Re: [PATCH v6 05/19] mm: memcontrol: decouple reference counting from page accounting

[Shakeel Butt <shakeelb@google.com>]

On Mon, Jun 8, 2020 at 4:07 PM Roman Gushchin <guro@fb.com> wrote:
>
> From: Johannes Weiner <hannes@cmpxchg.org>
>
> The reference counting of a memcg is currently coupled directly to how
> many 4k pages are charged to it. This doesn't work well with Roman's
> new slab controller, which maintains pools of objects and doesn't want
> to keep an extra balance sheet for the pages backing those objects.
>
> This unusual refcounting design (reference counts usually track
> pointers to an object) is only for historical reasons: memcg used to
> not take any css references and simply stalled offlining until all
> charges had been reparented and the page counters had dropped to
> zero. When we got rid of the reparenting requirement, the simple
> mechanical translation was to take a reference for every charge.
>
> More historical context can be found in commit e8ea14cc6ead ("mm:
> memcontrol: take a css reference for each charged page"),
> commit 64f219938941 ("mm: memcontrol: remove obsolete kmemcg pinning
> tricks") and commit b2052564e66d ("mm: memcontrol: continue cache
> reclaim from offlined groups").
>
> The new slab controller exposes the limitations in this scheme, so
> let's switch it to a more idiomatic reference counting model based on
> actual kernel pointers to the memcg:
>
> - The per-cpu stock holds a reference to the memcg its caching
>
> - User pages hold a reference for their page->mem_cgroup. Transparent
>   huge pages will no longer acquire tail references in advance, we'll
>   get them if needed during the split.
>
> - Kernel pages hold a reference for their page->mem_cgroup
>
> - Pages allocated in the root cgroup will acquire and release css
>   references for simplicity. css_get() and css_put() optimize that.
>
> - The current memcg_charge_slab() already hacked around the per-charge
>   references; this change gets rid of that as well.
>
> Roman:
> 1) Rebased on top of the current mm tree: added css_get() in
>    mem_cgroup_charge(), dropped mem_cgroup_try_charge() part
> 2) I've reformatted commit references in the commit log to make
>    checkpatch.pl happy.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Acked-by: Roman Gushchin <guro@fb.com>
> ---
>  mm/memcontrol.c | 37 +++++++++++++++++++++----------------
>  mm/slab.h       |  2 --
>  2 files changed, 21 insertions(+), 18 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d18bf93e0f19..80282b2e8b7f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2094,13 +2094,17 @@ static void drain_stock(struct memcg_stock_pcp *stock)
>  {
>         struct mem_cgroup *old = stock->cached;
>
> +       if (!old)
> +               return;
> +
>         if (stock->nr_pages) {
>                 page_counter_uncharge(&old->memory, stock->nr_pages);
>                 if (do_memsw_account())
>                         page_counter_uncharge(&old->memsw, stock->nr_pages);
> -               css_put_many(&old->css, stock->nr_pages);
>                 stock->nr_pages = 0;
>         }
> +
> +       css_put(&old->css);
>         stock->cached = NULL;
>  }
>
> @@ -2136,6 +2140,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>         stock = this_cpu_ptr(&memcg_stock);
>         if (stock->cached != memcg) { /* reset if necessary */
>                 drain_stock(stock);
> +               css_get(&memcg->css);
>                 stock->cached = memcg;
>         }
>         stock->nr_pages += nr_pages;
> @@ -2594,12 +2599,10 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>         page_counter_charge(&memcg->memory, nr_pages);
>         if (do_memsw_account())
>                 page_counter_charge(&memcg->memsw, nr_pages);
> -       css_get_many(&memcg->css, nr_pages);
>
>         return 0;
>
>  done_restock:
> -       css_get_many(&memcg->css, batch);
>         if (batch > nr_pages)
>                 refill_stock(memcg, batch - nr_pages);
>
> @@ -2657,8 +2660,6 @@ static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
>         page_counter_uncharge(&memcg->memory, nr_pages);
>         if (do_memsw_account())
>                 page_counter_uncharge(&memcg->memsw, nr_pages);
> -
> -       css_put_many(&memcg->css, nr_pages);
>  }
>  #endif
>
> @@ -2964,6 +2965,7 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
>                 if (!ret) {
>                         page->mem_cgroup = memcg;
>                         __SetPageKmemcg(page);
> +                       return 0;
>                 }
>         }
>         css_put(&memcg->css);
> @@ -2986,12 +2988,11 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
>         VM_BUG_ON_PAGE(mem_cgroup_is_root(memcg), page);
>         __memcg_kmem_uncharge(memcg, nr_pages);
>         page->mem_cgroup = NULL;
> +       css_put(&memcg->css);
>
>         /* slab pages do not have PageKmemcg flag set */
>         if (PageKmemcg(page))
>                 __ClearPageKmemcg(page);
> -
> -       css_put_many(&memcg->css, nr_pages);
>  }
>  #endif /* CONFIG_MEMCG_KMEM */
>
> @@ -3003,13 +3004,16 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
>   */
>  void mem_cgroup_split_huge_fixup(struct page *head)
>  {
> +       struct mem_cgroup *memcg = head->mem_cgroup;
>         int i;
>
>         if (mem_cgroup_disabled())

if (mem_cgroup_disabled() || !memcg)?

>                 return;
>
> -       for (i = 1; i < HPAGE_PMD_NR; i++)
> -               head[i].mem_cgroup = head->mem_cgroup;
> +       for (i = 1; i < HPAGE_PMD_NR; i++) {
> +               css_get(&memcg->css);
> +               head[i].mem_cgroup = memcg;
> +       }
>  }
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>
> @@ -5454,7 +5458,10 @@ static int mem_cgroup_move_account(struct page *page,
>          */
>         smp_mb();
>
> -       page->mem_cgroup = to;  /* caller should have done css_get */
> +       css_get(&to->css);
> +       css_put(&from->css);
> +
> +       page->mem_cgroup = to;
>
>         __unlock_page_memcg(from);
>
> @@ -6540,6 +6547,7 @@ int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
>         if (ret)
>                 goto out_put;
>
> +       css_get(&memcg->css);
>         commit_charge(page, memcg);
>
>         local_irq_disable();
> @@ -6594,9 +6602,6 @@ static void uncharge_batch(const struct uncharge_gather *ug)
>         __this_cpu_add(ug->memcg->vmstats_percpu->nr_page_events, ug->nr_pages);
>         memcg_check_events(ug->memcg, ug->dummy_page);
>         local_irq_restore(flags);
> -
> -       if (!mem_cgroup_is_root(ug->memcg))
> -               css_put_many(&ug->memcg->css, ug->nr_pages);
>  }
>
>  static void uncharge_page(struct page *page, struct uncharge_gather *ug)
> @@ -6634,6 +6639,7 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
>
>         ug->dummy_page = page;
>         page->mem_cgroup = NULL;
> +       css_put(&ug->memcg->css);
>  }
>
>  static void uncharge_list(struct list_head *page_list)
> @@ -6739,8 +6745,8 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
>         page_counter_charge(&memcg->memory, nr_pages);
>         if (do_memsw_account())
>                 page_counter_charge(&memcg->memsw, nr_pages);
> -       css_get_many(&memcg->css, nr_pages);
>
> +       css_get(&memcg->css);
>         commit_charge(newpage, memcg);
>
>         local_irq_save(flags);
> @@ -6977,8 +6983,7 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>         mem_cgroup_charge_statistics(memcg, page, -nr_entries);
>         memcg_check_events(memcg, page);
>
> -       if (!mem_cgroup_is_root(memcg))
> -               css_put_many(&memcg->css, nr_entries);
> +       css_put(&memcg->css);
>  }
>
>  /**
> diff --git a/mm/slab.h b/mm/slab.h
> index 633eedb6bad1..8a574d9361c1 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -373,9 +373,7 @@ static __always_inline int memcg_charge_slab(struct page *page,
>         lruvec = mem_cgroup_lruvec(memcg, page_pgdat(page));
>         mod_lruvec_state(lruvec, cache_vmstat_idx(s), nr_pages << PAGE_SHIFT);
>
> -       /* transer try_charge() page references to kmem_cache */
>         percpu_ref_get_many(&s->memcg_params.refcnt, nr_pages);
> -       css_put_many(&memcg->css, nr_pages);
>  out:
>         css_put(&memcg->css);
>         return ret;
> --
> 2.25.4
>