Re: [PATCH v2] mm/mempolicy: fix lock contention on mems_allowed

[Muchun Song <muchun.song@linux.dev>]

> On Aug 11, 2022, at 20:41, Abel Wu <wuyun.abel@bytedance.com> 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.
> 
> 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) {
>  mpol_set_nodemask(pol) {
> 				      task_lock(t); // t could be A
>    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)
>  -------------------------------------------------------
>  pol = mpol_new();
>  mmap_write_lock(A->mm);
> 				cpuset_being_rebound = cpusetA;
> 				update_tasks_nodemask(cpusetA) {
> 				  foreach t in cpusetA {
> 				    cpuset_change_task_nodemask(t) {
>  mpol_set_nodemask(pol) {
> 				      task_lock(t); // t could be A
>    mask = f(A->mems_allowed);
> 				      update t->mems_allowed;
>    pol.create(pol, mask);
> 				      task_unlock(t);
>  }
> 				    }
>  foreach v in A->mm {
>    if (cpuset_being_rebound == cpusetA)
>      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).
> 
> Fixes: 78b132e9bae9 ("mm/mempolicy: remove or narrow the lock on current")
> Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>

Reviewed-by: Muchun Song <songmuchun@bytedance.com>

Thanks.