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

[Michal Hocko]

fix the lkml address (fat fingers, sorry)

On Thu 11-08-22 16:06:37, Michal Hocko wrote:
> [Cc Wei Yang who is author of 78b132e9bae9]
> 
> On Thu 11-08-22 20:41:57, 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.
> > 
> > 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.
> 
> Just to clarify. With an unfortunate timing and those two nodemasks
> overlap the end user effect could be a premature OOM because some nodes
> wouldn't be considered, right?
> 
> > 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@xxxxxxxxxxxxx>
> 
> The fix looks correct.
> 
> > ---
> >  mm/mempolicy.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index d39b01fd52fe..61e4e6f5cfe8 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -855,12 +855,14 @@ static long do_set_mempolicy(unsigned short mode, unsigned short flags,
> >  		goto out;
> >  	}
> >  
> > +	task_lock(current);
> >  	ret = mpol_set_nodemask(new, nodes, scratch);
> >  	if (ret) {
> > +		task_unlock(current);
> >  		mpol_put(new);
> >  		goto out;
> >  	}
> > -	task_lock(current);
> > +
> >  	old = current->mempolicy;
> >  	current->mempolicy = new;
> >  	if (new && new->mode == MPOL_INTERLEAVE)
> > -- 
> > 2.31.1
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs