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    SubjectRe: [PATCH 1/2] memcg: fix page_referencies cgroup filter on global reclaim
    Johannes Weiner wrote:
    > On Tue, Feb 21, 2012 at 03:38:04PM +0400, Konstantin Khlebnikov wrote:
    >> Johannes Weiner wrote:
    >>> On Wed, Feb 15, 2012 at 08:28:30PM +0400, Konstantin Khlebnikov wrote:
    >>>> Global memory reclaimer should't skip referencies for any pages,
    >>>> even if they are shared between different cgroups.
    >>>
    >>> Agreed: if we reclaim from one memcg because of its limit, we want to
    >>> reclaim those pages that this group is not using. If it's used by
    >>> someone else, it should be evicted and refaulted by the group that
    >>> needs it.
    >>>
    >>> If we reclaim globally, all references are "true" because we want to
    >>> evict those pages that are not used by any cgroup.
    >>>
    >>> But if we reclaim a hierarchical subgroup, we don't want to evict
    >>> pages that are shared among this hierarchy, either, even if the memcg
    >>> that has the page charged to it is not using it. Bouncing the page
    >>> around the hierarchy is not sensible, because it does not solve the
    >>> problem of the parent hitting its limit when the sibling group will
    >>> refault it in a blink of an eye. It should only be evicted if the
    >>> memcg that's not using it nears its own limit, because only in that
    >>> case would reclaiming the page remedy the situation.
    >>>
    >>>> This patch adds scan_control->current_mem_cgroup, which points to currently
    >>>> shrinking sub-cgroup in hierarchy, at global reclaim it always NULL.
    >>>
    >>> So to be consistent, I'm wondering if we should pass
    >>> sc->target_mem_cgroup - the limit-hitting hierarchy root - to
    >>> page_referenced() and then have mm_match_cgroup() do a
    >>> mem_cgroup_same_or_subtree() check to see if the vma is in the
    >>> hierarchy rooted at sc->target_mem_cgroup.
    >>>
    >>> Global reclaim is handled automatically, because mm_match_cgroup() is
    >>> not checked when the passed memcg is NULL, which sc->target_mem_cgroup
    >>> is for global reclaim.
    >>
    >> Also we can try to recharge page to other cgroup, if we found in rmap another its user
    >> outsize of currently shrinking hierarchy, page there is isolated, so at the end we will
    >> insert page directly to its lru.
    >
    > Not sure if it's worth the trouble, but that could work.
    >
    >> But the main purpose of this patch for me is killing mz->mem_cgroup dereference,
    >> because I plan to replace mz with direct reference to lruvec, which will be memcg-free object.
    >
    > I like that plan. How about this patch then?

    I sent my version. See patch "[PATCH v3 02/21] memcg: make mm_match_cgroup() hirarchical"

    one note below

    >
    > ---
    > From: Johannes Weiner<hannes@cmpxchg.org>
    > Subject: mm: memcg: count pte references from every member of the reclaimed hierarchy
    >
    > The rmap walker checking page table references has historically
    > ignored references from VMAs that were not part of the memcg that was
    > being reclaimed during memcg hard limit reclaim.
    >
    > When transitioning global reclaim to memcg hierarchy reclaim, I missed
    > that bit and now references from outside a memcg are ignored even
    > during global reclaim.
    >
    > Reverting back to traditional behaviour - count all references during
    > global reclaim and only mind references of the memcg being reclaimed
    > during limit reclaim would be one option.
    >
    > However, the more generic idea is to ignore references exactly then
    > when they are outside the hierarchy that is currently under reclaim;
    > because only then will their reclamation be of any use to help the
    > pressure situation. It makes no sense to ignore references from a
    > sibling memcg and then evict a page that will be immediately refaulted
    > by that sibling which contributes to the same usage of the common
    > ancestor under reclaim.
    >
    > The solution: make the rmap walker ignore references from VMAs that
    > are not part of the hierarchy that is being reclaimed.
    >
    > Flat limit reclaim will stay the same, hierarchical limit reclaim will
    > mind the references only to pages that the hierarchy owns. Global
    > reclaim, since it reclaims from all memcgs, will be fixed to regard
    > all references.
    >
    > Reported-by: Konstantin Khlebnikov<khlebnikov@openvz.org>
    > Signed-off-by: Johannes Weiner<hannes@cmpxchg.org>
    > ---
    > include/linux/memcontrol.h | 3 ++-
    > mm/memcontrol.c | 2 +-
    > mm/vmscan.c | 6 ++++--
    > 3 files changed, 7 insertions(+), 4 deletions(-)
    >
    > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
    > index 8537c5d..ba9455d 100644
    > --- a/include/linux/memcontrol.h
    > +++ b/include/linux/memcontrol.h
    > @@ -78,6 +78,7 @@ extern void mem_cgroup_uncharge_page(struct page *page);
    > extern void mem_cgroup_uncharge_cache_page(struct page *page);
    >
    > extern void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask);
    > +bool mem_cgroup_same_or_subtree(const struct mem_cgroup *, struct mem_cgroup *);
    > int task_in_mem_cgroup(struct task_struct *task, const struct mem_cgroup *memcg);
    >
    > extern struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page);
    > @@ -91,7 +92,7 @@ int mm_match_cgroup(const struct mm_struct *mm, const struct mem_cgroup *cgroup)
    > rcu_read_lock();
    > memcg = mem_cgroup_from_task(rcu_dereference((mm)->owner));
    > rcu_read_unlock();
    > - return cgroup == memcg;
    > + return mem_cgroup_same_or_subtree(cgroup, memcg);
    > }

    seems like this memcg pointer is rcu-protected, so it seems broken.

    >
    > extern struct cgroup_subsys_state *mem_cgroup_css(struct mem_cgroup *memcg);
    > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > index e4be95a..f91d762 100644
    > --- a/mm/memcontrol.c
    > +++ b/mm/memcontrol.c
    > @@ -1044,7 +1044,7 @@ struct lruvec *mem_cgroup_lru_move_lists(struct zone *zone,
    > * Checks whether given mem is same or in the root_mem_cgroup's
    > * hierarchy subtree
    > */
    > -static bool mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
    > +bool mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
    > struct mem_cgroup *memcg)
    > {
    > if (root_memcg != memcg) {
    > diff --git a/mm/vmscan.c b/mm/vmscan.c
    > index c631234..120646e 100644
    > --- a/mm/vmscan.c
    > +++ b/mm/vmscan.c
    > @@ -708,7 +708,8 @@ static enum page_references page_check_references(struct page *page,
    > int referenced_ptes, referenced_page;
    > unsigned long vm_flags;
    >
    > - referenced_ptes = page_referenced(page, 1, mz->mem_cgroup,&vm_flags);
    > + referenced_ptes = page_referenced(page, 1, sc->target_mem_cgroup,
    > + &vm_flags);
    > referenced_page = TestClearPageReferenced(page);
    >
    > /* Lumpy reclaim - ignore references */
    > @@ -1710,7 +1711,8 @@ static void shrink_active_list(unsigned long nr_pages,
    > continue;
    > }
    >
    > - if (page_referenced(page, 0, mz->mem_cgroup,&vm_flags)) {
    > + if (page_referenced(page, 0, sc->target_mem_cgroup,
    > + &vm_flags)) {
    > nr_rotated += hpage_nr_pages(page);
    > /*
    > * Identify referenced, file-backed active pages and



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