Re: [PATCH 1/1] mm: Fix memory leak on mm_init error handling

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2023-03-30 15:42, Andrew Morton wrote:
> On Thu, 30 Mar 2023 09:38:22 -0400 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
>> commit f1a7941243c1 ("mm: convert mm's rss stats into percpu_counter")
>> introduces a memory leak by missing a call to destroy_context() when a
>> percpu_counter fails to allocate.
>>
>> Before introducing the per-cpu counter allocations, init_new_context()
>> was the last call that could fail in mm_init(), and thus there was no
>> need to ever invoke destroy_context() in the error paths. Adding the
>> following percpu counter allocations adds error paths after
>> init_new_context(), which means its associated destroy_context() needs
>> to be called when percpu counters fail to allocate.
>>
>> ...
>>
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1171,6 +1171,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>>   fail_pcpu:
>>   	while (i > 0)
>>   		percpu_counter_destroy(&mm->rss_stat[--i]);
>> +	destroy_context(mm);
>>   fail_nocontext:
>>   	mm_free_pgd(mm);
>>   fail_nopgd:
> 
> Is there really a leak?  I wasn't able to find a version of
> init_new_context() which performs allocation.

AFAIU, at least on powerpc:

arch/powerpc/mm/book3s64/mmu_context.c: init_new_context() calls 
radix__init_new_context() or hash__init_new_context() which
leak IDs through ida_alloc_range.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com