On Mon 02-08-21 18:00:10, Miaohe Lin wrote: > On 2021/8/2 14:43, Michal Hocko wrote: > > On Sat 31-07-21 10:05:51, Miaohe Lin wrote: > >> On 2021/7/30 14:44, Michal Hocko wrote: > >>> On Thu 29-07-21 20:12:43, Roman Gushchin wrote: > >>>> On Thu, Jul 29, 2021 at 08:57:54PM +0800, Miaohe Lin wrote: > >>>>> rtpn might be NULL in very rare case. We have better to check it before > >>>>> dereferencing it. Since memcg can live with NULL rb_tree_per_node in > >>>>> soft_limit_tree, warn this case and continue. > >>>>> > >>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx> > >>>>> --- > >>>>> mm/memcontrol.c | 2 ++ > >>>>> 1 file changed, 2 insertions(+) > >>>>> > >>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c > >>>>> index 5b4592d1e0f2..70a32174e7c4 100644 > >>>>> --- a/mm/memcontrol.c > >>>>> +++ b/mm/memcontrol.c > >>>>> @@ -7109,6 +7109,8 @@ static int __init mem_cgroup_init(void) > >>>>> rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL, > >>>>> node_online(node) ? node : NUMA_NO_NODE); > >>>>> > >>>>> + if (WARN_ON_ONCE(!rtpn)) > >>>>> + continue; > >>>> > >>>> I also really doubt that it makes any sense to continue in this case. > >>>> If this allocations fails (at the very beginning of the system's life, it's an __init function), > >>>> something is terribly wrong and panic'ing on a NULL-pointer dereference sounds like > >>>> a perfect choice. > >>> > >>> Moreover this is 24B allocation during early boot. Kernel will OOM and > >>> panic when not being able to find any victim. I do not think we need to > >> > >> Agree with you. But IMO it may not be a good idea to leave the rtpn without NULL check. We should defend > >> it though it could hardly happen. But I'm not insist on this check. I will drop this patch if you insist. > > > > It is not that I would insist. I just do not see any point in the code > > churn. This check is not going to ever trigger and there is nothing you > > can do to recover anyway so crashing the kernel is likely the only > > choice left. > > > > I hope I get the point now. What you mean is nothing we can do to recover and panic'ing on a > NULL-pointer dereference is a perfect choice ? Should we declare that we leave the rtpn without > NULL check on purpose like below ? > > Many thanks. > > @@ -7109,8 +7109,12 @@ static int __init mem_cgroup_init(void) > rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL, > node_online(node) ? node : NUMA_NO_NODE); > > - if (WARN_ON_ONCE(!rtpn)) > - continue; > + /* > + * If this allocation fails (at the very beginning of the > + * system's life, it's an __init function), something is > + * terribly wrong and panic'ing on a NULL-pointer > + * dereference sounds like a perfect choice. > + */ I am not really sure this is really worth it. Really we do not really want to have similar comments all over the early init code, do we? > rtpn->rb_root = RB_ROOT; > rtpn->rb_rightmost = NULL; > spin_lock_init(&rtpn->lock); -- Michal Hocko SUSE Labs
