Re: [PATCH 1/3] Slab infrastructure for array operations

[David Rientjes <rientjes@google.com>]

On Tue, 10 Feb 2015, Christoph Lameter wrote:

> This patch adds the basic infrastructure for alloc / free operations
> on pointer arrays. It includes a fallback function that can perform
> the array operations using the single alloc and free that every
> slab allocator performs.
> 
> Allocators must define _HAVE_SLAB_ALLOCATOR_OPERATIONS in their
> header files in order to implement their own fast version for
> these array operations.
> 
> Array operations allow a reduction of the processing overhead
> during allocation and therefore speed up acquisition of larger
> amounts of objects.
> 

This doesn't apply to -mm because of commits f707780a2121 ("slab: embed 
memcg_cache_params to kmem_cache") and 0d48f42820db ("memcg: free 
memcg_caches slot on css offline"), but it should be trivial to resolve.

> Signed-off-by: Christoph Lameter <cl@linux.com>
> 
> Index: linux/include/linux/slab.h
> ===================================================================
> --- linux.orig/include/linux/slab.h
> +++ linux/include/linux/slab.h
> @@ -123,6 +123,7 @@ struct kmem_cache *memcg_create_kmem_cac
>  void kmem_cache_destroy(struct kmem_cache *);
>  int kmem_cache_shrink(struct kmem_cache *);
>  void kmem_cache_free(struct kmem_cache *, void *);
> +void kmem_cache_free_array(struct kmem_cache *, size_t, void **);
>  
>  /*
>   * Please use this macro to create slab caches. Simply specify the
> @@ -289,6 +290,8 @@ static __always_inline int kmalloc_index
>  
>  void *__kmalloc(size_t size, gfp_t flags);
>  void *kmem_cache_alloc(struct kmem_cache *, gfp_t flags);
> +int kmem_cache_alloc_array(struct kmem_cache *, gfp_t gfpflags,
> +				size_t nr, void **);
>  
>  #ifdef CONFIG_NUMA
>  void *__kmalloc_node(size_t size, gfp_t flags, int node);
> Index: linux/mm/slab_common.c
> ===================================================================
> --- linux.orig/mm/slab_common.c
> +++ linux/mm/slab_common.c
> @@ -105,6 +105,83 @@ static inline int kmem_cache_sanity_chec
>  }
>  #endif
>  
> +/*
> + * Fallback function that just calls kmem_cache_alloc
> + * for each element. This may be used if not all
> + * objects can be allocated or as a generic fallback
> + * if the allocator cannot support buik operations.
> + */
> +int __kmem_cache_alloc_array(struct kmem_cache *s,
> +		gfp_t flags, size_t nr, void **p)
> +{
> +	int i;
> +
> +	for (i = 0; i < nr; i++) {
> +		void *x = kmem_cache_alloc(s, flags);
> +
> +		if (!x)
> +			return i;
> +		p[i] = x;
> +	}
> +	return i;
> +}

If size_t is unsigned long and i is int and i overflows then bad things 
happen.  I don't expect that we'll have any callers that have such large 
values of nr, but it shouldn't index negatively into an array.

> +
> +int kmem_cache_alloc_array(struct kmem_cache *s,
> +		gfp_t flags, size_t nr, void **p)
> +{
> +	int i = 0;
> +
> +#ifdef _HAVE_SLAB_ALLOCATOR_ARRAY_OPERATIONS
> +	/*
> +	 * First extract objects from partial lists in order to
> +	 * avoid further fragmentation.
> +	 */
> +	i += slab_array_alloc_from_partial(s, nr - i, p + i);
> +
> +	/*
> +	 * If there are still a larger number of objects to be allocated
> +	 * use the page allocator directly.
> +	 */
> +	if (nr - i > objects_per_slab_page(s))
> +		i += slab_array_alloc_from_page_allocator(s,
> +				flags, nr - i, p + i);
> +
> +	/* Get per cpu objects that may be available */
> +	if (i < nr)
> +		i += slab_array_alloc_from_local(s, nr - i, p + i);
> +
> +#endif

This patch is referencing functions that don't exist and can do so since 
it's not compiled, but I think this belongs in the next patch.  I also 
think that this particular implementation may be slub-specific so I would 
have expected just a call to an allocator-defined
__kmem_cache_alloc_array() here with i = __kmem_cache_alloc_array().

If that's done, then slab and slob can just define a dummy inline 
__kmem_cache_alloc_array() functions that 
return 0 instead of using _HAVE_SLAB_ALLOCATOR_ARRAY_OPERATIONS at all.

> +	/*
> +	 * If a fully filled array has been requested then fill it
> +	 * up if there are objects missing using the regular kmem_cache_alloc()
> +	 */
> +	if (i < nr)
> +		i += __kmem_cache_alloc_array(s, flags, nr - i, p + i);
> +
> +	return i;
> +}
> +EXPORT_SYMBOL(kmem_cache_alloc_array);
> +
> +/*
> + * Fallback function for objects that an allocator does not want
> + * to deal with or for allocators that do not support bulk operations.
> + */
> +void __kmem_cache_free_array(struct kmem_cache *s, size_t nr, void **p)
> +{
> +	int i;
> +
> +	for (i = 0; i < nr; i++)
> +		kmem_cache_free(s, p[i]);
> +}
> +
> +#ifndef _HAVE_SLAB_ALLOCATOR_ARRAY_OPERATIONS
> +void kmem_cache_free_array(struct kmem_cache *s, size_t nr, void **p)
> +{
> +	__kmem_cache_free_array(s, nr, p);
> +}
> +EXPORT_SYMBOL(kmem_cache_free_array);
> +#endif
> +

Hmm, not sure why the allocator would be required to do the 
EXPORT_SYMBOL() if it defines kmem_cache_free_array() itself.  This 
becomes simpler if you remove _HAVE_SLAB_ALLOCATOR_ARRAY_OPERATIONS 
entirely and just have slab and slob do __kmem_cache_free_array() 
behavior.

>  #ifdef CONFIG_MEMCG_KMEM
>  static int memcg_alloc_cache_params(struct mem_cgroup *memcg,
>  		struct kmem_cache *s, struct kmem_cache *root_cache)
> Index: linux/mm/slab.h
> ===================================================================
> --- linux.orig/mm/slab.h
> +++ linux/mm/slab.h
> @@ -69,6 +69,9 @@ extern struct kmem_cache *kmem_cache;
>  unsigned long calculate_alignment(unsigned long flags,
>  		unsigned long align, unsigned long size);
>  
> +/* Determine the number of objects per slab page */
> +unsigned objects_per_slab_page(struct kmem_cache *);

Seems like it should be in the next patch.

> +
>  #ifndef CONFIG_SLOB
>  /* Kmalloc array related functions */
>  void create_kmalloc_caches(unsigned long);
> @@ -362,4 +365,12 @@ void *slab_next(struct seq_file *m, void
>  void slab_stop(struct seq_file *m, void *p);
>  int memcg_slab_show(struct seq_file *m, void *p);
>  
> +void __kmem_cache_free_array(struct kmem_cache *s, int nr, void **p);
> +void __kmem_cache_alloc_array(struct kmem_cache *s, gfp_t flags, int nr, void **p);

Longer than 80 chars.

> +
> +int slab_array_alloc_from_partial(struct kmem_cache *s, size_t nr, void **p);
> +int slab_array_alloc_from_local(struct kmem_cache *s, size_t nr, void **p);
> +int slab_array_alloc_from_page_allocator(struct kmem_cache *s, gfp_t flags,
> +					size_t nr, void **p);
> +
>  #endif /* MM_SLAB_H */
> Index: linux/mm/slub.c
> ===================================================================
> --- linux.orig/mm/slub.c
> +++ linux/mm/slub.c
> @@ -332,6 +332,11 @@ static inline int oo_objects(struct kmem
>  	return x.x & OO_MASK;
>  }
>  
> +unsigned objects_per_slab_page(struct kmem_cache *s)
> +{
> +	return oo_objects(s->oo);
> +}
> +
>  /*
>   * Per slab locking using the pagelock
>   */

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