Re: [PATCH v1 3/4] mm: introduce page memcg flags

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Sep 24, 2020 at 01:39:05PM -0700, Roman Gushchin wrote:
> 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@fb.com>
> > > ---
> > >  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.

Fair enough, it's a bit dense.

MEMCG_DATA_OBJCGS could work too. It wouldn't introduce a new prefix
and would relate to the field those flags belong to.

> > > @@ -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.

Does anybody need the bit numbers? You can make them masks directly:

enum memcg_data_flags {
	MEMCG_DATA_OBJCGS	= (1 << 0),
	...
}

and do memcg_data | MEMCG_DATA_OBJCGS.

cgroup-defs.h alone has 3 examples of this. It's very common.

> 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?

The bitops are for shared state, that's why set_bit(), clear_bit(),
test_bit() provide atomicity, and the __ versions of them usually
indicate that outside locking is provided.

Grep for __clear_bit() and most of the time it's on a shared data
structure and surrounded by some sort of lock or atomic context.

Why would you want to replace a single | expression with an RMW
transaction involving three statements and a function call to
__set_bit()?