[Devel] Re: [RFC][-mm PATCH 4/8] Memory controller memory accounting (v3)

[Paul Menage]

On 7/20/07, Balbir Singh <balbir at linux.vnet.ibm.com> wrote:

> +void __always_inline unlock_meta_page(struct page *page)
> +{
> +       bit_spin_unlock(PG_metapage, &page->flags);
> +}

Maybe add a BUG_ON(!test_bit(PG_metapage, &page->flags)) at least for
development?

> +       mem = rcu_dereference(mm->mem_container);
> +       /*
> +        * For every charge from the container, increment reference
> +        * count
> +        */
> +       css_get(&mem->css);
> +       rcu_read_unlock();

It's not clear to me that this is safe.

If

> +
> +       /*
> +        * If we created the meta_page, we should free it on exceeding
> +        * the container limit.
> +        */
> +       if (res_counter_charge(&mem->res, 1)) {
> +               css_put(&mem->css);
> +               goto free_mp;
> +       }
> +
> +       lock_meta_page(page);
> +       /*
> +        * Check if somebody else beat us to allocating the meta_page
> +        */
> +       if (page_get_meta_page(page)) {

I think you need to add something like

  kfree(mp);
  mp = page_get_meta_page(page);

otherwise you're going to leak the new but unneeded metapage.

> +               atomic_inc(&mp->ref_cnt);
> +               res_counter_uncharge(&mem->res, 1);
> +               goto done;
> +       }
> +
> +       atomic_set(&mp->ref_cnt, 1);
> +       mp->mem_container = mem;
> +       mp->page = page;
> +       page_assign_meta_page(page, mp);

Would it make sense to have the "mp->page = page" be part of
page_assign_meta_page() for consistency?

> +err:
> +       unlock_meta_page(page);
> +       return -ENOMEM;

The only jump to err: is from a location where the metapage is already
unlocked. Maybe scrap err: and just do a return -ENOMEM when the
allocation fails?

> +out_uncharge:
> +       mem_container_uncharge(page_get_meta_page(page));

Wanting to call mem_container_uncharge() on a page and hence having to
call page_get_meta_page() seems to be more common than wanting to call
it on a meta page that you already have available. Maybe make
mem_container_uncharge() be a wrapper that take a struct page and does
something like mem_container_uncharge_mp(page_get_meta_page(page))
where mem_container_uncharge_mp() is the raw meta-page version?

Paul
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