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    Subject[PATCH] memcg: clean up migration
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    This patch cleans up unncessary BUG_ON check and confusing
    charge variable.

    That's because memcg charge/uncharge could be handled by
    mem_cgroup_[prepare/end] migration itself so charge local variable
    in unmap_and_move lost the role since we introduced 01b1ae63c2.

    And mem_cgroup_prepare_migratio return 0 if only it is successful.
    Otherwise, it jumps to unlock label to clean up so BUG_ON(charge)
    isn;t meaningless.

    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
    Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
    ---
    mm/memcontrol.c | 1 +
    mm/migrate.c | 14 ++++----------
    2 files changed, 5 insertions(+), 10 deletions(-)

    diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    index 2fc97fc..6832926 100644
    --- a/mm/memcontrol.c
    +++ b/mm/memcontrol.c
    @@ -2872,6 +2872,7 @@ static inline int mem_cgroup_move_swap_account(swp_entry_t entry,
    /*
    * Before starting migration, account PAGE_SIZE to mem_cgroup that the old
    * page belongs to.
    + * Return 0 if charge is successful. Otherwise return -errno.
    */
    int mem_cgroup_prepare_migration(struct page *page,
    struct page *newpage, struct mem_cgroup **ptr, gfp_t gfp_mask)
    diff --git a/mm/migrate.c b/mm/migrate.c
    index eb083a6..737c2e5 100644
    --- a/mm/migrate.c
    +++ b/mm/migrate.c
    @@ -622,7 +622,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
    int *result = NULL;
    struct page *newpage = get_new_page(page, private, &result);
    int remap_swapcache = 1;
    - int charge = 0;
    struct mem_cgroup *mem;
    struct anon_vma *anon_vma = NULL;

    @@ -637,9 +636,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
    if (unlikely(split_huge_page(page)))
    goto move_newpage;

    - /* prepare cgroup just returns 0 or -ENOMEM */
    rc = -EAGAIN;
    -
    if (!trylock_page(page)) {
    if (!force)
    goto move_newpage;
    @@ -678,13 +675,11 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
    }

    /* charge against new page */
    - charge = mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL);
    - if (charge == -ENOMEM) {
    - rc = -ENOMEM;
    + rc = mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL);
    + if (rc)
    goto unlock;
    - }
    - BUG_ON(charge);

    + rc = -EAGAIN;
    if (PageWriteback(page)) {
    if (!force || !sync)
    goto uncharge;
    @@ -767,8 +762,7 @@ skip_unmap:
    drop_anon_vma(anon_vma);

    uncharge:
    - if (!charge)
    - mem_cgroup_end_migration(mem, page, newpage, rc == 0);
    + mem_cgroup_end_migration(mem, page, newpage, rc == 0);
    unlock:
    unlock_page(page);

    --
    1.7.1


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