Re: [PATCH] mm, slub: place the trace before freeing memory in kmem_cache_free()

[Yunfeng Ye <yeyunfeng@huawei.com>]

On 2021/11/2 15:03, John Hubbard wrote:
> On 10/30/21 03:11, Yunfeng Ye wrote:
>> After the memory is freed, it may be allocated by other CPUs and has
>> been recorded by trace. So the timing sequence of the memory tracing is
>> inaccurate.
>>
>> For example, we expect the following timing sequeuce:
>>
>>      CPU 0                 CPU 1
>>
>>    (1) alloc xxxxxx
>>    (2) free  xxxxxx
>>                           (3) alloc xxxxxx
>>                           (4) free  xxxxxx
>>
>> However, the following timing sequence may occur:
>>
>>      CPU 0                 CPU 1
>>
>>    (1) alloc xxxxxx
>>                           (2) alloc xxxxxx
>>    (3) free  xxxxxx
>>                           (4) free  xxxxxx
>>
>> So place the trace before freeing memory in kmem_cache_free().
> 
> Hi Yunfeng,
> 
> Like Muchun, I had some difficulty with the above description, but
> now I think I get it. :)
> 
> In order to make it easier for others, how about this wording and subject
> line, instead:
> 
Ok,I will modify the description in the next version patch.

Thanks.
> 
> mm, slub: emit the "free" trace report before freeing memory in kmem_cache_free()
> 
> After the memory is freed, it can be immediately allocated by other
> CPUs, before the "free" trace report has been emitted. This causes
> inaccurate traces.
> 
> For example, if the following sequence of events occurs:
> 
>     CPU 0                 CPU 1
> 
>   (1) alloc xxxxxx
>   (2) free  xxxxxx
>                          (3) alloc xxxxxx
>                          (4) free  xxxxxx
> 
> ...then they will be inaccurately reported via tracing, so that they
> appear to have happened in this order. This makes it look like CPU 1
> somehow managed to allocate mmemory that CPU 0 still had allocated for
> itself:
> 
>     CPU 0                 CPU 1
> 
>   (1) alloc xxxxxx
>                          (2) alloc xxxxxx
>   (3) free  xxxxxx
>                          (4) free  xxxxxx
> 
> In order to avoid this, emit the "free xxxxxx" tracing report just
> before the actual call to free the memory, instead of just after it.
> 
> 
>>
>> Signed-off-by: Yunfeng Ye <yeyunfeng@huawei.com>
>> ---
>>   mm/slub.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 432145d7b4ec..427e62034c3f 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -3526,8 +3526,8 @@ void kmem_cache_free(struct kmem_cache *s, void *x)
>>       s = cache_from_obj(s, x);
>>       if (!s)
>>           return;
>> -    slab_free(s, virt_to_head_page(x), x, NULL, 1, _RET_IP_);
>>       trace_kmem_cache_free(_RET_IP_, x, s->name);
>> +    slab_free(s, virt_to_head_page(x), x, NULL, 1, _RET_IP_);
>>   }
>>   EXPORT_SYMBOL(kmem_cache_free);
>>
> 
> ...the diffs seem correct, too, but I'm not exactly a slub reviewer, so
> take that for what it's worth.
> 
> 
> thanks,