Re: [PATCH RFC 1/4] mm, slab: move memcg charging to post-alloc hook

[Roman Gushchin <roman.gushchin@linux.dev>]

On Fri, Mar 01, 2024 at 06:07:08PM +0100, Vlastimil Babka wrote:
> The MEMCG_KMEM integration with slab currently relies on two hooks
> during allocation. memcg_slab_pre_alloc_hook() determines the objcg and
> charges it, and memcg_slab_post_alloc_hook() assigns the objcg pointer
> to the allocated object(s).
> 
> As Linus pointed out, this is unnecessarily complex. Failing to charge
> due to memcg limits should be rare, so we can optimistically allocate
> the object(s) and do the charging together with assigning the objcg
> pointer in a single post_alloc hook. In the rare case the charging
> fails, we can free the object(s) back.
> 
> This simplifies the code (no need to pass around the objcg pointer) and
> potentially allows to separate charging from allocation in cases where
> it's common that the allocation would be immediately freed, and the
> memcg handling overhead could be saved.
> 
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Link: https://lore.kernel.org/all/CAHk-=whYOOdM7jWy5jdrAm8LxcgCMFyk2bt8fYYvZzM4U-zAQA@mail.gmail.com/
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Nice cleanup, Vlastimil!
Couple of small nits below, but otherwise, please, add my

Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>

Thanks!

 ---
>  mm/slub.c | 180 +++++++++++++++++++++++++++-----------------------------------
>  1 file changed, 77 insertions(+), 103 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 2ef88bbf56a3..7022a1246bab 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1897,23 +1897,36 @@ static inline size_t obj_full_size(struct kmem_cache *s)
>  	return s->size + sizeof(struct obj_cgroup *);
>  }
>  
> -/*
> - * Returns false if the allocation should fail.
> - */
> -static bool __memcg_slab_pre_alloc_hook(struct kmem_cache *s,
> -					struct list_lru *lru,
> -					struct obj_cgroup **objcgp,
> -					size_t objects, gfp_t flags)
> +static bool __memcg_slab_post_alloc_hook(struct kmem_cache *s,
> +					 struct list_lru *lru,
> +					 gfp_t flags, size_t size,
> +					 void **p)
>  {
> +	struct obj_cgroup *objcg;
> +	struct slab *slab;
> +	unsigned long off;
> +	size_t i;
> +
>  	/*
>  	 * The obtained objcg pointer is safe to use within the current scope,
>  	 * defined by current task or set_active_memcg() pair.
>  	 * obj_cgroup_get() is used to get a permanent reference.
>  	 */
> -	struct obj_cgroup *objcg = current_obj_cgroup();
> +	objcg = current_obj_cgroup();
>  	if (!objcg)
>  		return true;
>  
> +	/*
> +	 * slab_alloc_node() avoids the NULL check, so we might be called with a
> +	 * single NULL object. kmem_cache_alloc_bulk() aborts if it can't fill
> +	 * the whole requested size.
> +	 * return success as there's nothing to free back
> +	 */
> +	if (unlikely(*p == NULL))
> +		return true;

Probably better to move this check up? current_obj_cgroup() != NULL check is more
expensive.

> +
> +	flags &= gfp_allowed_mask;
> +
>  	if (lru) {
>  		int ret;
>  		struct mem_cgroup *memcg;
> @@ -1926,71 +1939,51 @@ static bool __memcg_slab_pre_alloc_hook(struct kmem_cache *s,
>  			return false;
>  	}
>  
> -	if (obj_cgroup_charge(objcg, flags, objects * obj_full_size(s)))
> +	if (obj_cgroup_charge(objcg, flags, size * obj_full_size(s)))
>  		return false;
>  
> -	*objcgp = objcg;
> +	for (i = 0; i < size; i++) {
> +		slab = virt_to_slab(p[i]);

Not specific to this change, but I wonder if it makes sense to introduce virt_to_slab()
variant without any extra checks for this and similar cases, where we know for sure
that p resides on a slab page. What do you think?