_task(struct cgroup_subsys *ss,
>>  +                                       struct cgroup *cgrp,
>>  +                                       struct cgroup *old_cgrp,
>>  +                                       struct task_struct *p)
>>  +{
>>  +       struct mm_struct *mm;
>>  +       struct rlimit_cgroup *rcg, *old_rcg;
>>  +
>>  +       mm = get_task_mm(p);
>>  +       if (mm == NULL)
>>  +               return;
>>  +
>>  +       rcu_read_lock();
>>  +       if (p != rcu_dereference(mm->owner))
>>  +               goto out;
>>  +
>>  +       rcg = rlimit_cgroup_from_cgrp(cgrp);
>>  +       old_rcg = rlimit_cgroup_from_cgrp(old_cgrp);
>>  +
>>  +       if (rcg == old_rcg)
>>  +               goto out;
>>  +
>>  +       if (res_counter_charge(&rcg->as_res, (mm->total_vm << PAGE_SHIFT)))
>>  +               goto out;
>>  +       res_counter_uncharge(&old_rcg->as_res, (mm->total_vm << PAGE_SHIFT));
>>  +out:
>>  +       rcu_read_unlock();
>>  +       mmput(mm);
>>  +}
>>  +
> 
> Since you need to protect against concurrent charges, and against
> concurrent mm ownership changes, I think you should just do this under
> task_lock(p).
> 
>>  +static void rlimit_cgroup_mm_owner_changed(struct cgroup_subsys *ss,
>>  +                                               struct cgroup *cgrp,
>>  +                                               struct cgroup *old_cgrp,
>>  +                                               struct task_struct *p)
>>  +{
>>  +       struct rlimit_cgroup *rcg, *old_rcg;
>>  +       struct mm_struct *mm = get_task_mm(p);
>>  +
>>  +       BUG_ON(!mm);
>>  +       rcg = rlimit_cgroup_from_cgrp(cgrp);
>>  +       old_rcg = rlimit_cgroup_from_cgrp(old_cgrp);
>>  +       if (res_counter_charge(&rcg->as_res, (mm->total_vm << PAGE_SHIFT)))
>>  +               goto out;
>>  +       res_counter_uncharge(&old_rcg->as_res, (mm->total_vm << PAGE_SHIFT));
>>  +out:
>>  +       mmput(mm);
>>  +}
>>  +
> 
> Also needs to task_lock(p) to prevent concurrent charges or cgroup
> reassignments?
> 
> Paul
>