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...