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

From: John Hubbard
Date: Tue Nov 02 2021 - 14:37:18 EST


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@xxxxxxxxxx>


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@xxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
---
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);