Re: [PATCH v6 1/4] mm/slub: enable debugging memory wasting of kmalloc

[Vlastimil Babka]

On 11/3/22 17:57, Vlastimil Babka wrote:
> On 11/3/22 15:36, Hyeonggon Yoo wrote:
>> On Thu, Nov 03, 2022 at 10:16:12PM +0800, Feng Tang wrote:
>>> On Thu, Nov 03, 2022 at 09:33:28AM +0100, Vlastimil Babka wrote:
>>> [...]
>>> > >> AFAICS before this patch, we "survive" "kmem_cache *s" being NULL as
>>> > >> slab_pre_alloc_hook() will happen to return NULL and we bail out from
>>> > >> slab_alloc_node(). But this is a side-effect, not an intended protection.
>>> > >> Also the CONFIG_TRACING variant of kmalloc_trace() would have called
>>> > >> trace_kmalloc dereferencing s->size anyway even before this patch.
>>> > >> 
>>> > >> I don't think we should add WARNS in the slab hot paths just to prevent this
>>> > >> rare error of using slab too early. At most VM_WARN... would be acceptable
>>> > >> but still not necessary as crashing immediately from a NULL pointer is
>>> > >> sufficient.
>>> > >> 
>>> > >> So IMHO mips should fix their soc init, 
>>> > > 
>>> > > Yes, for the mips fix, John has proposed to defer the calling of prom_soc_init(),
>>> > > which looks reasonable.
>>> > > 
>>> > >> and we should look into the
>>> > >> CONFIG_TRACING=n variant of kmalloc_trace(), to pass orig_size properly.
>>> > > 
>>> > > You mean check if the pointer is NULL and bail out early. 
>>> > 
>>> > No I mean here:
>>> > 
>>> > #else /* CONFIG_TRACING */
>>> > /* Save a function call when CONFIG_TRACING=n */
>>> > static __always_inline __alloc_size(3)                                   
>>> > void *kmalloc_trace(struct kmem_cache *s, gfp_t flags, size_t size)
>>> > {       
>>> >         void *ret = kmem_cache_alloc(s, flags);
>>> >                     
>>> >         ret = kasan_kmalloc(s, ret, size, flags);
>>> >         return ret;
>>> > }
>>> > 
>>> > we call kmem_cache_alloc() and discard the size parameter, so it will assume
>>> > s->object_size (and as the side-effect, crash if s is NULL). We shouldn't
>>> > add "s is NULL?" checks, but fix passing the size - probably switch to
>>> > __kmem_cache_alloc_node()? and in the following kmalloc_node_trace() analogically.
>>>  
>>> Got it, thanks! I might have missed it during some rebasing for the
>>> kmalloc wastage debug patch.
>> 
>> That was good catch and I missed too!
>> But FYI I'm suggesting to drop CONFIG_TRACING=n variant:
>> 
>> https://lore.kernel.org/linux-mm/20221101222520.never.109-kees@xxxxxxxxxx/T/#m20ecf14390e406247bde0ea9cce368f469c539ed
>> 
>> Any thoughts?
> 
> I'll get to it, also I think we were pondering that within your series too,
> but I wanted to postpone in case somebody objects to the extra function call
> it creates.
> But that would be for 6.2 anyway while I'll collect the fix here for 6.1.

On second thought, the fix is making the inlined kmalloc_trace() expand to a
call that had 2 parameters and now it has 5, which seems to me like a worse
thing (code bloat) than the function call. With the other reasons to ditch
the CONFIG_TRACING=n variant I'm inclined to just do it right now.

>>> 
>>> How about the following fix?
>>> 
>>> Thanks,
>>> Feng
>>> 
>>> ---
>>> From 9f9fa9da8946fd44625f873c0f51167357075be1 Mon Sep 17 00:00:00 2001
>>> From: Feng Tang <feng.tang@xxxxxxxxx>
>>> Date: Thu, 3 Nov 2022 21:32:10 +0800
>>> Subject: [PATCH] mm/slub: Add missing orig_size parameter for wastage debug
>>> 
>>> commit 6edf2576a6cc ("mm/slub: enable debugging memory wasting of
>>> kmalloc") was introduced for debugging kmalloc memory wastage,
>>> and it missed to pass the original request size for kmalloc_trace()
>>> and kmalloc_node_trace() in CONFIG_TRACING=n path.
>>> 
>>> Fix it by using __kmem_cache_alloc_node() with correct original
>>> request size.
>>> 
>>> Fixes: 6edf2576a6cc ("mm/slub: enable debugging memory wasting of kmalloc")
>>> Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
>>> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
>>> ---
>>>  include/linux/slab.h | 9 +++++++--
>>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>> 
>>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>>> index 90877fcde70b..9691afa569e1 100644
>>> --- a/include/linux/slab.h
>>> +++ b/include/linux/slab.h
>>> @@ -469,6 +469,9 @@ void *__kmalloc_node(size_t size, gfp_t flags, int node) __assume_kmalloc_alignm
>>>  							 __alloc_size(1);
>>>  void *kmem_cache_alloc_node(struct kmem_cache *s, gfp_t flags, int node) __assume_slab_alignment
>>>  									 __malloc;
>>> +void *__kmem_cache_alloc_node(struct kmem_cache *s, gfp_t flags, int node,
>>> +				size_t orig_size, unsigned long caller) __assume_slab_alignment
>>> +									 __malloc;
>>>  
>>>  #ifdef CONFIG_TRACING
>>>  void *kmalloc_trace(struct kmem_cache *s, gfp_t flags, size_t size)
>>> @@ -482,7 +485,8 @@ void *kmalloc_node_trace(struct kmem_cache *s, gfp_t gfpflags,
>>>  static __always_inline __alloc_size(3)
>>>  void *kmalloc_trace(struct kmem_cache *s, gfp_t flags, size_t size)
>>>  {
>>> -	void *ret = kmem_cache_alloc(s, flags);
>>> +	void *ret = __kmem_cache_alloc_node(s, flags, NUMA_NO_NODE,
>>> +					    size, _RET_IP_);
>>>  
>>>  	ret = kasan_kmalloc(s, ret, size, flags);
>>>  	return ret;
>>> @@ -492,7 +496,8 @@ static __always_inline __alloc_size(4)
>>>  void *kmalloc_node_trace(struct kmem_cache *s, gfp_t gfpflags,
>>>  			 int node, size_t size)
>>>  {
>>> -	void *ret = kmem_cache_alloc_node(s, gfpflags, node);
>>> +	void *ret = __kmem_cache_alloc_node(s, gfpflags, node,
>>> +					    size, _RET_IP_);
>>>  
>>>  	ret = kasan_kmalloc(s, ret, size, gfpflags);
>>>  	return ret;
>>> -- 
>>> 2.34.1
>>> 
>>> 
>>> 
>> 
>