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

From: Michal Hocko
Date: Fri Aug 12 2022 - 03:07:35 EST


On Thu 11-08-22 16:11:23, Michal Hocko wrote:
> 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.

Forgot
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> >
> > > ---
> > > 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

--
Michal Hocko
SUSE Labs