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Date:   Tue, 2 Nov 2021 11:37:11 -0700
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Subject: Re: [PATCH v2] mm, slub: emit the "free" trace report before freeing
 memory in kmem_cache_free()
Content-Language: en-US
To:     Yunfeng Ye <yeyunfeng@huawei.com>, cl@linux.com,
        penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com,
        Andrew Morton <akpm@linux-foundation.org>, vbabka@suse.cz,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc:     songmuchun@bytedance.com, willy@infradead.org, wuxu.wu@huawei.com,
        Hewenliang <hewenliang4@huawei.com>
References: <a6824ebe-a0ad-fedb-ada3-c362f9c8f363@huawei.com>
From:   John Hubbard <jhubbard@nvidia.com>
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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);
>