Re: [PATCH] mm/cgroup: avoid panic when init with low memory
From: Laurent Dufour
Date: Mon Feb 20 2017 - 12:09:56 EST
On 20/02/2017 14:01, Michal Hocko wrote:
> On Wed 15-02-17 11:36:09, Laurent Dufour wrote:
>> The system may panic when initialisation is done when almost all the
>> memory is assigned to the huge pages using the kernel command line
>> parameter hugepage=xxxx. Panic may occur like this:
> I am pretty sure the system might blow up in many other ways when you
> misconfigure it and pull basically all the memory out. Anyway...
>> This is a chicken and egg issue where the kernel try to get free
>> memory when allocating per node data in mem_cgroup_init(), but in that
>> path mem_cgroup_soft_limit_reclaim() is called which assumes that
>> these data are allocated.
>> As mem_cgroup_soft_limit_reclaim() is best effort, it should return
>> when these data are not yet allocated.
> ... this makes some sense. Especially when there is no soft limit
> configured. So this is a good step. I would just like to ask you to go
> one step further. Can we make the whole soft reclaim thing uninitialized
> until the soft limit is actually set? Soft limit is not used in cgroup
> v2 at all and I would strongly discourage it in v1 as well. We will save
> few bytes as a bonus.
Hi Michal, and thanks for the review.
I'm not familiar with that part of the kernel, so to be sure we are on
the same line, are you suggesting to set soft_limit_tree at the first
time mem_cgroup_write() is called to set a soft_limit field ?
Obviously, all callers to soft_limit_tree_node() and
soft_limit_tree_from_page() will have to check for the return pointer to
>> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
>> mm/memcontrol.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 1fd6affcdde7..213f96b2f601 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -2556,7 +2556,7 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
>> * is empty. Do it lockless to prevent lock bouncing. Races
>> * are acceptable as soft limit is best effort anyway.
>> - if (RB_EMPTY_ROOT(&mctz->rb_root))
>> + if (!mctz || RB_EMPTY_ROOT(&mctz->rb_root))
>> return 0;