Re: [PATCH v4 4/4] mm: convert page kmemcg type to a page memcg flag

[Johannes Weiner]

On Thu, Oct 01, 2020 at 07:00:36PM +0200, Michal Koutný wrote:
> Hi.
> 
> On Wed, Sep 30, 2020 at 05:27:10PM -0700, Roman Gushchin <guro@xxxxxx> wrote:
> > @@ -369,8 +371,12 @@ enum page_memcg_data_flags {
> >   */
> >  static inline struct mem_cgroup *page_memcg(struct page *page)
> >  {
> > +	unsigned long memcg_data = page->memcg_data;
> > +
> >  	VM_BUG_ON_PAGE(PageSlab(page), page);
> > -	return (struct mem_cgroup *)page->memcg_data;
> > +	VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_OBJCGS, page);
> > +
> > +	return (struct mem_cgroup *)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> >  }
> Shouldn't this change go also into page_memcg_rcu()? (I don't think the
> current single user (workingset_activation() would pass a non-slab
> kernel page but for consistency sake.)

+1

> Alternatively, I'm thinking why (in its single use) is there
> page_memcg_rcu() a separate function to page_memcg() (cross memcg page
> migration?).

It goes back to commit 55779ec759ccc3c12b917b3712a7716e1140c652.

The activation code is the only path where page migration is not
excluded. Because unlike with page state statistics, we don't really
mind a race when counting an activation event.

I do think there is a bug, though: mem_cgroup_move_account() should
use WRITE_ONCE() on page->mem_cgroup.