Re: [RFC PATCH 4/5] mm, hugetlb: get rid of surplus page accounting tricks

[Mike Kravetz]

On 12/13/2017 11:50 PM, Michal Hocko wrote:
> On Wed 13-12-17 16:45:55, Mike Kravetz wrote:
>> On 12/04/2017 06:01 AM, Michal Hocko wrote:
>>> From: Michal Hocko <mhocko@xxxxxxxx>
>>>
>>> alloc_surplus_huge_page increases the pool size and the number of
>>> surplus pages opportunistically to prevent from races with the pool size
>>> change. See d1c3fb1f8f29 ("hugetlb: introduce nr_overcommit_hugepages
>>> sysctl") for more details.
>>>
>>> The resulting code is unnecessarily hairy, cause code duplication and
>>> doesn't allow to share the allocation paths. Moreover pool size changes
>>> tend to be very seldom so optimizing for them is not really reasonable.
>>> Simplify the code and allow to allocate a fresh surplus page as long as
>>> we are under the overcommit limit and then recheck the condition after
>>> the allocation and drop the new page if the situation has changed. This
>>> should provide a reasonable guarantee that an abrupt allocation requests
>>> will not go way off the limit.
>>>
>>> If we consider races with the pool shrinking and enlarging then we
>>> should be reasonably safe as well. In the first case we are off by one
>>> in the worst case and the second case should work OK because the page is
>>> not yet visible. We can waste CPU cycles for the allocation but that
>>> should be acceptable for a relatively rare condition.
>>>
>>> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
>>> ---
>>>  mm/hugetlb.c | 60 +++++++++++++++++++++---------------------------------------
>>>  1 file changed, 21 insertions(+), 39 deletions(-)
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index a1b8b2888ec9..0c7dc269b6c0 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -1538,62 +1538,44 @@ int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
>>>  static struct page *__alloc_surplus_huge_page(struct hstate *h, gfp_t gfp_mask,
>>>  		int nid, nodemask_t *nmask)
>>>  {
>>> -	struct page *page;
>>> -	unsigned int r_nid;
>>> +	struct page *page = NULL;
>>>  
>>>  	if (hstate_is_gigantic(h))
>>>  		return NULL;
>>>  
>>> -	/*
>>> -	 * Assume we will successfully allocate the surplus page to
>>> -	 * prevent racing processes from causing the surplus to exceed
>>> -	 * overcommit
>>> -	 *
>>> -	 * This however introduces a different race, where a process B
>>> -	 * tries to grow the static hugepage pool while alloc_pages() is
>>> -	 * called by process A. B will only examine the per-node
>>> -	 * counters in determining if surplus huge pages can be
>>> -	 * converted to normal huge pages in adjust_pool_surplus(). A
>>> -	 * won't be able to increment the per-node counter, until the
>>> -	 * lock is dropped by B, but B doesn't drop hugetlb_lock until
>>> -	 * no more huge pages can be converted from surplus to normal
>>> -	 * state (and doesn't try to convert again). Thus, we have a
>>> -	 * case where a surplus huge page exists, the pool is grown, and
>>> -	 * the surplus huge page still exists after, even though it
>>> -	 * should just have been converted to a normal huge page. This
>>> -	 * does not leak memory, though, as the hugepage will be freed
>>> -	 * once it is out of use. It also does not allow the counters to
>>> -	 * go out of whack in adjust_pool_surplus() as we don't modify
>>> -	 * the node values until we've gotten the hugepage and only the
>>> -	 * per-node value is checked there.
>>> -	 */
>>>  	spin_lock(&hugetlb_lock);
>>> -	if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages) {
>>> -		spin_unlock(&hugetlb_lock);
>>> -		return NULL;
>>> -	} else {
>>> -		h->nr_huge_pages++;
>>> -		h->surplus_huge_pages++;
>>> -	}
>>> +	if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages)
>>> +		goto out_unlock;
>>>  	spin_unlock(&hugetlb_lock);
>>>  
>>>  	page = __hugetlb_alloc_buddy_huge_page(h, gfp_mask, nid, nmask);
>>> +	if (!page)
>>> +		goto out_unlock;
>>>  
>>>  	spin_lock(&hugetlb_lock);
>>> -	if (page) {
>>> +	/*
>>> +	 * We could have raced with the pool size change.
>>> +	 * Double check that and simply deallocate the new page
>>> +	 * if we would end up overcommiting the surpluses. Abuse
>>> +	 * temporary page to workaround the nasty free_huge_page
>>> +	 * codeflow
>>> +	 */
>>> +	if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages) {
>>> +		SetPageHugeTemporary(page);
>>> +		put_page(page);
>>> +		page = NULL;
>>> +	} else {
>>> +		h->surplus_huge_pages_node[page_to_nid(page)]++;
>>> +		h->surplus_huge_pages++;
>>>  		INIT_LIST_HEAD(&page->lru);
>>>  		r_nid = page_to_nid(page);
>>>  		set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
>>>  		set_hugetlb_cgroup(page, NULL);
>>> -		/*
>>> -		 * We incremented the global counters already
>>> -		 */
>>>  		h->nr_huge_pages_node[r_nid]++;
>>>  		h->surplus_huge_pages_node[r_nid]++;
>>> -	} else {
>>> -		h->nr_huge_pages--;
>>> -		h->surplus_huge_pages--;
>>
>> In the case of a successful surplus allocation, the following counters
>> are incremented:
>>
>> h->surplus_huge_pages_node[page_to_nid(page)]++;
>> h->surplus_huge_pages++;
>> h->nr_huge_pages_node[r_nid]++;
>> h->surplus_huge_pages_node[r_nid]++;
>>
>> Looks like per-node surplus_huge_pages_node is incremented twice, and
>> global nr_huge_pages is not incremented at all.
>>
>> Also, you removed r_nid so I'm guessing this will not compile?
> 
> Ups a hickup during the rebase/split up. The following code removes all
> this so I haven't noticed. Thanks for catching that!
> The incremental diff
> ---
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 41d2d9082f0d..3c16cde72ceb 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1565,8 +1565,10 @@ static struct page *__alloc_surplus_huge_page(struct hstate *h, gfp_t gfp_mask,
>  		put_page(page);
>  		page = NULL;
>  	} else {
> -		h->surplus_huge_pages_node[page_to_nid(page)]++;
> +		int r_nid;
> +
>  		h->surplus_huge_pages++;
> +		h->nr_huge_pages++;
>  		INIT_LIST_HEAD(&page->lru);
>  		r_nid = page_to_nid(page);
>  		set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> 

With the incremental diff,

Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
-- 
Mike Kravetz