Re: [PATCH] fork: fix oops after fork failure

[Glauber Costa <glommer@parallels.com>]

On 08/23/2012 06:45 PM, Michal Hocko wrote:
> On Thu 23-08-12 16:38:50, Michal Hocko wrote:
>> On Thu 23-08-12 16:33:12, Michal Hocko wrote:
>>> On Thu 23-08-12 17:08:46, Glauber Costa wrote:
>>>> When we want to duplicate a new process, dup_task_struct() will undergo
>>>> a series of allocations. If alloc_thread_info_node() fails, we call
>>>> free_task_struct() and return.
>>>>
>>>> This seems right, but it is not. free_task_struct() will not only free
>>>> the task struct from the kmem_cache, but will also call
>>>> arch_release_task_struct(). The problem is that this function is
>>>> supposed to undo whatever arch-specific work done by
>>>> arch_dup_task_struct(), that is not yet called at this point.  The
>>>> particular problem I ran accross was that in x86, we will arrive at
>>>> fpu_free() without having ever allocated it.
>>>>
>>>> This code is very ancient, and according to git, it is there since the
>>>> pre-git era. But forks don't fail that often, so that made it well
>>>> hidden.
>>>>
>>>> Signed-off-by: Glauber Costa <glommer@parallels.com>
>>>> Reported-by: Frederic Weisbecker <fweisbec@redhat.com>
>>>> ---
>>>>  kernel/fork.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>>> index 152d023..b397435 100644
>>>> --- a/kernel/fork.c
>>>> +++ b/kernel/fork.c
>>>> @@ -299,7 +299,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
>>>>  
>>>>  	ti = alloc_thread_info_node(tsk, node);
>>>>  	if (!ti) {
>>>> -		free_task_struct(tsk);
>>>> +		kmem_cache_free(task_struct_cachep, tsk);
>>>
>>> What about ia64 (or !CONFIG_ARCH_THREAD_INFO_ALLOCATOR in general) which
>>> doesn't allocate thread_info at all?
>>
>> Hit send button too fast. Should read (or CONFIG_ARCH_THREAD_INFO_ALLOCATOR)
>> ia64 will not fail obviously and there is no other arch which would
>> define own thread infor allocators but there might be some in future.
> 
> Bahh, and I should have been looking at CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
> instead. Anyway ia64 uses page allocator directly so kmem_cache_free is
> not appropriate.
> 
Yes, you are right. Thanks for spotting this