Re: [PATCH v2] mm, slub: emit the "free" trace report before freeing memory in kmem_cache_free()

[John Hubbard <jhubbard@nvidia.com>]

On 11/2/21 04:43, Yunfeng Ye wrote:
> 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:
> 
>      CPU 0                 CPU 1
> 
>    (1) alloc xxxxxx
>                           (2) alloc xxxxxx
>    (3) free  xxxxxx
>                           (4) free  xxxxxx
> 
> This makes it look like CPU 1 somehow managed to allocate mmemory that


I see I created a typo for you, sorry about that: s/mmemory/memory/

But anyway, the wording looks good now. Please feel free to add:

Reviewed-by: John Hubbard <jhubbard@nvidia.com>


thanks,
-- 
John Hubbard
NVIDIA

> CPU 0 still had allocated for itself.
> 
> 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>
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> v1 -> v2:
>   - Modify the description
>   - Add "Reviewed-by"
> 
>   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);
>