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



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