The lowest bit in page->memcg_data is used to distinguish between struct memory_cgroup pointer and a pointer to a objcgs array. All checks and modifications of this bit are open-coded.
Let's formalize it using page memcg flags, defined in page_memcg_flags enum and replace all open-coded accesses with test_bit()/__set_bit(). Few additional flags might be added later. Flags are intended to be mutually exclusive. Signed-off-by: Roman Gushchin <[email protected]> --- include/linux/memcontrol.h | 29 +++++++++++++++++++---------- 1 file changed, 19 insertions(+), 10 deletions(-) diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index ab3ea3e90583..9a49f1e1c0c7 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -343,6 +343,11 @@ struct mem_cgroup { extern struct mem_cgroup *root_mem_cgroup; +enum page_memcg_flags { + /* page->memcg_data is a pointer to an objcgs vector */ + PG_MEMCG_OBJ_CGROUPS, +}; + /* * page_mem_cgroup - get the memory cgroup associated with a page * @page: a pointer to the page struct @@ -371,13 +376,7 @@ static inline struct mem_cgroup *page_mem_cgroup_check(struct page *page) { unsigned long memcg_data = page->memcg_data; - /* - * 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 doesn't belong - * to any specific memory cgroup. - */ - if (memcg_data & 0x1UL) + if (test_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data)) return NULL; return (struct mem_cgroup *)memcg_data; @@ -422,7 +421,13 @@ static inline void clear_page_mem_cgroup(struct page *page) */ static inline struct obj_cgroup **page_obj_cgroups(struct page *page) { - return (struct obj_cgroup **)(page->memcg_data & ~0x1UL); + unsigned long memcg_data = page->memcg_data; + + VM_BUG_ON_PAGE(memcg_data && !test_bit(PG_MEMCG_OBJ_CGROUPS, + &memcg_data), page); + __clear_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data); + + return (struct obj_cgroup **)memcg_data; } /* @@ -437,7 +442,7 @@ static inline struct obj_cgroup **page_obj_cgroups_check(struct page *page) { unsigned long memcg_data = page->memcg_data; - if (memcg_data && (memcg_data & 0x1UL)) + if (memcg_data && test_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data)) return (struct obj_cgroup **)memcg_data; return NULL; @@ -453,7 +458,11 @@ static inline struct obj_cgroup **page_obj_cgroups_check(struct page *page) static inline bool set_page_obj_cgroups(struct page *page, struct obj_cgroup **objcgs) { - return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs | 0x1UL); + unsigned long memcg_data = (unsigned long)objcgs; + + __set_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data); + + return !cmpxchg(&page->memcg_data, 0, memcg_data); } /* -- 2.26.2
