Re: [PATCH v2 5/8] hugetlb: call update_and_free_page without hugetlb_lock

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/3/30 7:23, 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@oracle.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Reviewed-by: Muchun Song <songmuchun@bytedance.com>

Looks good to me. Thanks!
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>

> ---
>  mm/hugetlb.c | 32 +++++++++++++++++++++++++++-----
>  1 file changed, 27 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 16beabbbbe49..dec7bd0dc63d 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1451,16 +1451,18 @@ static void __free_huge_page(struct page *page)
>  
>  	if (HPageTemporary(page)) {
>  		remove_hugetlb_page(h, page, false);
> +		spin_unlock(&hugetlb_lock);
>  		update_and_free_page(h, page);
>  	} else if (h->surplus_huge_pages_node[nid]) {
>  		/* remove the page from active list */
>  		remove_hugetlb_page(h, page, true);
> +		spin_unlock(&hugetlb_lock);
>  		update_and_free_page(h, page);
>  	} else {
>  		arch_clear_hugepage_flags(page);
>  		enqueue_huge_page(h, page);
> +		spin_unlock(&hugetlb_lock);
>  	}
> -	spin_unlock(&hugetlb_lock);
>  }
>  
>  /*
> @@ -1741,7 +1743,13 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
>  				list_entry(h->hugepage_freelists[node].next,
>  					  struct page, lru);
>  			remove_hugetlb_page(h, page, acct_surplus);
> +			/*
> +			 * unlock/lock around update_and_free_page is temporary
> +			 * and will be removed with subsequent patch.
> +			 */
> +			spin_unlock(&hugetlb_lock);
>  			update_and_free_page(h, page);
> +			spin_lock(&hugetlb_lock);
>  			ret = 1;
>  			break;
>  		}
> @@ -1810,8 +1818,9 @@ int dissolve_free_huge_page(struct page *page)
>  		}
>  		remove_hugetlb_page(h, page, false);
>  		h->max_huge_pages--;
> +		spin_unlock(&hugetlb_lock);
>  		update_and_free_page(h, head);
> -		rc = 0;
> +		return 0;
>  	}
>  out:
>  	spin_unlock(&hugetlb_lock);
> @@ -2674,22 +2683,35 @@ static void try_to_free_low(struct hstate *h, unsigned long count,
>  						nodemask_t *nodes_allowed)
>  {
>  	int i;
> +	struct page *page, *next;
> +	LIST_HEAD(page_list);
>  
>  	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);
> +			list_add(&page->lru, &page_list);
>  		}
>  	}
> +
> +out:
> +	spin_unlock(&hugetlb_lock);
> +	list_for_each_entry_safe(page, next, &page_list, 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,
>