Re: [PATCH 09/12] mm, slub: split out allocations from pre/post hooks

[Vlastimil Babka]

On 11/27/22 11:54, Hyeonggon Yoo wrote:
> On Mon, Nov 21, 2022 at 06:11:59PM +0100, Vlastimil Babka wrote:
>> In the following patch we want to introduce CONFIG_SLUB_TINY allocation
>> paths that don't use the percpu slab. To prepare, refactor the
>> allocation functions:
>> 
>> Split out __slab_alloc_node() from slab_alloc_node() where the former
>> does the actual allocation and the latter calls the pre/post hooks.
>> 
>> Analogically, split out __kmem_cache_alloc_bulk() from
>> kmem_cache_alloc_bulk().
>> 
>> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>> ---
>>  mm/slub.c | 127 +++++++++++++++++++++++++++++++++---------------------
>>  1 file changed, 77 insertions(+), 50 deletions(-)
> 
> [...]
> 
>> +
>> +/* Note that interrupts must be enabled when calling this function. */
>> +int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>> +			  void **p)
>> +{
>> +	int i;
>> +	struct obj_cgroup *objcg = NULL;
>> +
>> +	/* memcg and kmem_cache debug support */
>> +	s = slab_pre_alloc_hook(s, NULL, &objcg, size, flags);
>> +	if (unlikely(!s))
>> +		return false;
>> +
>> +	i = __kmem_cache_alloc_bulk(s, flags, size, p, objcg);
>> +
>> +	/*
>> +	 * memcg and kmem_cache debug support and memory initialization.
>> +	 * Done outside of the IRQ disabled fastpath loop.
>> +	 */
>> +	if (i != 0)
>> +		slab_post_alloc_hook(s, objcg, flags, size, p,
>> +				slab_want_init_on_alloc(flags, s));
> 
> This patch looks mostly good but wondering what happens if someone calls it with size == 0
> so it does not call slab_post_alloc_hook()?

Hmm looks like in that case we miss doing obj_cgroup_put(objcg) in
memcg_slab_post_alloc_hook(). Good catch, thanks!

Fixing up, also a "return false" from int function
(existed also before this patch).

----8<----
diff --git a/mm/slub.c b/mm/slub.c
index b74c4664160e..88f0ce49caab 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3888,10 +3888,13 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
 	int i;
 	struct obj_cgroup *objcg = NULL;
 
+	if (!size)
+		return 0;
+
 	/* memcg and kmem_cache debug support */
 	s = slab_pre_alloc_hook(s, NULL, &objcg, size, flags);
 	if (unlikely(!s))
-		return false;
+		return 0;
 
 	i = __kmem_cache_alloc_bulk(s, flags, size, p, objcg);