[Devel] Re: [RFC PATCH 5/5] refresh VM committed space after a task migration

[Andrea Righi]

Dave Hansen wrote:
> On Tue, 2008-06-10 at 01:33 +0200, Andrea Righi wrote:
>> +       preempt_disable();
>> +       committed = atomic_long_read(&p->vm_committed_space);
>> +       atomic_long_sub(committed, &old_mem->vmacct.vm_committed_space);
>> +       atomic_long_add(committed, &mem->vmacct.vm_committed_space);
>> +       preempt_enable();
>>  out:
>>         mmput(mm);
>>  }
> 
> Why bother with the preempt stuff here?  What does the actually protect
> against?  I assume that you're trying to keep other tasks that might run
> on this CPU from seeing weird, inconsistent numbers in here.  Is there
> some other looks that keeps *other* cpus from seeing this?
> 
> In any case, I think it needs a big, fat comment.

Yes, true, mem_cgroup_move_task() is called after the task->cgroups
pointer has been changed. So, even if task changes its committed space
between the atomic_long_sub() and atomic_long_add() it will be correctly
accounted in the new cgroup.

-Andrea
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