Re: [External] Re: [PATCH v3 3/4] mm: memcontrol: use obj_cgroup APIs to charge kmem pages

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Mar 12, 2021 at 05:22:55PM +0800, Muchun Song wrote:
> On Thu, Mar 11, 2021 at 6:05 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > > @@ -6828,7 +6857,7 @@ static void uncharge_batch(const struct uncharge_gather *ug)
> > >
> > >  static void uncharge_page(struct page *page, struct uncharge_gather *ug)
> > >  {
> > > -     unsigned long nr_pages;
> > > +     unsigned long nr_pages, nr_kmem;
> > >       struct mem_cgroup *memcg;
> > >
> > >       VM_BUG_ON_PAGE(PageLRU(page), page);
> > > @@ -6836,34 +6865,44 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
> > >       if (!page_memcg_charged(page))
> > >               return;
> > >
> > > +     nr_pages = compound_nr(page);
> > >       /*
> > >        * Nobody should be changing or seriously looking at
> > > -      * page memcg at this point, we have fully exclusive
> > > -      * access to the page.
> > > +      * page memcg or objcg at this point, we have fully
> > > +      * exclusive access to the page.
> > >        */
> > > -     memcg = page_memcg_check(page);
> > > +     if (PageMemcgKmem(page)) {
> > > +             struct obj_cgroup *objcg;
> > > +
> > > +             objcg = page_objcg(page);
> > > +             memcg = obj_cgroup_memcg_get(objcg);
> > > +
> > > +             page->memcg_data = 0;
> > > +             obj_cgroup_put(objcg);
> > > +             nr_kmem = nr_pages;
> > > +     } else {
> > > +             memcg = page_memcg(page);
> > > +             page->memcg_data = 0;
> > > +             nr_kmem = 0;
> > > +     }
> >
> > Why is all this moved above the uncharge_batch() call?
> 
> Before calling obj_cgroup_put(), we need set page->memcg_data
> to zero. So I move "page->memcg_data = 0" to here.

Yeah, it makes sense to keep those together, but we can move them both
down to after the uncharge, right?

> > It separates the pointer manipulations from the refcounting, which
> > makes the code very difficult to follow.
> >
> > > +
> > >       if (ug->memcg != memcg) {
> > >               if (ug->memcg) {
> > >                       uncharge_batch(ug);
> > >                       uncharge_gather_clear(ug);
> > >               }
> > >               ug->memcg = memcg;
> > > +             ug->dummy_page = page;
> >
> > Why this change?
> 
> Just like ug->memcg, we do not need to set
> ug->dummy_page in every loop.

Ah, okay. That's a reasonable change, it's just confusing because I
thought this was a requirement for the new code to work. But I didn't
see how it relied on that, and it made me think I'm not understanding
your code ;) It's better to split that into a separate patch.

> I will rework the code in the next version.

Thanks!