Re: [Patch v3] mm: thp: grab the lock before manipulation defer list

[David Rientjes]

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?