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

[Hyeonggon Yoo]

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?

> 
> 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
> 
> 
> 

-- 
Thanks,
Hyeonggon