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SubjectRe: [PATCH v2 16/28] mm: memcg/slab: allocate obj_cgroups for non-root slab pages
On Mon, Jan 27, 2020 at 09:34:41AM -0800, Roman Gushchin wrote:
> Allocate and release memory to store obj_cgroup pointers for each
> non-root slab page. Reuse page->mem_cgroup pointer to store a pointer
> to the allocated space.
>
> To distinguish between obj_cgroups and memcg pointers in case
> when it's not obvious which one is used (as in page_cgroup_ino()),
> let's always set the lowest bit in the obj_cgroup case.
>
> Signed-off-by: Roman Gushchin <guro@fb.com>
> ---
> include/linux/mm.h | 25 ++++++++++++++++++--
> include/linux/mm_types.h | 5 +++-
> mm/memcontrol.c | 5 ++--
> mm/slab.c | 3 ++-
> mm/slab.h | 51 +++++++++++++++++++++++++++++++++++++++-
> mm/slub.c | 2 +-
> 6 files changed, 83 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 080f8ac8bfb7..65224becc4ca 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1264,12 +1264,33 @@ static inline void set_page_links(struct page *page, enum zone_type zone,
> #ifdef CONFIG_MEMCG
> static inline struct mem_cgroup *page_memcg(struct page *page)
> {
> - return page->mem_cgroup;
> + struct mem_cgroup *memcg = page->mem_cgroup;
> +
> + /*
> + * The lowest bit set means that memcg isn't a valid memcg pointer,
> + * but a obj_cgroups pointer. In this case the page is shared and
> + * isn't charged to any specific memory cgroup. Return NULL.
> + */
> + if ((unsigned long) memcg & 0x1UL)
> + memcg = NULL;
> +
> + return memcg;

That should really WARN instead of silently returning NULL. Which
callsite optimistically asks a page's cgroup when it has no idea
whether that page is actually a userpage or not?

> static inline struct mem_cgroup *page_memcg_rcu(struct page *page)
> {
> + struct mem_cgroup *memcg = READ_ONCE(page->mem_cgroup);
> +
> WARN_ON_ONCE(!rcu_read_lock_held());
> - return READ_ONCE(page->mem_cgroup);
> +
> + /*
> + * The lowest bit set means that memcg isn't a valid memcg pointer,
> + * but a obj_cgroups pointer. In this case the page is shared and
> + * isn't charged to any specific memory cgroup. Return NULL.
> + */
> + if ((unsigned long) memcg & 0x1UL)
> + memcg = NULL;
> +
> + return memcg;

Same here.

> }
> #else
> static inline struct mem_cgroup *page_memcg(struct page *page)
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 270aa8fd2800..5102f00f3336 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -198,7 +198,10 @@ struct page {
> atomic_t _refcount;
>
> #ifdef CONFIG_MEMCG
> - struct mem_cgroup *mem_cgroup;
> + union {
> + struct mem_cgroup *mem_cgroup;
> + struct obj_cgroup **obj_cgroups;
> + };

Since you need the casts in both cases anyway, it's safer (and
simpler) to do

unsigned long mem_cgroup;

to prevent accidental direct derefs in future code.

Otherwise, this patch looks good to me!

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