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Thu, 24 Sep 2020 13:02:54 -0700 (PDT) Date: Thu, 24 Sep 2020 16:01:22 -0400 From: Johannes Weiner To: Roman Gushchin Cc: Andrew Morton , Shakeel Butt , Michal Hocko , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-team@fb.com Subject: Re: [PATCH v1 3/4] mm: introduce page memcg flags Message-ID: <20200924200122.GC329853@cmpxchg.org> References: <20200922203700.2879671-1-guro@fb.com> <20200922203700.2879671-4-guro@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200922203700.2879671-4-guro@fb.com> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000067, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 > --- > 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? > @@ -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.