mm, slab: add kerneldocs for common SLAB_ flags

We have many SLAB_ flags but many are used only internally, by kunit
tests or debugging subsystems cooperating with slab, or are set
according to slab_debug boot parameter.

Create kerneldocs for the commonly used flags that may be passed to
kmem_cache_create(). SLAB_TYPESAFE_BY_RCU already had a detailed
description, so turn it to a kerneldoc. Add some details for
SLAB_ACCOUNT, SLAB_RECLAIM_ACCOUNT and SLAB_HWCACHE_ALIGN. Reference
them from the __kmem_cache_create_args() kerneldoc.

Signed-off-by: Vlastimil Babka <[email protected]>

Author: tehcaster

include/linux/slab.h

@@ -77,7 +77,17 @@ enum _slab_flag_bits {
 #define SLAB_POISON		__SLAB_FLAG_BIT(_SLAB_POISON)
 /* Indicate a kmalloc slab */
 #define SLAB_KMALLOC		__SLAB_FLAG_BIT(_SLAB_KMALLOC)
-/* Align objs on cache lines */
+/**
+ * define SLAB_HWCACHE_ALIGN - Align objects on cache line boundaries.
+ *
+ * Sufficiently large objects are aligned on cache line boundary. For object
+ * size smaller than a half of cache line size, the alignment is on the half of
+ * cache line size. In general, if object size is smaller than 1/2^n of cache
+ * line size, the alignment is adjusted to 1/2^n.
+ *
+ * If explicit alignment is also requested by the respective
+ * &struct kmem_cache_args field, the greater of both is alignments is applied.
+ */
 #define SLAB_HWCACHE_ALIGN	__SLAB_FLAG_BIT(_SLAB_HWCACHE_ALIGN)
 /* Use GFP_DMA memory */
 #define SLAB_CACHE_DMA		__SLAB_FLAG_BIT(_SLAB_CACHE_DMA)
@@ -87,8 +97,8 @@ enum _slab_flag_bits {
 #define SLAB_STORE_USER		__SLAB_FLAG_BIT(_SLAB_STORE_USER)
 /* Panic if kmem_cache_create() fails */
 #define SLAB_PANIC		__SLAB_FLAG_BIT(_SLAB_PANIC)
-/*
- * SLAB_TYPESAFE_BY_RCU - **WARNING** READ THIS!
+/**
+ * define SLAB_TYPESAFE_BY_RCU - **WARNING** READ THIS!
  *
  * This delays freeing the SLAB page by a grace period, it does _NOT_
  * delay object freeing. This means that if you do kmem_cache_free()
@@ -99,20 +109,22 @@ enum _slab_flag_bits {
  * stays valid, the trick to using this is relying on an independent
  * object validation pass. Something like:
  *
- * begin:
- *  rcu_read_lock();
- *  obj = lockless_lookup(key);
- *  if (obj) {
- *    if (!try_get_ref(obj)) // might fail for free objects
- *      rcu_read_unlock();
- *      goto begin;
+ * ::
+ *
+ *  begin:
+ *   rcu_read_lock();
+ *   obj = lockless_lookup(key);
+ *   if (obj) {
+ *     if (!try_get_ref(obj)) // might fail for free objects
+ *       rcu_read_unlock();
+ *       goto begin;
  *
- *    if (obj->key != key) { // not the object we expected
- *      put_ref(obj);
- *      rcu_read_unlock();
- *      goto begin;
- *    }
- *  }
+ *     if (obj->key != key) { // not the object we expected
+ *       put_ref(obj);
+ *       rcu_read_unlock();
+ *       goto begin;
+ *     }
+ *   }
  *  rcu_read_unlock();
  *
  * This is useful if we need to approach a kernel structure obliquely,
@@ -137,7 +149,6 @@ enum _slab_flag_bits {
  *
  * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU.
  */
-/* Defer freeing slabs to RCU */
 #define SLAB_TYPESAFE_BY_RCU	__SLAB_FLAG_BIT(_SLAB_TYPESAFE_BY_RCU)
 /* Trace allocations and frees */
 #define SLAB_TRACE		__SLAB_FLAG_BIT(_SLAB_TRACE)
@@ -170,7 +181,12 @@ enum _slab_flag_bits {
 #else
 # define SLAB_FAILSLAB		__SLAB_FLAG_UNUSED
 #endif
-/* Account to memcg */
+/**
+ * define SLAB_ACCOUNT - Account allocations to memcg.
+ *
+ * All object allocations from this cache will be memcg accounted, regardless of
+ * __GFP_ACCOUNT being or not being passed to individual allocations.
+ */
 #ifdef CONFIG_MEMCG
 # define SLAB_ACCOUNT		__SLAB_FLAG_BIT(_SLAB_ACCOUNT)
 #else
@@ -197,7 +213,13 @@ enum _slab_flag_bits {
 #endif
 
 /* The following flags affect the page allocator grouping pages by mobility */
-/* Objects are reclaimable */
+/**
+ * define SLAB_RECLAIM_ACCOUNT - Objects are reclaimable.
+ *
+ * Use this flag for caches that have an associated shrinker. As a result, slab
+ * pages are allocated with __GFP_RECLAIMABLE, which affects grouping pages by
+ * mobility, and are accounted in SReclaimable counter in /proc/meminfo
+ */
 #ifndef CONFIG_SLUB_TINY
 #define SLAB_RECLAIM_ACCOUNT	__SLAB_FLAG_BIT(_SLAB_RECLAIM_ACCOUNT)
 #else

mm/slab_common.c

@@ -254,11 +254,23 @@ static struct kmem_cache *create_cache(const char *name,
  * @object_size: The size of objects to be created in this cache.
  * @args: Additional arguments for the cache creation (see
  *        &struct kmem_cache_args).
- * @flags: See %SLAB_* flags for an explanation of individual @flags.
+ * @flags: See the desriptions of individual flags. The common ones are listed
+ *         in the description below.
  *
  * Not to be called directly, use the kmem_cache_create() wrapper with the same
  * parameters.
  *
+ * Commonly used @flags:
+ *
+ * &SLAB_ACCOUNT - Account allocations to memcg.
+ *
+ * &SLAB_HWCACHE_ALIGN - Align objects on cache line boundaries.
+ *
+ * &SLAB_RECLAIM_ACCOUNT - Objects are reclaimable.
+ *
+ * &SLAB_TYPESAFE_BY_RCU - Slab page (not individual objects) freeing delayed
+ * by a grace period - see the full description before using.
+ *
  * Context: Cannot be called within a interrupt, but can be interrupted.
  *
  * Return: a pointer to the cache on success, NULL on failure.