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    SubjectRe: [patch 07/11] mm: vmscan: convert unevictable page rescue scanner to per-memcg LRU lists
    On Mon 12-09-11 12:57:24, Johannes Weiner wrote:
    > The global per-zone LRU lists are about to go away on memcg-enabled
    > kernels, the unevictable page rescue scanner must be able to find its
    > pages on the per-memcg LRU lists.
    >
    > Signed-off-by: Johannes Weiner <jweiner@redhat.com>

    The patch is correct but I guess the original implementation of
    scan_zone_unevictable_pages is buggy (see bellow). This should be
    addressed separatelly, though.

    Reviewed-by: Michal Hocko <mhocko@suse.cz>

    > ---
    > include/linux/memcontrol.h | 3 ++
    > mm/memcontrol.c | 11 ++++++++
    > mm/vmscan.c | 61 ++++++++++++++++++++++++++++---------------
    > 3 files changed, 54 insertions(+), 21 deletions(-)
    >
    [...]
    > --- a/mm/memcontrol.c
    > +++ b/mm/memcontrol.c
    [...]
    > @@ -3490,32 +3501,40 @@ void scan_mapping_unevictable_pages(struct address_space *mapping)
    > #define SCAN_UNEVICTABLE_BATCH_SIZE 16UL /* arbitrary lock hold batch size */
    > static void scan_zone_unevictable_pages(struct zone *zone)
    > {
    > - struct list_head *l_unevictable = &zone->lru[LRU_UNEVICTABLE].list;
    > - unsigned long scan;
    > - unsigned long nr_to_scan = zone_page_state(zone, NR_UNEVICTABLE);
    > -
    > - while (nr_to_scan > 0) {
    > - unsigned long batch_size = min(nr_to_scan,
    > - SCAN_UNEVICTABLE_BATCH_SIZE);
    > -
    > - spin_lock_irq(&zone->lru_lock);
    > - for (scan = 0; scan < batch_size; scan++) {
    > - struct page *page = lru_to_page(l_unevictable);
    > + struct mem_cgroup *mem;
    >
    > - if (!trylock_page(page))
    > - continue;
    > + mem = mem_cgroup_iter(NULL, NULL, NULL);
    > + do {
    > + struct mem_cgroup_zone mz = {
    > + .mem_cgroup = mem,
    > + .zone = zone,
    > + };
    > + unsigned long nr_to_scan;
    >
    > - prefetchw_prev_lru_page(page, l_unevictable, flags);
    > + nr_to_scan = zone_nr_lru_pages(&mz, LRU_UNEVICTABLE);
    > + while (nr_to_scan > 0) {
    > + unsigned long batch_size;
    > + unsigned long scan;
    >
    > - if (likely(PageLRU(page) && PageUnevictable(page)))
    > - check_move_unevictable_page(page, zone);
    > + batch_size = min(nr_to_scan,
    > + SCAN_UNEVICTABLE_BATCH_SIZE);
    > + spin_lock_irq(&zone->lru_lock);
    > + for (scan = 0; scan < batch_size; scan++) {
    > + struct page *page;
    >
    > - unlock_page(page);
    > + page = lru_tailpage(&mz, LRU_UNEVICTABLE);
    > + if (!trylock_page(page))
    > + continue;

    We are not moving to the next page so we will try it again in the next
    round while we already increased the scan count. In the end we will
    missed some pages.

    > + if (likely(PageLRU(page) &&
    > + PageUnevictable(page)))
    > + check_move_unevictable_page(page, zone);
    > + unlock_page(page);
    > + }
    > + spin_unlock_irq(&zone->lru_lock);
    > + nr_to_scan -= batch_size;
    > }
    > - spin_unlock_irq(&zone->lru_lock);
    > -
    > - nr_to_scan -= batch_size;
    > - }
    > + mem = mem_cgroup_iter(NULL, mem, NULL);
    > + } while (mem);
    > }
    >
    >
    > --
    > 1.7.6
    >

    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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