Re: [PATCH v1 2/4] mm: memcontrol/slab: use helpers to access slab page's memcg_data

[Johannes Weiner]

On Tue, Sep 22, 2020 at 01:36:58PM -0700, Roman Gushchin wrote:
> To gather all direct accesses to struct page's memcg_data field
> in one place, let's introduce 4 new helper functions to use in
> the slab accounting code:
>   struct obj_cgroup **page_obj_cgroups(struct page *page);
>   struct obj_cgroup **page_obj_cgroups_check(struct page *page);
>   bool set_page_obj_cgroups(struct page *page, struct obj_cgroup **objcgs);
>   void clear_page_obj_cgroups(struct page *page);
> 
> They are similar to the corresponding API for generic pages, except
> that the setter can return false, indicating that the value has been
> already set from a different thread.
> 
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> ---
>  include/linux/memcontrol.h | 80 ++++++++++++++++++++++++++++++++++++++
>  mm/memcontrol.c            |  4 +-
>  mm/slab.h                  | 27 ++-----------
>  3 files changed, 85 insertions(+), 26 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 3313e7c21534..ab3ea3e90583 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -409,6 +409,86 @@ static inline void clear_page_mem_cgroup(struct page *page)
>  	page->memcg_data = 0;
>  }
>  
> +#ifdef CONFIG_MEMCG_KMEM
> +/*
> + * page_obj_cgroups - get the object cgroups vector associated with a page
> + * @page: a pointer to the page struct
> + *
> + * Returns a pointer to the object cgroups vector associated with the page,
> + * or NULL. This function assumes that the page is known to have an
> + * associated object cgroups vector. It's not safe to call this function
> + * against pages, which might have an associated memory cgroup: e.g.
> + * kernel stack pages.
> + */
> +static inline struct obj_cgroup **page_obj_cgroups(struct page *page)
> +{
> +	return (struct obj_cgroup **)(page->memcg_data & ~0x1UL);

page_mem_cgroup() has a VM_BUG_ON() if the page is in fact a slab
type. Should this also check that if memcg_data is set, it MUST have
the lower bit set?

In line with page_memcg(), I think page_objcgs() would be nicer.

As in 'memcg_reparent_objcgs()' :-) :-) :-)

> @@ -2923,7 +2923,7 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
>  	 * Memcg membership data for each individual object is saved in
>  	 * the page->obj_cgroups.
>  	 */
> -	if (page_has_obj_cgroups(page)) {
> +	if (page_obj_cgroups_check(page)) {
>  		struct obj_cgroup *objcg;
>  		unsigned int off;

Similar to the previous patch: do we have anybody passing in objects
that aren't actual objects in the obj_cgroup sense?