Re: [linus:master] [bpf] c930472552: WARNING:at_kernel/bpf/memalloc.c:#bpf_mem_alloc_init

Hou Tao
2023-11-18T13:14:34Z

Hi,

On 11/8/2023 12:04 AM, Yonghong Song wrote:
>
> On 11/7/23 2:43 AM, Hou Tao wrote:
>> Hi,
>>
>> On 11/4/2023 12:49 AM, Yonghong Song wrote:
>>> On 11/2/23 11:54 PM, Hou Tao wrote:
>>>> Hi,
>>>>
>>>> On 11/3/2023 12:08 AM, Yonghong Song wrote:
>>>>> On 11/2/23 6:40 AM, Hou Tao wrote:
>>>>>> Hi Alexei,
>>>>>>
>>>>>> On 10/31/2023 4:01 PM, Hou Tao wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> On 10/30/2023 10:11 PM, kernel test robot wrote:
>>>>>>>> hi, Hou Tao,
>>>>>>>>
>>>>>>>> we noticed a WARN_ONCE added in this commit was hit in our tests.
>>>>>>>> FYI.
>>>>>>>>
>>>>>>>>
>> SNIP
>>>>>> I see what has happened. The problem is twofold:
>>>>>> (1) The object_size of kmalloc-cg-96 is adjust from 96 to 128 due to
>>>>>> slab merge in __kmem_cache_alias(). For SLAB, SLAB_HWCACHE_ALIGN is
>>>>>> enabled by default for kmalloc slab, so align is 64 and size is 128
>>>>>> for
>>>>>> kmalloc-cg-96. So when unit_alloc() does kmalloc_node(96,
>>>>>> __GFP_ACCOUNT,
>>>>>> node), ksize() will return 128 instead of 96 for the returned
>>>>>> pointer.
>>>>>> SLUB has a similar merge logic, but because its align is 8 under
>>>>>> x86-64,
>>>>>> so the warning doesn't happen for i386 + SLUB, but I think the
>>>>>> similar
>>>>>> problem may exist for other architectures.
>>>>>> (2) kmalloc_size_roundup() returns the object_size of kmalloc-96
>>>>>> instead
>>>>>> of kmalloc-cg-96, so bpf_mem_cache_adjust_size() doesn't adjust
>>>>>> size_index accordingly. The reason why the object_size of
>>>>>> kmalloc-96 is
>>>>>> 96 instead of 128 is that there is slab merge for kmalloc-96.
>>>>>>
>>>>>> About how to fix the problem, I have two ideas:
>>>>>> The first is to introduce kmalloc_size_roundup_flags(), so
>>>>>> bpf_mem_cache_adjust_size() could use
>>>>>> kmalloc_size_roundup_flags(size,
>>>>>> __GFP_ACCOUNT) to get the object_size of kmalloc-cg-xxx. It could
>>>>>> fix
>>>>>> the warning for now, but the warning may pop-up occasionally due to
>>>>>> SLUB
>>>>>> merge and unusual slab align. The second is just using the
>>>>>> bpf_mem_cache
>>>>>> pointer to get the unit_size which is saved before the to-be-free
>>>>>> pointer. Its downside is that it may can not be able to skip the
>>>>>> free
>>>>>> operation for pointer which is not allocated from bpf ma, but I
>>>>>> think it
>>>>>> is acceptable. I prefer the latter solution. What do you think ?
>>>>> Is it possible that in bpf_mem_cache_adjust_size(), we do a series of
>>>>> kmalloc (for supported bucket size) and call ksize() to get the
>>>>> actual
>>>>> allocated object size. So eventually all possible allocated object
>>>>> sizes
>>>>> will be used for size_index[]. This will avoid all kind of special
>>>>> corner cases due to config/macro/arch etc. WDYT?
>>>> It is basically the same as the first proposed solution and it has the
>>>> same flaw. The problem is that slab merge can happen in any time,
>>>> so the
>>>> return value of ksize() may change even all passed pointers are
>>>> allocated from the same slab. Considering the following case:
>>>> during the
>>>> invocation of bpf_mem_cache_adjust_size() or the initialization of
>>>> bpf_global_ma, there is no slab merge and ksize() for a 96-bytes
>>>> object
>>>> returns 96. But after these invocations, a new slab created by a
>>>> kernel
>>>> module is merged to kmalloc-cg-96 and the object_size of kmalloc-cg-96
>>>> is adjust from 96 to 128 (which is possible for x86-64 + CONFIG_SLAB,
>>>> because it is alignment requirement is 64 for 96-bytes slab). So
>>>> soon or
>>> So, the object_size for allocated objects in that is adjusted from 96
>>> to 128
>>> while previously allocated objects should have no change, it is merely
>>> ksize(old_obj)
>>> previous return 96, now returns 128, right? Okay, so this is indeed a
>>> problem
>>> since we use ksize() to decide the bucket.
>> Yes. The object_size of underlying slab changes, so the return value of
>> ksize() will change as well.
>>>
>>>> later, when bpf_global_ma frees a 96-byte-sized pointer which is
>>>> allocated from a bpf_mem_cache in which unit_size is 96,
>>>> bpf_mem_free()
>>>> will free the pointer through a bpf_mem_cache in which unit_size is
>>>> 128,
>>>> because the return value of ksize() changes. Maybe we should
>>>> introduce a
>>>> new API in mm which returns size instead of object_size of underlying
>>>> slab, so the return value will not change due to slab merge.
>>> In this case, to avoid the warning, indeed we need to use '96' instead
>>> of '128'.
>>> So use the original ksize() return value is indeed a solution.
>>> We could use the mechanism similar to percpu alloc to save '96' in the
>>> memory.
>> We have already saved the pointer of bpf_mem_cache in the extra space
>> (aka LLIST_NODE_SZ) which is allocated together with the returned
>> pointer, so I think we could use bpf_mem_cache->unit_size to get the
>> size of the free pointer directly. I will check whether or not there is
>> performance degradation before posting the patch.

Post for status update.

There is no benchmark for bpf_mem_free(), so I am stilling write the
benchmark for bpf_mem_alloc() and bpf_mem_free(). After the benchmark is
ready (I think it will be next week), I will post the fix patch.
>
> See:
>
> bpf_local_storage.c:            err =
> bpf_mem_alloc_init(&smap->selem_ma, smap->elem_size, false);
> bpf_local_storage.c:            err =
> bpf_mem_alloc_init(&smap->storage_ma, sizeof(struct
> bpf_local_storage), false);
> core.c: ret = bpf_mem_alloc_init(&bpf_global_ma, 0, false);
> core.c: ret = bpf_mem_alloc_init(&bpf_global_percpu_ma, 0, true);
> cpumask.c:      ret = bpf_mem_alloc_init(&bpf_cpumask_ma,
> sizeof(struct bpf_cpumask), false);
> hashtab.c:              err = bpf_mem_alloc_init(&htab->ma,
> htab->elem_size, false);
> hashtab.c:                      err = bpf_mem_alloc_init(&htab->pcpu_ma,
> memalloc.c:int bpf_mem_alloc_init(struct bpf_mem_alloc *ma, int size,
> bool percpu)
>
> Some 'size' parameter in core.c is zero.
> Not sure how exactly you will resolve this issue based on
> bpf_mem_cache->unit_size. But looking forward to your patch!
>
>>
>> Regards,
>> Tao
>>
>
> .