Re: [PATCH] mm/mempolicy: fix lock contention on mems_allowed

[Michal Hocko]

On Thu 11-08-22 16:43:28, Abel Wu wrote:
> On 8/9/22 8:11 PM, Michal Hocko Wrote:
> > On Tue 09-08-22 18:49:27, Abel Wu wrote:
> > > The mems_allowed field can be modified by other tasks, so it
> > > isn't safe to access it with alloc_lock unlocked even in the
> > > current process context.
> > 
> > It would be useful to describe the racing scenario and the effect it
> > would have. 78b132e9bae9 hasn't really explained thinking behind and why
> > it was considered safe to drop the lock. I assume it was based on the
> > fact that the operation happens on the current task but this is hard to
> > tell.
> > 
> 
> Sorry for my poor description. Say there are two tasks: A from cpusetA
> is performing set_mempolicy(2), and B is changing cpusetA's cpuset.mems.
> 
>     A (set_mempolicy)		B (echo xx > cpuset.mems)
> 
>     pol = mpol_new();
> 				update_tasks_nodemask(cpusetA) {
> 				  foreach t in cpusetA {
> 				    cpuset_change_task_nodemask(t) {
> 				      task_lock(t); // t could be A
>     mpol_set_nodemask(pol) {
>       new = f(A->mems_allowed);
> 				      update t->mems_allowed;
>       pol.create(pol, new);
>     }
> 				      task_unlock(t);
>     task_lock(A);
>     A->mempolicy = pol;
>     task_unlock(A);
> 				    }
> 				  }
> 				}
> 
> In this case A's pol->nodes is computed by old mems_allowed, and could
> be inconsistent with A's new mems_allowed.
> 
> While it is different when replacing vmas' policy: the pol->nodes is
> gone wild only when current_cpuset_is_being_rebound():
> 
>     A (mbind)			B (echo xx > cpuset.mems)
> 
> 				cpuset_being_rebound = cpusetA;
> 				update_tasks_nodemask(cpusetA) {
> 				  foreach t in cpusetA {
> 				    cpuset_change_task_nodemask(t) {
> 				      task_lock(t); // t could be A
>     pol = mpol_new();
>     mmap_write_lock(A->mm);
>     mpol_set_nodemask(pol) {
>       mask = f(A->mems_allowed);
> 				      update t->mems_allowed;
>       pol.create(pol, mask);
>     }
> 				      task_unlock(t);
> 				    }
>     foreach v in A->mm {
>       if (current_cpuset_is_being_rebound())
>         pol.rebind(pol, cpuset.mems);
>       v->vma_policy = pol;
>     }
>     mmap_write_unlock(A->mm);
> 				    mmap_write_lock(t->mm);
> 				    mpol_rebind_mm(t->mm);
> 				    mmap_write_unlock(t->mm);
> 				  }
> 				}
> 				cpuset_being_rebound = NULL;
> 
> In this case, the cpuset.mems, which has already done updating, is
> finally used for calculating pol->nodes, rather than A->mems_allowed.
> So it is OK to call mpol_set_nodemask() with alloc_lock unlocked when
> doing mbind(2).

Please add this to the patch changelog.

Thanks!
-- 
Michal Hocko
SUSE Labs