[PATCH 7/7] mm/slab: document kfree() as allowed for kmem_cache_alloc() objects

Vlastimil Babka
2023-03-10T10:32:28Z

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@suse.cz>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
---
Documentation/core-api/memory-allocation.rst | 15 +++++++++++----
include/linux/rcupdate.h | 6 ++++--
mm/slab_common.c | 5 +----
3 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
index 5954ddf6ee13..f9e8d352ed67 100644
--- a/Documentation/core-api/memory-allocation.rst
+++ b/Documentation/core-api/memory-allocation.rst
@@ -170,7 +170,14 @@ 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`
+or also by `kfree` or `kvfree`, which can be more convenient as it does
+not require the kmem_cache pointed.
+The rules for _bulk and _rcu flavors of freeing functions are analogical.
+
+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