Re: [RFC PATCH] mm/vmalloc: fix vmalloc which may return null if called with __GFP_NOFAIL

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2024/5/8 23:31, Hailong Liu wrote:
> On Wed, 08. May 23:10, Gao Xiang wrote:
>> Hi,
>>
>> On 2024/5/8 22:43, Hailong Liu wrote:
>>> On Wed, 08. May 21:41, Gao Xiang wrote:
>>>>
>>>> +Cc Michal,
>>>>
>>>> On 2024/5/8 20:58, hailong.liu@oppo.com wrote:
>>>>> From: "Hailong.Liu" <hailong.liu@oppo.com>
>>>>>
>>>>> Commit a421ef303008 ("mm: allow !GFP_KERNEL allocations for kvmalloc")
>>>>> includes support for __GFP_NOFAIL, but it presents a conflict with
>>>>> commit dd544141b9eb ("vmalloc: back off when the current task is
>>>>> OOM-killed"). A possible scenario is as belows:
>>>>>
>>>>> process-a
>>>>> kvcalloc(n, m, GFP_KERNEL | __GFP_NOFAIL)
>>>>>        __vmalloc_node_range()
>>>>> 	__vmalloc_area_node()
>>>>> 	    vm_area_alloc_pages()
>>>>>                --> oom-killer send SIGKILL to process-a
>>>>>                if (fatal_signal_pending(current)) break;
>>>>> --> return NULL;
>>>>>
>>>>> to fix this, do not check fatal_signal_pending() in vm_area_alloc_pages()
>>>>> if __GFP_NOFAIL set.
>>>>>
>>>>> Reported-by: Oven <liyangouwen1@oppo.com>
>>>>> Signed-off-by: Hailong.Liu <hailong.liu@oppo.com>
>>>>
>>>> Why taging this as RFC here?  It seems a corner-case fix of
>>>> commit a421ef303008
>>>>
>>>> Thanks,
>>>> Gao Xiang
>>>>
>>>
>>> Hi Gao Xiang:
>>>
>>> RFC here to wait for a better way to handle this case :).
>>> IMO, if vmalloc support __GFP_NOFAIL it should not return
>>> null even system is deadlock on memory.
>>
>> The starting point is that kmalloc doesn't support __GFP_NOFAIL
>> if order > 1 (even for very short temporary uses), see:
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/page_alloc.c?h=v6.8#n2896
>>
>> but it is possible if we have such page pointer array (since two
>> (order-1) pages can only keep 1024 8-byte entries, it can happen
>> if compression ratios are high), and kvmalloc(__GFP_NOFAIL) has
>> already been supported for almost two years, it will fallback to
>> order-0 allocation as described in commit e9c3cda4d86e
>> ("mm, vmalloc: fix high order __GFP_NOFAIL allocations").
>>
>> With my limited understanding, I'm not sure why it can cause
>> deadlock here since it will fallback to order-0 allocation then,
>> and such allocation is just for short temporary uses again
>> because kmalloc doesn't support order > 1 short memory
>> allocation strictly.
>>
> 
> deadlock on memory meands there is a memory leak causing
> system to be unable to allocate memory not actual
> *deadlock*.

Where is memory leak? If it's caused by kvmalloc(__GFP_NOFAIL)
callers, then it's bugs of callers and we should fix the callers.

Also why kmalloc(__GFP_NOFAIL) (for example, also order-0
allocation) differs?

Thanks,
Gao Xiang


> 
>> Thanks,
>> Gao Xiang
>>
> 
> --
> 
> Best Regards,
> Hailong.