Re: [patch 8/8] mm: make per-memcg lru lists exclusive

Christoph Hellwig
2011-06-07T12:42:31Z

On Wed, Jun 01, 2011 at 08:25:19AM +0200, Johannes Weiner wrote:
> All lru list walkers have been converted to operate on per-memcg
> lists, the global per-zone lists are no longer required.
>
> This patch makes the per-memcg lists exclusive and removes the global
> lists from memcg-enabled kernels.
>
> The per-memcg lists now string up page descriptors directly, which
> unifies/simplifies the list isolation code of page reclaim as well as
> it saves a full double-linked list head for each page in the system.
>
> At the core of this change is the introduction of the lruvec
> structure, an array of all lru list heads. It exists for each zone
> globally, and for each zone per memcg. All lru list operations are
> now done in generic code against lruvecs, with the memcg lru list
> primitives only doing accounting and returning the proper lruvec for
> the currently scanned memcg on isolation, or for the respective page
> on putback.

Wouldn't it be simpler if we always have a stub mem_cgroup_per_zone
structure even for non-memcg kernels, and always operate on a
single instance per node of those for non-memcg kernels? In effect the
lruvec almost is something like that, just adding another layer of
abstraction.

> static inline struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index 8f7d247..43d5d9f 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -25,23 +25,27 @@ static inline void
> __add_page_to_lru_list(struct zone *zone, struct page *page, enum lru_list l,
> struct list_head *head)
> {
> + /* NOTE: Caller must ensure @head is on the right lruvec! */
> + mem_cgroup_lru_add_list(zone, page, l);
> list_add(&page->lru, head);
>
> __mod_zone_page_state(zone, NR_LRU_BASE + l, hpage_nr_pages(page));
> - mem_cgroup_add_lru_list(page, l);
> }

This already has been a borderline-useful function before, but with the
new changes it's not a useful helper. Either add the code surrounding
it includeing the PageLRU check and the normal add_page_to_lru_list
into a new page_update_lru_pos or similar helper, or just opencode these
bits in the only caller with a comment documenting why we are doing it.

I would tend towards the opencoding variant.