On Thu, Sep 24, 2020 at 04:01:22PM -0400, Johannes Weiner wrote: > On Tue, Sep 22, 2020 at 01:36:59PM -0700, Roman Gushchin wrote: > > 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 <guro@xxxxxx> > > --- > > 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, > > How about enum memcg_data_flags and PGMEMCG_OBJCG? Honestly I prefer the original names. I'm ok with enum memcg_data_flags, if you prefer it. PGMEMCG_OBJCG looks bulky with too many letters without a separator, also we use object cgroups (plural) everywhere, like OBJCGS vs OBJCG. PG_MEMCG_OBJCGS works for me. > > > @@ -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); > > The flag names make sense to me, but this shouldn't be using test_bit, > __clear_bit, __set_bit etc. on local variables. It suggests that it's > modifying some shared/global state, when it's just masking out a bit > during a read. We usually just open-code the bitwise ops for that. It will be way more bulky otherwise, all those memcg_data & (1UL << PG_MEMCG_OBJ_CGROUPS) etc. I don't see why these bitops helpers can't be used on local variables. Is the preference to not use them this way documented anywhere? Thanks!