On Thu 16-01-20 14:01:59, David Rientjes wrote:
> On Thu, 16 Jan 2020, Kirill Tkhai wrote:
>
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index c5b5f74cfd4d..6450bbe394e2 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -5360,10 +5360,12 @@ static int mem_cgroup_move_account(struct page *page,
> > > }
> > >
> > > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > > - if (compound && !list_empty(page_deferred_list(page))) {
> > > + if (compound) {
> > > spin_lock(&from->deferred_split_queue.split_queue_lock);
> > > - list_del_init(page_deferred_list(page));
> > > - from->deferred_split_queue.split_queue_len--;
> > > + if (!list_empty(page_deferred_list(page))) {
> > > + list_del_init(page_deferred_list(page));
> > > + from->deferred_split_queue.split_queue_len--;
> > > + }
> > > spin_unlock(&from->deferred_split_queue.split_queue_lock);
> > > }
> > > #endif
> > > @@ -5377,11 +5379,13 @@ static int mem_cgroup_move_account(struct page *page,
> > > page->mem_cgroup = to;
> > >
> > > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > > - if (compound && list_empty(page_deferred_list(page))) {
> > > + if (compound) {
> > > spin_lock(&to->deferred_split_queue.split_queue_lock);
> > > - list_add_tail(page_deferred_list(page),
> > > - &to->deferred_split_queue.split_queue);
> > > - to->deferred_split_queue.split_queue_len++;
> > > + if (list_empty(page_deferred_list(page))) {
> > > + list_add_tail(page_deferred_list(page),
> > > + &to->deferred_split_queue.split_queue);
> > > + to->deferred_split_queue.split_queue_len++;
> > > + }
> > > spin_unlock(&to->deferred_split_queue.split_queue_lock);
> > > }
> > > #endif
> >
> > The patch looks OK for me. But there is another question. I forget, why we unconditionally
> > add a page with empty deferred list to deferred_split_queue. Shouldn't we also check that
> > it was initially in the list? Something like:
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index d4394ae4e5be..0be0136adaa6 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5289,6 +5289,7 @@ static int mem_cgroup_move_account(struct page *page,
> > struct pglist_data *pgdat;
> > unsigned long flags;
> > unsigned int nr_pages = compound ? hpage_nr_pages(page) : 1;
> > + bool split = false;
> > int ret;
> > bool anon;
> >
> > @@ -5346,6 +5347,7 @@ static int mem_cgroup_move_account(struct page *page,
> > if (!list_empty(page_deferred_list(page))) {
> > list_del_init(page_deferred_list(page));
> > from->deferred_split_queue.split_queue_len--;
> > + split = true;
> > }
> > spin_unlock(&from->deferred_split_queue.split_queue_lock);
> > }
> > @@ -5360,7 +5362,7 @@ static int mem_cgroup_move_account(struct page *page,
> > page->mem_cgroup = to;
> >
> > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > - if (compound) {
> > + if (compound && split) {
> > spin_lock(&to->deferred_split_queue.split_queue_lock);
> > if (list_empty(page_deferred_list(page))) {
> > list_add_tail(page_deferred_list(page),
> >
>
> I think that's a good point, especially considering that the current code
> appears to unconditionally place any compound page on the deferred split
> queue of the destination memcg. The correct list that it should appear
> on, I believe, depends on whether the pmd has been split for the process
> being moved: note the MC_TARGET_PAGE caveat in
> mem_cgroup_move_charge_pte_range() that does not move the charge for
> compound pages with split pmds. So when mem_cgroup_move_account() is
> called with compound == true, we're moving the charge of the entire
> compound page: why would it appear on that memcg's deferred split queue?
I believe Kirill asked how do we know that the page should be actually
added to the deferred list just from the list_empty check. In other
words what if the page hasn't been split at all?
--
Michal Hocko
SUSE Labs
