Re: [PATCH v2 4/5] kasan, mm: integrate slab init_on_alloc with HW_TAGS

[Marco Elver]

On Mon, Mar 08, 2021 at 04:55PM +0100, Andrey Konovalov wrote:
> This change uses the previously added memory initialization feature
> of HW_TAGS KASAN routines for slab memory when init_on_alloc is enabled.
> 
> With this change, memory initialization memset() is no longer called
> when both HW_TAGS KASAN and init_on_alloc are enabled. Instead, memory
> is initialized in KASAN runtime.
> 
> The memory initialization memset() is moved into slab_post_alloc_hook()
> that currently directly follows the initialization loop. A new argument
> is added to slab_post_alloc_hook() that indicates whether to initialize
> the memory or not.

This is a pretty intrusive change to the internal slab APIs. However, I
think this is a positive cleanup, removing some code duplication, so I
hope this is the right thing to do.

> To avoid discrepancies with which memory gets initialized that can be
> caused by future changes, both KASAN hook and initialization memset()
> are put together and a warning comment is added.
> 
> Combining setting allocation tags with memory initialization improves
> HW_TAGS KASAN performance when init_on_alloc is enabled.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>

The code looks fine to me, but there are some non-obvious changes to the
internal slab APIs, so I'd wait a bit more to see if we missed
something.

> ---
>  include/linux/kasan.h |  8 ++++----
>  mm/kasan/common.c     |  4 ++--
>  mm/slab.c             | 28 +++++++++++++---------------
>  mm/slab.h             | 17 +++++++++++++----
>  mm/slub.c             | 27 +++++++++++----------------
>  5 files changed, 43 insertions(+), 41 deletions(-)
> 
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index c89613caa8cf..85f2a8786606 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -226,12 +226,12 @@ static __always_inline void kasan_slab_free_mempool(void *ptr)
>  }
>  
>  void * __must_check __kasan_slab_alloc(struct kmem_cache *s,
> -				       void *object, gfp_t flags);
> +				       void *object, gfp_t flags, bool init);
>  static __always_inline void * __must_check kasan_slab_alloc(
> -				struct kmem_cache *s, void *object, gfp_t flags)
> +		struct kmem_cache *s, void *object, gfp_t flags, bool init)
>  {
>  	if (kasan_enabled())
> -		return __kasan_slab_alloc(s, object, flags);
> +		return __kasan_slab_alloc(s, object, flags, init);
>  	return object;
>  }
>  
> @@ -320,7 +320,7 @@ static inline bool kasan_slab_free(struct kmem_cache *s, void *object)
>  static inline void kasan_kfree_large(void *ptr) {}
>  static inline void kasan_slab_free_mempool(void *ptr) {}
>  static inline void *kasan_slab_alloc(struct kmem_cache *s, void *object,
> -				   gfp_t flags)
> +				   gfp_t flags, bool init)
>  {
>  	return object;
>  }
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 6107c795611f..7ea747b18c26 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -428,7 +428,7 @@ static void set_alloc_info(struct kmem_cache *cache, void *object,
>  }
>  
>  void * __must_check __kasan_slab_alloc(struct kmem_cache *cache,
> -					void *object, gfp_t flags)
> +					void *object, gfp_t flags, bool init)
>  {
>  	u8 tag;
>  	void *tagged_object;
> @@ -453,7 +453,7 @@ void * __must_check __kasan_slab_alloc(struct kmem_cache *cache,
>  	 * Unpoison the whole object.
>  	 * For kmalloc() allocations, kasan_kmalloc() will do precise poisoning.
>  	 */
> -	kasan_unpoison(tagged_object, cache->object_size, false);
> +	kasan_unpoison(tagged_object, cache->object_size, init);
>  
>  	/* Save alloc info (if possible) for non-kmalloc() allocations. */
>  	if (kasan_stack_collection_enabled())
> diff --git a/mm/slab.c b/mm/slab.c
> index 51fd424e0d6d..936dd686dec9 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3216,6 +3216,7 @@ slab_alloc_node(struct kmem_cache *cachep, gfp_t flags, int nodeid, size_t orig_
>  	void *ptr;
>  	int slab_node = numa_mem_id();
>  	struct obj_cgroup *objcg = NULL;
> +	bool init = false;
>  
>  	flags &= gfp_allowed_mask;
>  	cachep = slab_pre_alloc_hook(cachep, &objcg, 1, flags);
> @@ -3254,12 +3255,10 @@ slab_alloc_node(struct kmem_cache *cachep, gfp_t flags, int nodeid, size_t orig_
>    out:
>  	local_irq_restore(save_flags);
>  	ptr = cache_alloc_debugcheck_after(cachep, flags, ptr, caller);
> -
> -	if (unlikely(slab_want_init_on_alloc(flags, cachep)) && ptr)
> -		memset(ptr, 0, cachep->object_size);
> +	init = slab_want_init_on_alloc(flags, cachep);
>  
>  out_hooks:
> -	slab_post_alloc_hook(cachep, objcg, flags, 1, &ptr);
> +	slab_post_alloc_hook(cachep, objcg, flags, 1, &ptr, init);
>  	return ptr;
>  }
>  
> @@ -3301,6 +3300,7 @@ slab_alloc(struct kmem_cache *cachep, gfp_t flags, size_t orig_size, unsigned lo
>  	unsigned long save_flags;
>  	void *objp;
>  	struct obj_cgroup *objcg = NULL;
> +	bool init = false;
>  
>  	flags &= gfp_allowed_mask;
>  	cachep = slab_pre_alloc_hook(cachep, &objcg, 1, flags);
> @@ -3317,12 +3317,10 @@ slab_alloc(struct kmem_cache *cachep, gfp_t flags, size_t orig_size, unsigned lo
>  	local_irq_restore(save_flags);
>  	objp = cache_alloc_debugcheck_after(cachep, flags, objp, caller);
>  	prefetchw(objp);
> -
> -	if (unlikely(slab_want_init_on_alloc(flags, cachep)) && objp)
> -		memset(objp, 0, cachep->object_size);
> +	init = slab_want_init_on_alloc(flags, cachep);
>  
>  out:
> -	slab_post_alloc_hook(cachep, objcg, flags, 1, &objp);
> +	slab_post_alloc_hook(cachep, objcg, flags, 1, &objp, init);
>  	return objp;
>  }
>  
> @@ -3542,18 +3540,18 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>  
>  	cache_alloc_debugcheck_after_bulk(s, flags, size, p, _RET_IP_);
>  
> -	/* Clear memory outside IRQ disabled section */
> -	if (unlikely(slab_want_init_on_alloc(flags, s)))
> -		for (i = 0; i < size; i++)
> -			memset(p[i], 0, s->object_size);
> -
> -	slab_post_alloc_hook(s, objcg, flags, size, p);
> +	/*
> +	 * memcg and kmem_cache debug support and memory initialization.
> +	 * Done outside of the IRQ disabled section.
> +	 */
> +	slab_post_alloc_hook(s, objcg, flags, size, p,
> +				slab_want_init_on_alloc(flags, s));
>  	/* FIXME: Trace call missing. Christoph would like a bulk variant */
>  	return size;
>  error:
>  	local_irq_enable();
>  	cache_alloc_debugcheck_after_bulk(s, flags, i, p, _RET_IP_);
> -	slab_post_alloc_hook(s, objcg, flags, i, p);
> +	slab_post_alloc_hook(s, objcg, flags, i, p, false);
>  	__kmem_cache_free_bulk(s, i, p);
>  	return 0;
>  }
> diff --git a/mm/slab.h b/mm/slab.h
> index 076582f58f68..c6f0e55a674a 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -506,15 +506,24 @@ static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
>  }
>  
>  static inline void slab_post_alloc_hook(struct kmem_cache *s,
> -					struct obj_cgroup *objcg,
> -					gfp_t flags, size_t size, void **p)
> +					struct obj_cgroup *objcg, gfp_t flags,
> +					size_t size, void **p, bool init)
>  {
>  	size_t i;
>  
>  	flags &= gfp_allowed_mask;
> +
> +	/*
> +	 * As memory initialization might be integrated into KASAN,
> +	 * kasan_slab_alloc and initialization memset must be
> +	 * kept together to avoid discrepancies in behavior.
> +	 *
> +	 * As p[i] might get tagged, memset and kmemleak hook come after KASAN.
> +	 */
>  	for (i = 0; i < size; i++) {
> -		p[i] = kasan_slab_alloc(s, p[i], flags);
> -		/* As p[i] might get tagged, call kmemleak hook after KASAN. */
> +		p[i] = kasan_slab_alloc(s, p[i], flags, init);
> +		if (p[i] && init && !kasan_has_integrated_init())
> +			memset(p[i], 0, s->object_size);
>  		kmemleak_alloc_recursive(p[i], s->object_size, 1,
>  					 s->flags, flags);
>  	}
> diff --git a/mm/slub.c b/mm/slub.c
> index e26c274b4657..f53df23760e3 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2822,6 +2822,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
>  	struct page *page;
>  	unsigned long tid;
>  	struct obj_cgroup *objcg = NULL;
> +	bool init = false;
>  
>  	s = slab_pre_alloc_hook(s, &objcg, 1, gfpflags);
>  	if (!s)
> @@ -2899,12 +2900,10 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
>  	}
>  
>  	maybe_wipe_obj_freeptr(s, object);
> -
> -	if (unlikely(slab_want_init_on_alloc(gfpflags, s)) && object)
> -		memset(kasan_reset_tag(object), 0, s->object_size);
> +	init = slab_want_init_on_alloc(gfpflags, s);
>  
>  out:
> -	slab_post_alloc_hook(s, objcg, gfpflags, 1, &object);
> +	slab_post_alloc_hook(s, objcg, gfpflags, 1, &object, init);
>  
>  	return object;
>  }
> @@ -3356,20 +3355,16 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>  	c->tid = next_tid(c->tid);
>  	local_irq_enable();
>  
> -	/* Clear memory outside IRQ disabled fastpath loop */
> -	if (unlikely(slab_want_init_on_alloc(flags, s))) {
> -		int j;
> -
> -		for (j = 0; j < i; j++)
> -			memset(kasan_reset_tag(p[j]), 0, s->object_size);
> -	}
> -
> -	/* memcg and kmem_cache debug support */
> -	slab_post_alloc_hook(s, objcg, flags, size, p);
> +	/*
> +	 * memcg and kmem_cache debug support and memory initialization.
> +	 * Done outside of the IRQ disabled fastpath loop.
> +	 */
> +	slab_post_alloc_hook(s, objcg, flags, size, p,
> +				slab_want_init_on_alloc(flags, s));
>  	return i;
>  error:
>  	local_irq_enable();
> -	slab_post_alloc_hook(s, objcg, flags, i, p);
> +	slab_post_alloc_hook(s, objcg, flags, i, p, false);
>  	__kmem_cache_free_bulk(s, i, p);
>  	return 0;
>  }
> @@ -3579,7 +3574,7 @@ static void early_kmem_cache_node_alloc(int node)
>  	init_object(kmem_cache_node, n, SLUB_RED_ACTIVE);
>  	init_tracking(kmem_cache_node, n);
>  #endif
> -	n = kasan_slab_alloc(kmem_cache_node, n, GFP_KERNEL);
> +	n = kasan_slab_alloc(kmem_cache_node, n, GFP_KERNEL, false);
>  	page->freelist = get_freepointer(kmem_cache_node, n);
>  	page->inuse = 1;
>  	page->frozen = 0;
> -- 
> 2.30.1.766.gb4fecdf3b7-goog
>