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 enum page_memcg_data_flags. Additional flags might be added later. Signed-off-by: Roman Gushchin <[email protected]> --- include/linux/memcontrol.h | 35 ++++++++++++++++++++++++----------- 1 file changed, 24 insertions(+), 11 deletions(-) diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index d4d15c04bbaf..35f846c6b89b 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -343,6 +343,15 @@ struct mem_cgroup { extern struct mem_cgroup *root_mem_cgroup; +enum page_memcg_data_flags { + /* page->memcg_data is a pointer to an objcgs vector */ + MEMCG_DATA_OBJCGS, + /* the next bit after the last actual flag */ + __NR_MEMCG_DATA_FLAGS, +}; + +#define MEMCG_DATA_FLAGS_MASK ((0x1UL << __NR_MEMCG_DATA_FLAGS) - 1) + /* * page_memcg - get the memory cgroup associated with a page * @page: a pointer to the page struct @@ -404,13 +413,7 @@ static inline struct mem_cgroup *page_memcg_check(struct page *page) */ unsigned long memcg_data = READ_ONCE(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 (memcg_data & (0x1UL << MEMCG_DATA_OBJCGS)) return NULL; return (struct mem_cgroup *)memcg_data; @@ -459,7 +462,12 @@ static inline void clear_page_memcg(struct page *page) */ static inline struct obj_cgroup **page_objcgs(struct page *page) { - return (struct obj_cgroup **)(READ_ONCE(page->memcg_data) & ~0x1UL); + unsigned long memcg_data = READ_ONCE(page->memcg_data); + + VM_BUG_ON_PAGE(memcg_data && + !(memcg_data & (0x1UL << MEMCG_DATA_OBJCGS)), page); + + return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK); } /* @@ -474,8 +482,9 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page) { unsigned long memcg_data = READ_ONCE(page->memcg_data); - if (memcg_data && (memcg_data & 0x1UL)) - return (struct obj_cgroup **)(memcg_data & ~0x1UL); + if (memcg_data && (memcg_data & (0x1UL << MEMCG_DATA_OBJCGS))) + return (struct obj_cgroup **) + (memcg_data & ~MEMCG_DATA_FLAGS_MASK); return NULL; } @@ -490,7 +499,11 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page) static inline bool set_page_objcgs(struct page *page, struct obj_cgroup **objcgs) { - return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs | 0x1UL); + unsigned long memcg_data = (unsigned long)objcgs; + + memcg_data |= (0x1UL << MEMCG_DATA_OBJCGS); + + return !cmpxchg(&page->memcg_data, 0, memcg_data); } /* -- 2.26.2
