Re: [PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[Joonsoo Kim]

On Tue, Dec 08, 2020 at 05:12:59PM -0800, paulmck@xxxxxxxxxx wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> 
> There are kernel facilities such as per-CPU reference counts that give
> error messages in generic handlers or callbacks, whose messages are
> unenlightening.  In the case of per-CPU reference-count underflow, this
> is not a problem when creating a new use of this facility because in that
> case the bug is almost certainly in the code implementing that new use.
> However, trouble arises when deploying across many systems, which might
> exercise corner cases that were not seen during development and testing.
> Here, it would be really nice to get some kind of hint as to which of
> several uses the underflow was caused by.
> 
> This commit therefore exposes a mem_dump_obj() function that takes
> a pointer to memory (which must still be allocated if it has been
> dynamically allocated) and prints available information on where that
> memory came from.  This pointer can reference the middle of the block as
> well as the beginning of the block, as needed by things like RCU callback
> functions and timer handlers that might not know where the beginning of
> the memory block is.  These functions and handlers can use mem_dump_obj()
> to print out better hints as to where the problem might lie.
> 
> The information printed can depend on kernel configuration.  For example,
> the allocation return address can be printed only for slab and slub,
> and even then only when the necessary debug has been enabled.  For slab,
> build with CONFIG_DEBUG_SLAB=y, and either use sizes with ample space
> to the next power of two or use the SLAB_STORE_USER when creating the
> kmem_cache structure.  For slub, build with CONFIG_SLUB_DEBUG=y and
> boot with slub_debug=U, or pass SLAB_STORE_USER to kmem_cache_create()
> if more focused use is desired.  Also for slub, use CONFIG_STACKTRACE
> to enable printing of the allocation-time stack trace.
> 
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: <linux-mm@xxxxxxxxx>
> Reported-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> [ paulmck: Convert to printing and change names per Joonsoo Kim. ]
> [ paulmck: Move slab definition per Stephen Rothwell and kbuild test robot. ]
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

Introducing three functions, kmem_valid_obj(), kmem_provenance(),
mem_dump_obj() looks better than patchset v1. Nice work. Few comments
below.

> ---
>  include/linux/mm.h   |  2 ++
>  include/linux/slab.h |  2 ++
>  mm/slab.c            | 28 +++++++++++++++++++++
>  mm/slab.h            | 11 +++++++++
>  mm/slab_common.c     | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  mm/slob.c            |  7 ++++++
>  mm/slub.c            | 40 ++++++++++++++++++++++++++++++
>  mm/util.c            | 25 +++++++++++++++++++
>  8 files changed, 184 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ef360fe..1eea266 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3153,5 +3153,7 @@ unsigned long wp_shared_mapping_range(struct address_space *mapping,
>  
>  extern int sysctl_nr_trim_pages;
>  
> +void mem_dump_obj(void *object);
> +
>  #endif /* __KERNEL__ */
>  #endif /* _LINUX_MM_H */
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index dd6897f..169b511 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -186,6 +186,8 @@ void kfree(const void *);
>  void kfree_sensitive(const void *);
>  size_t __ksize(const void *);
>  size_t ksize(const void *);
> +bool kmem_valid_obj(void *object);
> +void kmem_dump_obj(void *object);
>  
>  #ifdef CONFIG_HAVE_HARDENED_USERCOPY_ALLOCATOR
>  void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
> diff --git a/mm/slab.c b/mm/slab.c
> index b111356..72b6743 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3602,6 +3602,34 @@ void *kmem_cache_alloc_node_trace(struct kmem_cache *cachep,
>  EXPORT_SYMBOL(kmem_cache_alloc_node_trace);
>  #endif
>  
> +void kmem_provenance(struct kmem_provenance *kpp)

To open up the possibility of future enhancement, name, provenance,
looks not good to me. This function could be used to extract various
object information so such as kmem_obj_info() looks better to me. Any
thought?

> +{
> +#ifdef DEBUG
> +	struct kmem_cache *cachep;
> +	void *object = kpp->kp_ptr;
> +	unsigned int objnr;
> +	void *objp;
> +	struct page *page = kpp->kp_page;
> +
> +	cachep = page->slab_cache;
> +	if (!(cachep->flags & SLAB_STORE_USER)) {
> +		kpp->kp_ret = NULL;
> +		goto nodebug;
> +	}
> +	objp = object - obj_offset(cachep);
> +	page = virt_to_head_page(objp);
> +	objnr = obj_to_index(cachep, page, objp);
> +	objp = index_to_obj(cachep, page, objnr);
> +	kpp->kp_objp = objp;
> +	kpp->kp_ret = *dbg_userword(cachep, objp);
> +nodebug:
> +#else
> +	kpp->kp_ret = NULL;
> +#endif
> +	if (kpp->kp_nstack)
> +		kpp->kp_stack[0] = NULL;
> +}
> +
>  static __always_inline void *
>  __do_kmalloc_node(size_t size, gfp_t flags, int node, unsigned long caller)
>  {
> diff --git a/mm/slab.h b/mm/slab.h
> index 6d7c6a5..28a41d5 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -630,4 +630,15 @@ static inline bool slab_want_init_on_free(struct kmem_cache *c)
>  	return false;
>  }
>  
> +#define KS_ADDRS_COUNT 16
> +struct kmem_provenance {
> +	void *kp_ptr;
> +	struct page *kp_page;
> +	void *kp_objp;
> +	void *kp_ret;
> +	void *kp_stack[KS_ADDRS_COUNT];
> +	int kp_nstack;
> +};
> +void kmem_provenance(struct kmem_provenance *kpp);
> +
>  #endif /* MM_SLAB_H */
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index f9ccd5d..09f0cbc 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -536,6 +536,75 @@ bool slab_is_available(void)
>  	return slab_state >= UP;
>  }
>  
> +/**
> + * kmem_valid_obj - does the pointer reference a valid slab object?
> + * @object: pointer to query.
> + *
> + * Return: %true if the pointer is to a not-yet-freed object from
> + * kmalloc() or kmem_cache_alloc(), either %true or %false if the pointer
> + * is to an already-freed object, and %false otherwise.
> + */
> +bool kmem_valid_obj(void *object)
> +{
> +	struct page *page;
> +
> +	if (!virt_addr_valid(object))
> +		return false;
> +	page = virt_to_head_page(object);
> +	return PageSlab(page);
> +}
> +EXPORT_SYMBOL_GPL(kmem_valid_obj);
> +
> +/**
> + * kmem_dump_obj - Print available slab provenance information
> + * @object: slab object for which to find provenance information.
> + *
> + * This function uses pr_cont(), so that the caller is expected to have
> + * printed out whatever preamble is appropriate.  The provenance information
> + * depends on the type of object and on how much debugging is enabled.
> + * For a slab-cache object, the fact that it is a slab object is printed,
> + * and, if available, the slab name, return address, and stack trace from
> + * the allocation of that object.
> + *
> + * This function will splat if passed a pointer to a non-slab object.
> + * If you are not sure what type of object you have, you should instead
> + * use mem_dump_obj().
> + */
> +void kmem_dump_obj(void *object)
> +{
> +	int i;
> +	struct page *page;
> +	struct kmem_provenance kp;
> +
> +	if (WARN_ON_ONCE(!virt_addr_valid(object)))
> +		return;
> +	page = virt_to_head_page(object);
> +	if (WARN_ON_ONCE(!PageSlab(page))) {
> +		pr_cont(" non-slab memory.\n");
> +		return;
> +	}
> +	kp.kp_ptr = object;
> +	kp.kp_page = page;
> +	kp.kp_nstack = KS_ADDRS_COUNT;

I hope that kmem_dump_obj() doesn't set any kp fields. It's the job
reserved for kmem_provenance().

> +	kmem_provenance(&kp);
> +	if (page->slab_cache)
> +		pr_cont(" slab %s", page->slab_cache->name);

Rather than accessing page->slab_cache, it's better to introduce
slab_cache field on kp and use it. Note that slob doesn't use
page->slab_cache. In slob, that field on struct page would be NULL so
it would not cause a problem. But using kp makes things clear.

> +	else
> +		pr_cont(" slab ");
> +	if (kp.kp_ret)
> +		pr_cont(" allocated at %pS\n", kp.kp_ret);
> +	else
> +		pr_cont("\n");
> +	if (kp.kp_stack[0]) {

This check would be useless since we check it on every iteration.
 
Thanks.