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.
