Re: [PATCH v2 6/6] mm/slab: document kfree() as allowed for kmem_cache_alloc() objects

[Mike Rapoport]

On Fri, Mar 17, 2023 at 11:43:07AM +0100, Vlastimil Babka wrote:
> This will make it easier to free objects in situations when they can
> come from either kmalloc() or kmem_cache_alloc(), and also allow
> kfree_rcu() for freeing objects from kmem_cache_alloc().
> 
> For the SLAB and SLUB allocators this was always possible so with SLOB
> gone, we can document it as supported.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>

Reviewed-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>

> ---
>  Documentation/core-api/memory-allocation.rst | 17 +++++++++++++----
>  include/linux/rcupdate.h                     |  6 ++++--
>  mm/slab_common.c                             |  5 +----
>  3 files changed, 18 insertions(+), 10 deletions(-)
> 
> diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
> index 5954ddf6ee13..1c58d883b273 100644
> --- a/Documentation/core-api/memory-allocation.rst
> +++ b/Documentation/core-api/memory-allocation.rst
> @@ -170,7 +170,16 @@ should be used if a part of the cache might be copied to the userspace.
>  After the cache is created kmem_cache_alloc() and its convenience
>  wrappers can allocate memory from that cache.
>  
> -When the allocated memory is no longer needed it must be freed. You can
> -use kvfree() for the memory allocated with `kmalloc`, `vmalloc` and
> -`kvmalloc`. The slab caches should be freed with kmem_cache_free(). And
> -don't forget to destroy the cache with kmem_cache_destroy().
> +When the allocated memory is no longer needed it must be freed.
> +
> +Objects allocated by `kmalloc` can be freed by `kfree` or `kvfree`. Objects
> +allocated by `kmem_cache_alloc` can be freed with `kmem_cache_free`, `kfree`
> +or `kvfree`, where the latter two might be more convenient thanks to not
> +needing the kmem_cache pointer.
> +
> +The same rules apply to _bulk and _rcu flavors of freeing functions.
> +
> +Memory allocated by `vmalloc` can be freed with `vfree` or `kvfree`.
> +Memory allocated by `kvmalloc` can be freed with `kvfree`.
> +Caches created by `kmem_cache_create` should be freed with
> +`kmem_cache_destroy` only after freeing all the allocated objects first.
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 094321c17e48..dcd2cf1e8326 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -976,8 +976,10 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
>   * either fall back to use of call_rcu() or rearrange the structure to
>   * position the rcu_head structure into the first 4096 bytes.
>   *
> - * Note that the allowable offset might decrease in the future, for example,
> - * to allow something like kmem_cache_free_rcu().
> + * The object to be freed can be allocated either by kmalloc() or
> + * kmem_cache_alloc().
> + *
> + * Note that the allowable offset might decrease in the future.
>   *
>   * The BUILD_BUG_ON check must not involve any function calls, hence the
>   * checks are done in macros here.
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 1522693295f5..607249785c07 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -989,12 +989,9 @@ EXPORT_SYMBOL(__kmalloc_node_track_caller);
>  
>  /**
>   * kfree - free previously allocated memory
> - * @object: pointer returned by kmalloc.
> + * @object: pointer returned by kmalloc() or kmem_cache_alloc()
>   *
>   * If @object is NULL, no operation is performed.
> - *
> - * Don't free memory not originally allocated by kmalloc()
> - * or you will run into trouble.
>   */
>  void kfree(const void *object)
>  {
> -- 
> 2.39.2
> 

-- 
Sincerely yours,
Mike.