linux-kernel - Re: [PATCH 5/8] hugetlb: call update_and_free_page without hugetlb

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Date:   Thu, 25 Mar 2021 11:55:03 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Rientjes <rientjes@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        HORIGUCHI NAOYA <naoya.horiguchi@....com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Waiman Long <longman@...hat.com>, Peter Xu <peterx@...hat.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Hillf Danton <hdanton@...a.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 5/8] hugetlb: call update_and_free_page without
 hugetlb_lock

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@...cle.com>

Acked-by: Michal Hocko <mhocko@...e.com>

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.

> +			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
> 

-- 
Michal Hocko
SUSE Labs