Re: [PATCH v3 2/3] mm: Charge active memcg when no mm is set

[Shakeel Butt <shakeelb@google.com>]

On Thu, Feb 20, 2020 at 8:52 AM Dan Schatzberg <schatzberg.dan@gmail.com> wrote:
>
> memalloc_use_memcg() worked for kernel allocations but was silently
> ignored for user pages.
>
> This patch establishes a precedence order for who gets charged:
>
> 1. If there is a memcg associated with the page already, that memcg is
>    charged. This happens during swapin.
>
> 2. If an explicit mm is passed, mm->memcg is charged. This happens
>    during page faults, which can be triggered in remote VMs (eg gup).
>
> 3. Otherwise consult the current process context. If it has configured
>    a current->active_memcg, use that.

What if css_tryget_online(current->active_memcg) in
get_mem_cgroup_from_current() fails? Do we want to change this to
css_tryget() and even if that fails should we fallback to
root_mem_cgroup or current->mm->memcg?

> Otherwise, current->mm->memcg.
>
> Previously, if a NULL mm was passed to mem_cgroup_try_charge (case 3) it
> would always charge the root cgroup. Now it looks up the current
> active_memcg first (falling back to charging the root cgroup if not
> set).
>
> Signed-off-by: Dan Schatzberg <schatzberg.dan@gmail.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Tejun Heo <tj@kernel.org>
> ---
>  mm/memcontrol.c | 11 ++++++++---
>  mm/shmem.c      |  2 +-
>  2 files changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6f6dc8712e39..b174aff4f069 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6317,7 +6317,8 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
>   * @compound: charge the page as compound or small page
>   *
>   * Try to charge @page to the memcg that @mm belongs to, reclaiming
> - * pages according to @gfp_mask if necessary.
> + * pages according to @gfp_mask if necessary. If @mm is NULL, try to
> + * charge to the active memcg.
>   *
>   * Returns 0 on success, with *@memcgp pointing to the charged memcg.
>   * Otherwise, an error code is returned.
> @@ -6361,8 +6362,12 @@ int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,
>                 }
>         }
>
> -       if (!memcg)
> -               memcg = get_mem_cgroup_from_mm(mm);
> +       if (!memcg) {
> +               if (!mm)
> +                       memcg = get_mem_cgroup_from_current();
> +               else
> +                       memcg = get_mem_cgroup_from_mm(mm);
> +       }
>
>         ret = try_charge(memcg, gfp_mask, nr_pages);
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index c8f7540ef048..7c7f5acf89d6 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1766,7 +1766,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
>         }
>
>         sbinfo = SHMEM_SB(inode->i_sb);
> -       charge_mm = vma ? vma->vm_mm : current->mm;
> +       charge_mm = vma ? vma->vm_mm : NULL;
>
>         page = find_lock_entry(mapping, index);
>         if (xa_is_value(page)) {
> --
> 2.17.1
>