Re: [PATCH 5/8] hugetlb: call update_and_free_page without hugetlb_lock

[Mike Kravetz]

On 3/25/21 3:55 AM, Michal Hocko wrote:
> On Wed 24-03-21 17:28:32, Mike Kravetz wrote:
>> With the introduction of remove_hugetlb_page(), there is no need for
>> update_and_free_page to hold the hugetlb lock.  Change all callers to
>> drop the lock before calling.
>>
>> With additional code modifications, this will allow loops which decrease
>> the huge page pool to drop the hugetlb_lock with each page to reduce
>> long hold times.
>>
>> The ugly unlock/lock cycle in free_pool_huge_page will be removed in
>> a subsequent patch which restructures free_pool_huge_page.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> 
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> 
> One minor thing below
> 
> [...]
>> @@ -2563,22 +2572,37 @@ static void try_to_free_low(struct hstate *h, unsigned long count,
>>  						nodemask_t *nodes_allowed)
>>  {
>>  	int i;
>> +	struct list_head page_list;
>> +	struct page *page, *next;
>>  
>>  	if (hstate_is_gigantic(h))
>>  		return;
>>  
>> +	/*
>> +	 * Collect pages to be freed on a list, and free after dropping lock
>> +	 */
>> +	INIT_LIST_HEAD(&page_list);
>>  	for_each_node_mask(i, *nodes_allowed) {
>> -		struct page *page, *next;
>>  		struct list_head *freel = &h->hugepage_freelists[i];
>>  		list_for_each_entry_safe(page, next, freel, lru) {
>>  			if (count >= h->nr_huge_pages)
>> -				return;
>> +				goto out;
>>  			if (PageHighMem(page))
>>  				continue;
>>  			remove_hugetlb_page(h, page, false);
>> -			update_and_free_page(h, page);
>> +			INIT_LIST_HEAD(&page->lru);
> 
> What is the point of rhis INIT_LIST_HEAD? Page has been removed from the
> list by remove_hugetlb_page so it can be added to a new one without any
> reinitialization.

remove_hugetlb_page just does a list_del.  list_del will poison the
pointers in page->lru.  The following list_add will then complain about
list corruption.

I could replace the list_del in remove_hugetlb_page with list_del_init.
However, not all callers of remove_hugetlb_page will be adding the page
to a list.  If we just call update_and_free_page, there is no need to
reinitialize the list pointers.

Might be better to just use list_del_init in remove_hugetlb_page to
avoid any questions like this.
-- 
Mike Kravetz

> 
>> +			list_add(&page->lru, &page_list);
>>  		}
>>  	}
>> +
>> +out:
>> +	spin_unlock(&hugetlb_lock);
>> +	list_for_each_entry_safe(page, next, &page_list, lru) {
>> +		list_del(&page->lru);
>> +		update_and_free_page(h, page);
>> +		cond_resched();
>> +	}
>> +	spin_lock(&hugetlb_lock);
>>  }
>>  #else
>>  static inline void try_to_free_low(struct hstate *h, unsigned long count,
>> -- 
>> 2.30.2
>>
>