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Date:   Mon, 3 Feb 2020 13:19:15 -0800
From:   Roman Gushchin <guro@fb.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
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        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 03:46:27PM -0500, Johannes Weiner wrote:
> On Mon, Feb 03, 2020 at 10:34:52AM -0800, Roman Gushchin wrote:
> > 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.
> 
> But that checks PageSlab() and implements memcg_from_slab_page() to
> handle that case properly. And that's what we expect all callsites to
> do: make sure that the question asked actually makes sense, instead of
> having the interface paper over bogus requests.
> 
> If that function is completely racy and PageSlab isn't stable, then it
> should really just open-code the lookup, rather than require weakening
> the interface for everybody else.

Why though?

Another example: process stack can be depending on the machine config and
platform a vmalloc allocation, a slab allocation or a "high-order slab allocation",
which is executed by the page allocator directly.

It's kinda nice to have a function that hides accounting details
and returns a valid memcg pointer for any kind of objects.

To me it seems to be a valid question:
for a given kernel object give me a pointer to the memory cgroup.

Why it's weakening?

Moreover, open-coding of this question leads to bugs like one fixed by
ec9f02384f60 ("mm: workingset: fix vmstat counters for shadow nodes").

> 
> > > >  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?
> 
> Personally, I don't think the suffix adds much. The type makes it so
> the compiler catches any accidental use, and access is very
> centralized so greppability doesn't matter much.