On Mon, Feb 03, 2020 at 01:27:56PM -0500, Johannes Weiner wrote:
> 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@xxxxxx>
> > ---
> > 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?
For instance, look at page_cgroup_ino() called from the
reading /proc/kpageflags.
>
> > 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.
Agree. Maybe even mem_cgroup_data?
>
> Otherwise, this patch looks good to me!
Thanks!
