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

[Mike Kravetz]

On 3/25/21 12:39 PM, Michal Hocko wrote:
> On Thu 25-03-21 10:12:05, Mike Kravetz wrote:
>> 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.
> 
> Are you sure? list_del followed by list_add is a normal API usage
> pattern AFAIK. INIT_LIST_HEAD is to do the first initialization before
> first use.

Sorry for the noise.  The INIT_LIST_HEAD is indeed unnecessary.

I must have got confused while looking at a corrupt list splat in
earlier code development.
-- 
Mike Kravetz