On 2021/8/2 18:42, Michal Hocko wrote: > 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? Maybe not. Will drop this patch. Thanks. > >> rtpn->rb_root = RB_ROOT; >> rtpn->rb_rightmost = NULL; >> spin_lock_init(&rtpn->lock); >
