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    Subject[PATCH 12/12] memcg: fix race at charging swap-in
    There is a small race in do_swap_page(). When the page swapped-in is charged,
    the mapcount can be greater than 0. But, at the same time some process (shares
    it ) call unmap and make mapcount 1->0 and the page is uncharged.

    For fixing this, I added a new interface.
    - precharge
    account to res_counter by PAGE_SIZE and try to free pages if necessary.
    - commit
    register page_cgroup and add to LRU if necessary.
    - cancel
    uncharge PAGE_SIZE because of do_swap_page failure.

    This protocol uses PCG_USED bit on page_cgroup for avoiding over accounting.
    Usual mem_cgroup_charge_common() does precharge -> commit at a time.

    These precharge/commit/cancel is useful and can be used for other places,
    - shmem, (and other places need precharge.)
    - migration
    - move_account(force_empty) etc...
    etc..we'll revisit later.

    Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

    include/linux/memcontrol.h | 21 +++++++
    mm/memcontrol.c | 135 +++++++++++++++++++++++++++++++--------------
    mm/memory.c | 6 +-
    3 files changed, 120 insertions(+), 42 deletions(-)

    Index: mmotm-2.6.27-rc7+/include/linux/memcontrol.h
    ===================================================================
    --- mmotm-2.6.27-rc7+.orig/include/linux/memcontrol.h
    +++ mmotm-2.6.27-rc7+/include/linux/memcontrol.h
    @@ -31,6 +31,13 @@ struct mm_struct;

    extern int mem_cgroup_charge(struct page *page, struct mm_struct *mm,
    gfp_t gfp_mask);
    +/* for swap handling */
    +extern int mem_cgroup_precharge(struct mm_struct *mm,
    + gfp_t gfp_mask, struct mem_cgroup **ptr);
    +extern void mem_cgroup_commit_charge_swap(struct page *page,
    + struct mem_cgroup *ptr);
    +extern void mem_cgroup_cancel_charge_swap(struct mem_cgroup *ptr);
    +
    extern int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
    gfp_t gfp_mask);
    extern void mem_cgroup_move_lists(struct page *page, enum lru_list lru);
    @@ -85,6 +92,20 @@ static inline int mem_cgroup_cache_charg
    return 0;
    }

    +static int mem_cgroup_precharge(struct mm_struct *mm,
    + gfp_t gfp_mask, struct mem_cgroup **ptr)
    +{
    + return 0;
    +}
    +
    +static void mem_cgroup_commit_charge_swap(struct page *page,
    + struct mem_cgroup *ptr)
    +{
    +}
    +static void mem_cgroup_cancel_charge_swap(struct mem_cgroup *ptr)
    +{
    +}
    +
    static inline void mem_cgroup_uncharge_page(struct page *page)
    {
    }
    Index: mmotm-2.6.27-rc7+/mm/memcontrol.c
    ===================================================================
    --- mmotm-2.6.27-rc7+.orig/mm/memcontrol.c
    +++ mmotm-2.6.27-rc7+/mm/memcontrol.c
    @@ -698,52 +698,44 @@ static void drain_page_cgroup_all(void)


    /*
    - * Charge the memory controller for page usage.
    - * Return
    - * 0 if the charge was successful
    - * < 0 if the cgroup is over its limit
    + * charge against mem_cgroup linked to this mm. (or *ptr)
    + *
    + * This just charge PAGE_SIZE and reduce memory usage if necessary.
    + *
    + * Pages on radix-tree is charged at radix-tree add/remove under lock.
    + * new pages are charged at allocation and both are guaranteed to be that
    + * there are no racy users. We does precharge->commit at once.
    + *
    + * About swapcache, we can't trust page->mapcount until it's mapped.
    + * Then we do precharge before map and commit/cancel after the mapping is
    + * established. (see below, we have commit_swap and cancel_swap)
    */
    -static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
    - gfp_t gfp_mask, enum charge_type ctype,
    - struct mem_cgroup *memcg)
    +
    +int mem_cgroup_precharge(struct mm_struct *mm,
    + gfp_t mask, struct mem_cgroup **ptr)
    {
    struct mem_cgroup *mem;
    - struct page_cgroup *pc;
    - unsigned long nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
    - struct mem_cgroup_per_zone *mz;
    - unsigned long flags;
    -
    - pc = lookup_page_cgroup(page);
    - /* can happen at boot */
    - if (unlikely(!pc))
    - return 0;
    - prefetchw(pc);
    - /*
    - * We always charge the cgroup the mm_struct belongs to.
    - * The mm_struct's mem_cgroup changes on task migration if the
    - * thread group leader migrates. It's possible that mm is not
    - * set, if so charge the init_mm (happens for pagecache usage).
    - */
    + int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;

    - if (likely(!memcg)) {
    - rcu_read_lock();
    + rcu_read_lock();
    + if (!*ptr) {
    mem = mem_cgroup_from_task(rcu_dereference(mm->owner));
    if (unlikely(!mem)) {
    rcu_read_unlock();
    - return 0;
    + return -ESRCH;
    }
    - rcu_read_unlock();
    + *ptr = mem;
    } else {
    - mem = memcg;
    + mem = *ptr;
    }
    + rcu_read_unlock();

    + css_get(&mem->css);
    while (unlikely(res_counter_charge(&mem->res, PAGE_SIZE))) {
    - if (!(gfp_mask & __GFP_WAIT))
    - goto out;
    -
    - if (try_to_free_mem_cgroup_pages(mem, gfp_mask))
    + if (!(mask & __GFP_WAIT))
    + goto nomem;
    + if (try_to_free_mem_cgroup_pages(mem, mask))
    continue;
    -
    /*
    * try_to_free_mem_cgroup_pages() might not give us a full
    * picture of reclaim. Some pages are reclaimed and might be
    @@ -755,16 +747,31 @@ static int mem_cgroup_charge_common(stru
    continue;

    if (!nr_retries--) {
    - mem_cgroup_out_of_memory(mem, gfp_mask);
    - goto out;
    + mem_cgroup_out_of_memory(mem, mask);
    + goto nomem;
    }
    }
    + return 0;
    +nomem:
    + css_put(&mem->css);
    + return -ENOMEM;
    +}

    +void mem_cgroup_commit_charge(struct page_cgroup *pc,
    + struct mem_cgroup *mem,
    + enum charge_type ctype)
    +{
    + struct mem_cgroup_per_zone *mz;
    + unsigned long flags;
    +
    + if (!mem)
    + return;
    preempt_disable();
    if (TestSetPageCgroupUsed(pc)) {
    res_counter_uncharge(&mem->res, PAGE_SIZE);
    + css_put(&mem->css);
    preempt_enable();
    - goto done;
    + return;
    }
    /*
    * page cgroup is *unused* now....but....
    @@ -786,14 +793,43 @@ static int mem_cgroup_charge_common(stru

    pc->mem_cgroup = mem;
    set_page_cgroup_lru(pc);
    + css_put(&mem->css);
    preempt_enable();
    +}

    -done:
    +/*
    + * Charge the memory controller for page usage.
    + * Return
    + * 0 if the charge was successful
    + * < 0 if the cgroup is over its limit
    + */
    +static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
    + gfp_t gfp_mask, enum charge_type ctype,
    + struct mem_cgroup *memcg)
    +{
    + struct page_cgroup *pc;
    + struct mem_cgroup *ptr = memcg;
    + int ret;
    +
    + pc = lookup_page_cgroup(page);
    + /* can happen at boot */
    + if (unlikely(!pc))
    + return 0;
    + prefetchw(pc);
    +
    + ret = mem_cgroup_precharge(mm, gfp_mask, &ptr);
    + if (likely(!ret)) {
    + mem_cgroup_commit_charge(pc, ptr, ctype);
    + return 0;
    + }
    + if (unlikely((ret == -ENOMEM)))
    + return ret;
    + /* ESRCH case */
    return 0;
    -out:
    - return -ENOMEM;
    }

    +
    +
    int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
    {
    if (mem_cgroup_subsys.disabled)
    @@ -806,7 +842,7 @@ int mem_cgroup_charge(struct page *page,
    * But page->mapping may have out-of-use anon_vma pointer,
    * detecit it by PageAnon() check. newly-mapped-anon's page->mapping
    * is NULL.
    - */
    + */
    if (page_mapped(page) || (page->mapping && !PageAnon(page)))
    return 0;
    if (unlikely(!mm))
    @@ -857,6 +893,25 @@ int mem_cgroup_cache_charge(struct page
    MEM_CGROUP_CHARGE_TYPE_SHMEM, NULL);
    }

    +
    +void mem_cgroup_commit_charge_swap(struct page *page, struct mem_cgroup *ptr)
    +{
    + struct page_cgroup *pc;
    + if (!ptr)
    + return;
    + pc = lookup_page_cgroup(page);
    + mem_cgroup_commit_charge(pc, ptr, MEM_CGROUP_CHARGE_TYPE_MAPPED);
    +}
    +
    +void mem_cgroup_cancel_charge_swap(struct mem_cgroup *mem)
    +{
    + if (!mem)
    + return;
    + res_counter_uncharge(&mem->res, PAGE_SIZE);
    + css_put(&mem->css);
    +}
    +
    +
    /*
    * uncharge if !page_mapped(page)
    */
    Index: mmotm-2.6.27-rc7+/mm/memory.c
    ===================================================================
    --- mmotm-2.6.27-rc7+.orig/mm/memory.c
    +++ mmotm-2.6.27-rc7+/mm/memory.c
    @@ -2287,6 +2287,7 @@ static int do_swap_page(struct mm_struct
    struct page *page;
    swp_entry_t entry;
    pte_t pte;
    + struct mem_cgroup *ptr = NULL;
    int ret = 0;

    if (!pte_unmap_same(mm, pmd, page_table, orig_pte))
    @@ -2323,7 +2324,7 @@ static int do_swap_page(struct mm_struct
    lock_page(page);
    delayacct_clear_flag(DELAYACCT_PF_SWAPIN);

    - if (mem_cgroup_charge(page, mm, GFP_KERNEL)) {
    + if (mem_cgroup_precharge(mm, GFP_KERNEL, &ptr) == -ENOMEM) {
    ret = VM_FAULT_OOM;
    goto out;
    }
    @@ -2353,6 +2354,7 @@ static int do_swap_page(struct mm_struct
    flush_icache_page(vma, page);
    set_pte_at(mm, address, page_table, pte);
    page_add_anon_rmap(page, vma, address);
    + mem_cgroup_commit_charge_swap(page, ptr);

    swap_free(entry);
    if (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
    @@ -2373,7 +2375,7 @@ unlock:
    out:
    return ret;
    out_nomap:
    - mem_cgroup_uncharge_page(page);
    + mem_cgroup_cancel_charge_swap(ptr);
    pte_unmap_unlock(page_table, ptl);
    unlock_page(page);
    page_cache_release(page);


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