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

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

[Wei Yang]

As all the other places, we grab the lock before manipulate the defer list.
Current implementation may face a race condition.

For example, the potential race would be:

    CPU1                      CPU2
    mem_cgroup_move_account   deferred_split_huge_page
      list_empty
                                lock
                                list_empty
                                list_add_tail
                                unlock
      lock
      # list_empty might not hold anymore
      list_add_tail
      unlock

When this sequence happens, the list_add_tail() in
mem_cgroup_move_account() corrupt the list since which is already been
added to some split_queue in split_huge_page_to_list().

Besides this, David Rientjes points out the split_queue_len would be in
a wrong state, which would be a significant issue for shrinkers.

Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")

Signed-off-by: Wei Yang <richardw.yang-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
Cc: <stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>    [5.4+]

---
v3:
  * remove all review/ack tag since rewrite the changelog
  * use deferred_split_huge_page as the example of race
  * add cc stable 5.4+ tag as suggested by David Rientjes

v2:
  * move check on compound outside suggested by Alexander
  * an example of the race condition, suggested by Michal
---
 mm/memcontrol.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

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
-- 
2.17.1

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

[Kirill Tkhai]

On 16.01.2020 04:31, Wei Yang wrote:
> As all the other places, we grab the lock before manipulate the defer list.
> Current implementation may face a race condition.
> 
> For example, the potential race would be:
> 
>     CPU1                      CPU2
>     mem_cgroup_move_account   deferred_split_huge_page
>       list_empty
>                                 lock
>                                 list_empty
>                                 list_add_tail
>                                 unlock
>       lock
>       # list_empty might not hold anymore
>       list_add_tail
>       unlock
> 
> When this sequence happens, the list_add_tail() in
> mem_cgroup_move_account() corrupt the list since which is already been
> added to some split_queue in split_huge_page_to_list().
> 
> Besides this, David Rientjes points out the split_queue_len would be in
> a wrong state, which would be a significant issue for shrinkers.
> 
> Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
> 
> Signed-off-by: Wei Yang <richardw.yang-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
> Cc: <stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>    [5.4+]
> 
> ---
> v3:
>   * remove all review/ack tag since rewrite the changelog
>   * use deferred_split_huge_page as the example of race
>   * add cc stable 5.4+ tag as suggested by David Rientjes
> 
> v2:
>   * move check on compound outside suggested by Alexander
>   * an example of the race condition, suggested by Michal
> ---
>  mm/memcontrol.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> 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),

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?

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

[Wei Yang]

On Thu, Jan 16, 2020 at 02:01:59PM -0800, 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?

Well, Kirill's change is easy to understand, while your statement here is a
bit hard for me. Seems I lack some knowledge about cgroup. I am sorry about
this. :-(

-- 
Wei Yang
Help you, Help me

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

[Michal Hocko]

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

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

[Kirill Tkhai]

On 17.01.2020 12:10, Michal Hocko wrote:
> 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 	(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?

Yes, I'm talking about this. Function mem_cgroup_move_account() adds every
huge page to the deferred list, while we need to do that only for pages,
which are queued for splitting...

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

[David Rientjes]

On Fri, 17 Jan 2020, Kirill Tkhai wrote:

> >> 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?
> 
> Yes, I'm talking about this. Function mem_cgroup_move_account() adds every
> huge page to the deferred list, while we need to do that only for pages,
> which are queued for splitting...
> 

Yup, and that appears broken before Wei's patch.  Since we only migrate 
charges of entire compound pages (we have a mapping pmd, the underlying 
page cannot be split), it should not appear on the deferred split queue 
for any memcg, right?

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

[Kirill Tkhai]

On 17.01.2020 12:32, David Rientjes wrote:
> On Fri, 17 Jan 2020, Kirill Tkhai wrote:
> 
>>>> 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?
>>
>> Yes, I'm talking about this. Function mem_cgroup_move_account() adds every
>> huge page to the deferred list, while we need to do that only for pages,
>> which are queued for splitting...
>>
> 
> Yup, and that appears broken before Wei's patch.  Since we only migrate 
> charges of entire compound pages (we have a mapping pmd, the underlying 
> page cannot be split), it should not appear on the deferred split queue 
> for any memcg, right?

Hm. Can't a huge page be mapped in two tasks:

1)the first task unmapped a part of page and initiated splitting,
2)the second task still refers the whole page,

then we move account for the second task?

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

[Michal Hocko]

On Fri 17-01-20 12:42:05, Kirill Tkhai wrote:
> On 17.01.2020 12:32, David Rientjes wrote:
> > On Fri, 17 Jan 2020, Kirill Tkhai wrote:
> > 
> >>>> 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?
> >>
> >> Yes, I'm talking about this. Function mem_cgroup_move_account() adds every
> >> huge page to the deferred list, while we need to do that only for pages,
> >> which are queued for splitting...
> >>
> > 
> > Yup, and that appears broken before Wei's patch.  Since we only migrate 
> > charges of entire compound pages (we have a mapping pmd, the underlying 
> > page cannot be split), it should not appear on the deferred split queue 
> > for any memcg, right?
> 
> Hm. Can't a huge page be mapped in two tasks:

It can but it will get charged to only of the initially. I haven't
checked the THP code in that aspect but from what I remember subpages
shouldn't refer to different memcgs. Kirill Shutemov?
-- 
Michal Hocko
SUSE Labs

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

[David Rientjes]

On Fri, 17 Jan 2020, Michal Hocko wrote:

> 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?
> 

Right, and I don't think that it necessarily is and the second 
conditional in Wei's patch will always succeed unless we have raced.  That 
patch is for a lock concern but I think Kirill's question has uncovered 
something more interesting.

Kirill S would definitely be best to answer Kirill T's question, but from 
my understanding when mem_cgroup_move_account() is called with 
compound == true that we always have an intact pmd (we never migrate 
partial page charges for pages on the deferred split queue with the 
current charge migration implementation) and thus the underlying page is 
not eligible to be split and shouldn't be on the deferred split queue.

In other words, a page being on the deferred split queue for a memcg 
should only happen when it is charged to that memcg.  (This wasn't the 
case when we only had per-node split queues.)  I think that's currently 
broken in mem_cgroup_move_account() before Wei's patch.

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

[Kirill A. Shutemov]

On Fri, Jan 17, 2020 at 01:31:50AM -0800, David Rientjes wrote:
> On Fri, 17 Jan 2020, Michal Hocko wrote:
> 
> > 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?
> > 
> 
> Right, and I don't think that it necessarily is and the second 
> conditional in Wei's patch will always succeed unless we have raced.  That 
> patch is for a lock concern but I think Kirill's question has uncovered 
> something more interesting.
> 
> Kirill S would definitely be best to answer Kirill T's question, but from 
> my understanding when mem_cgroup_move_account() is called with 
> compound == true that we always have an intact pmd (we never migrate 
> partial page charges for pages on the deferred split queue with the 
> current charge migration implementation) and thus the underlying page is 
> not eligible to be split and shouldn't be on the deferred split queue.
> 
> In other words, a page being on the deferred split queue for a memcg 
> should only happen when it is charged to that memcg.  (This wasn't the 
> case when we only had per-node split queues.)  I think that's currently 
> broken in mem_cgroup_move_account() before Wei's patch.

Right. It's broken indeed.

We are dealing with anon page here. And it cannot be on deferred list as
long as it's mapped with PMD. We cannot get compound == true &&
!list_empty() on the (first) enter to the function. Any PMD-mapped page
will be put onto deferred by the function. This is wrong.

The fix is not obvious.

This comment got in mem_cgroup_move_charge_pte_range() my attention:

			/*
			 * We can have a part of the split pmd here. Moving it
			 * can be done but it would be too convoluted so simply
			 * ignore such a partial THP and keep it in original
			 * memcg. There should be somebody mapping the head.
			 */

That's exactly the case we care about: PTE-mapped THP that has to be split
under load. We don't move charge of them between memcgs and therefore we
should not move the page to different memcg.

I guess this will do the trick :P

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c5b5f74cfd4d..e87ee4c10f6e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5359,14 +5359,6 @@ static int mem_cgroup_move_account(struct page *page,
 		__mod_lruvec_state(to_vec, NR_WRITEBACK, nr_pages);
 	}
 
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
-	if (compound && !list_empty(page_deferred_list(page))) {
-		spin_lock(&from->deferred_split_queue.split_queue_lock);
-		list_del_init(page_deferred_list(page));
-		from->deferred_split_queue.split_queue_len--;
-		spin_unlock(&from->deferred_split_queue.split_queue_lock);
-	}
-#endif
 	/*
 	 * It is safe to change page->mem_cgroup here because the page
 	 * is referenced, charged, and isolated - we can't race with
@@ -5376,16 +5368,6 @@ static int mem_cgroup_move_account(struct page *page,
 	/* caller should have done css_get */
 	page->mem_cgroup = to;
 
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
-	if (compound && list_empty(page_deferred_list(page))) {
-		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++;
-		spin_unlock(&to->deferred_split_queue.split_queue_lock);
-	}
-#endif
-
 	spin_unlock_irqrestore(&from->move_lock, flags);
 
 	ret = 0;
-- 
 Kirill A. Shutemov

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

[David Rientjes]

On Fri, 17 Jan 2020, Kirill A. Shutemov wrote:

> > Right, and I don't think that it necessarily is and the second 
> > conditional in Wei's patch will always succeed unless we have raced.  That 
> > patch is for a lock concern but I think Kirill's question has uncovered 
> > something more interesting.
> > 
> > Kirill S would definitely be best to answer Kirill T's question, but from 
> > my understanding when mem_cgroup_move_account() is called with 
> > compound == true that we always have an intact pmd (we never migrate 
> > partial page charges for pages on the deferred split queue with the 
> > current charge migration implementation) and thus the underlying page is 
> > not eligible to be split and shouldn't be on the deferred split queue.
> > 
> > In other words, a page being on the deferred split queue for a memcg 
> > should only happen when it is charged to that memcg.  (This wasn't the 
> > case when we only had per-node split queues.)  I think that's currently 
> > broken in mem_cgroup_move_account() before Wei's patch.
> 
> Right. It's broken indeed.
> 
> We are dealing with anon page here. And it cannot be on deferred list as
> long as it's mapped with PMD. We cannot get compound == true &&
> !list_empty() on the (first) enter to the function. Any PMD-mapped page
> will be put onto deferred by the function. This is wrong.
> 
> The fix is not obvious.
> 
> This comment got in mem_cgroup_move_charge_pte_range() my attention:
> 
> 			/*
> 			 * We can have a part of the split pmd here. Moving it
> 			 * can be done but it would be too convoluted so simply
> 			 * ignore such a partial THP and keep it in original
> 			 * memcg. There should be somebody mapping the head.
> 			 */
> 
> That's exactly the case we care about: PTE-mapped THP that has to be split
> under load. We don't move charge of them between memcgs and therefore we
> should not move the page to different memcg.
> 
> I guess this will do the trick :P
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c5b5f74cfd4d..e87ee4c10f6e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5359,14 +5359,6 @@ static int mem_cgroup_move_account(struct page *page,
>  		__mod_lruvec_state(to_vec, NR_WRITEBACK, nr_pages);
>  	}
>  
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	if (compound && !list_empty(page_deferred_list(page))) {
> -		spin_lock(&from->deferred_split_queue.split_queue_lock);
> -		list_del_init(page_deferred_list(page));
> -		from->deferred_split_queue.split_queue_len--;
> -		spin_unlock(&from->deferred_split_queue.split_queue_lock);
> -	}
> -#endif
>  	/*
>  	 * It is safe to change page->mem_cgroup here because the page
>  	 * is referenced, charged, and isolated - we can't race with
> @@ -5376,16 +5368,6 @@ static int mem_cgroup_move_account(struct page *page,
>  	/* caller should have done css_get */
>  	page->mem_cgroup = to;
>  
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	if (compound && list_empty(page_deferred_list(page))) {
> -		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++;
> -		spin_unlock(&to->deferred_split_queue.split_queue_lock);
> -	}
> -#endif
> -
>  	spin_unlock_irqrestore(&from->move_lock, flags);
>  
>  	ret = 0;

Yeah, this is what I was thinking as well.  When 
PageTransHuge(page) == true and there's a mapping pmd, the charge gets 
moved but the page shouldn't appear on any deferred split queue; when 
there isn't a mapped pmd, it should already be on a queue but the charge 
doesn't get moved so no change in which queue is needed.

There was no deferred split handling in mem_cgroup_move_account() needed 
for per-node deferred split queues either so this is purely an issue for 
commit 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware") 
so I think we need your patch and it should be annotated for stable 5.4+.

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

[Yang Shi]

On 1/17/20 7:38 AM, Kirill A. Shutemov wrote:
> On Fri, Jan 17, 2020 at 01:31:50AM -0800, David Rientjes wrote:
>> On Fri, 17 Jan 2020, Michal Hocko wrote:
>>
>>> 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?
>>>
>> Right, and I don't think that it necessarily is and the second
>> conditional in Wei's patch will always succeed unless we have raced.  That
>> patch is for a lock concern but I think Kirill's question has uncovered
>> something more interesting.
>>
>> Kirill S would definitely be best to answer Kirill T's question, but from
>> my understanding when mem_cgroup_move_account() is called with
>> compound == true that we always have an intact pmd (we never migrate
>> partial page charges for pages on the deferred split queue with the
>> current charge migration implementation) and thus the underlying page is
>> not eligible to be split and shouldn't be on the deferred split queue.
>>
>> In other words, a page being on the deferred split queue for a memcg
>> should only happen when it is charged to that memcg.  (This wasn't the
>> case when we only had per-node split queues.)  I think that's currently
>> broken in mem_cgroup_move_account() before Wei's patch.
> Right. It's broken indeed.

Hmm... Yes, definitely. I wasn't realized this at the first place.

>
> We are dealing with anon page here. And it cannot be on deferred list as
> long as it's mapped with PMD. We cannot get compound == true &&
> !list_empty() on the (first) enter to the function. Any PMD-mapped page
> will be put onto deferred by the function. This is wrong.
>
> The fix is not obvious.
>
> This comment got in mem_cgroup_move_charge_pte_range() my attention:
>
> 			/*
> 			 * We can have a part of the split pmd here. Moving it
> 			 * can be done but it would be too convoluted so simply
> 			 * ignore such a partial THP and keep it in original
> 			 * memcg. There should be somebody mapping the head.
> 			 */
>
> That's exactly the case we care about: PTE-mapped THP that has to be split
> under load. We don't move charge of them between memcgs and therefore we
> should not move the page to different memcg.
>
> I guess this will do the trick :P

It seems correct to me. In addition, memcg move charge just move PMD 
mapped THP, the THP should be never on the deferred split queue of 
"from" if it is PMD mapped, so actually we don't have to move it to the 
deferred split queue of "to".

>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c5b5f74cfd4d..e87ee4c10f6e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5359,14 +5359,6 @@ static int mem_cgroup_move_account(struct page *page,
>   		__mod_lruvec_state(to_vec, NR_WRITEBACK, nr_pages);
>   	}
>   
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	if (compound && !list_empty(page_deferred_list(page))) {
> -		spin_lock(&from->deferred_split_queue.split_queue_lock);
> -		list_del_init(page_deferred_list(page));
> -		from->deferred_split_queue.split_queue_len--;
> -		spin_unlock(&from->deferred_split_queue.split_queue_lock);
> -	}
> -#endif
>   	/*
>   	 * It is safe to change page->mem_cgroup here because the page
>   	 * is referenced, charged, and isolated - we can't race with
> @@ -5376,16 +5368,6 @@ static int mem_cgroup_move_account(struct page *page,
>   	/* caller should have done css_get */
>   	page->mem_cgroup = to;
>   
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	if (compound && list_empty(page_deferred_list(page))) {
> -		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++;
> -		spin_unlock(&to->deferred_split_queue.split_queue_lock);
> -	}
> -#endif
> -
>   	spin_unlock_irqrestore(&from->move_lock, flags);
>   
>   	ret = 0;

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

[Wei Yang]

On Fri, Jan 17, 2020 at 11:17:38AM -0800, Yang Shi wrote:
>
>
>On 1/17/20 7:38 AM, Kirill A. Shutemov wrote:
>> On Fri, Jan 17, 2020 at 01:31:50AM -0800, David Rientjes wrote:
>> > On Fri, 17 Jan 2020, Michal Hocko wrote:
>> > 
>> > > 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?
>> > > 
>> > Right, and I don't think that it necessarily is and the second
>> > conditional in Wei's patch will always succeed unless we have raced.  That
>> > patch is for a lock concern but I think Kirill's question has uncovered
>> > something more interesting.
>> > 
>> > Kirill S would definitely be best to answer Kirill T's question, but from
>> > my understanding when mem_cgroup_move_account() is called with
>> > compound == true that we always have an intact pmd (we never migrate
>> > partial page charges for pages on the deferred split queue with the
>> > current charge migration implementation) and thus the underlying page is
>> > not eligible to be split and shouldn't be on the deferred split queue.
>> > 
>> > In other words, a page being on the deferred split queue for a memcg
>> > should only happen when it is charged to that memcg.  (This wasn't the
>> > case when we only had per-node split queues.)  I think that's currently
>> > broken in mem_cgroup_move_account() before Wei's patch.
>> Right. It's broken indeed.
>
>Hmm... Yes, definitely. I wasn't realized this at the first place.
>
>> 
>> We are dealing with anon page here. And it cannot be on deferred list as
>> long as it's mapped with PMD. We cannot get compound == true &&
>> !list_empty() on the (first) enter to the function. Any PMD-mapped page
>> will be put onto deferred by the function. This is wrong.
>> 
>> The fix is not obvious.
>> 
>> This comment got in mem_cgroup_move_charge_pte_range() my attention:
>> 
>> 			/*
>> 			 * We can have a part of the split pmd here. Moving it
>> 			 * can be done but it would be too convoluted so simply
>> 			 * ignore such a partial THP and keep it in original
>> 			 * memcg. There should be somebody mapping the head.
>> 			 */
>> 
>> That's exactly the case we care about: PTE-mapped THP that has to be split
>> under load. We don't move charge of them between memcgs and therefore we
>> should not move the page to different memcg.
>> 
>> I guess this will do the trick :P
>
>It seems correct to me. In addition, memcg move charge just move PMD mapped
>THP, the THP should be never on the deferred split queue of "from" if it is
>PMD mapped, so actually we don't have to move it to the deferred split queue
>of "to".
>

Well, I got the point.

Since Kirill S found the correct solution, should I prepare v3 or Kirill will
send it?

>> 
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index c5b5f74cfd4d..e87ee4c10f6e 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -5359,14 +5359,6 @@ static int mem_cgroup_move_account(struct page *page,
>>   		__mod_lruvec_state(to_vec, NR_WRITEBACK, nr_pages);
>>   	}
>> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> -	if (compound && !list_empty(page_deferred_list(page))) {
>> -		spin_lock(&from->deferred_split_queue.split_queue_lock);
>> -		list_del_init(page_deferred_list(page));
>> -		from->deferred_split_queue.split_queue_len--;
>> -		spin_unlock(&from->deferred_split_queue.split_queue_lock);
>> -	}
>> -#endif
>>   	/*
>>   	 * It is safe to change page->mem_cgroup here because the page
>>   	 * is referenced, charged, and isolated - we can't race with
>> @@ -5376,16 +5368,6 @@ static int mem_cgroup_move_account(struct page *page,
>>   	/* caller should have done css_get */
>>   	page->mem_cgroup = to;
>> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> -	if (compound && list_empty(page_deferred_list(page))) {
>> -		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++;
>> -		spin_unlock(&to->deferred_split_queue.split_queue_lock);
>> -	}
>> -#endif
>> -
>>   	spin_unlock_irqrestore(&from->move_lock, flags);
>>   	ret = 0;

-- 
Wei Yang
Help you, Help me

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

[Kirill A. Shutemov]

On Fri, Jan 17, 2020 at 10:18:59PM +0000, Wei Yang wrote:
> On Fri, Jan 17, 2020 at 11:17:38AM -0800, Yang Shi wrote:
> >
> >
> >On 1/17/20 7:38 AM, Kirill A. Shutemov wrote:
> >> On Fri, Jan 17, 2020 at 01:31:50AM -0800, David Rientjes wrote:
> >> > On Fri, 17 Jan 2020, Michal Hocko wrote:
> >> > 
> >> > > 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?
> >> > > 
> >> > Right, and I don't think that it necessarily is and the second
> >> > conditional in Wei's patch will always succeed unless we have raced.  That
> >> > patch is for a lock concern but I think Kirill's question has uncovered
> >> > something more interesting.
> >> > 
> >> > Kirill S would definitely be best to answer Kirill T's question, but from
> >> > my understanding when mem_cgroup_move_account() is called with
> >> > compound == true that we always have an intact pmd (we never migrate
> >> > partial page charges for pages on the deferred split queue with the
> >> > current charge migration implementation) and thus the underlying page is
> >> > not eligible to be split and shouldn't be on the deferred split queue.
> >> > 
> >> > In other words, a page being on the deferred split queue for a memcg
> >> > should only happen when it is charged to that memcg.  (This wasn't the
> >> > case when we only had per-node split queues.)  I think that's currently
> >> > broken in mem_cgroup_move_account() before Wei's patch.
> >> Right. It's broken indeed.
> >
> >Hmm... Yes, definitely. I wasn't realized this at the first place.
> >
> >> 
> >> We are dealing with anon page here. And it cannot be on deferred list as
> >> long as it's mapped with PMD. We cannot get compound == true &&
> >> !list_empty() on the (first) enter to the function. Any PMD-mapped page
> >> will be put onto deferred by the function. This is wrong.
> >> 
> >> The fix is not obvious.
> >> 
> >> This comment got in mem_cgroup_move_charge_pte_range() my attention:
> >> 
> >> 			/*
> >> 			 * We can have a part of the split pmd here. Moving it
> >> 			 * can be done but it would be too convoluted so simply
> >> 			 * ignore such a partial THP and keep it in original
> >> 			 * memcg. There should be somebody mapping the head.
> >> 			 */
> >> 
> >> That's exactly the case we care about: PTE-mapped THP that has to be split
> >> under load. We don't move charge of them between memcgs and therefore we
> >> should not move the page to different memcg.
> >> 
> >> I guess this will do the trick :P
> >
> >It seems correct to me. In addition, memcg move charge just move PMD mapped
> >THP, the THP should be never on the deferred split queue of "from" if it is
> >PMD mapped, so actually we don't have to move it to the deferred split queue
> >of "to".
> >
> 
> Well, I got the point.
> 
> Since Kirill S found the correct solution, should I prepare v3 or Kirill will
> send it?

Go ahead. With my Suggested-by.

-- 
 Kirill A. Shutemov