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Date:   Mon, 3 Feb 2020 10:34:52 -0800
From:   Roman Gushchin <guro@fb.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
CC:     <linux-mm@kvack.org>, Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
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        <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
        Bharata B Rao <bharata@linux.ibm.com>,
        Yafang Shao <laoar.shao@gmail.com>
Subject: Re: [PATCH v2 16/28] mm: memcg/slab: allocate obj_cgroups for
 non-root slab pages
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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@fb.com>
> > ---
> >  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!