Re: [PATCH] mm/cgroup: avoid panic when init with low memory

[Laurent Dufour]

On 20/02/2017 18:42, Michal Hocko wrote:
> On Mon 20-02-17 18:09:43, Laurent Dufour wrote:
>> 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 ?
> 
> yes
> 
>> Obviously, all callers to soft_limit_tree_node() and
>> soft_limit_tree_from_page() will have to check for the return pointer to
>> be NULL.
> 
> All callers that need to access the tree unconditionally, yes. Which is
> the case anyway, right? I haven't checked the check you have added is
> sufficient, but we shouldn't have that many of them because some code
> paths are called only when the soft limit is enabled.

You're right there are not so much callers to fix.
I'll send a new series containing the previous patch fixing the initial
panic and another one delaying the data allocation.