linux-kernel - Re: [PATCH v6 4/8] hugetlb: perform vmemmap restoration on a list of pages

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Date:   Fri, 29 Sep 2023 15:10:18 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Muchun Song <songmuchun@...edance.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Rientjes <rientjes@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Barry Song <21cnbao@...il.com>, Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Xiongchun Duan <duanxiongchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v6 4/8] hugetlb: perform vmemmap restoration on a list of
 pages

On 09/25/23 16:48, Mike Kravetz wrote:
<snip>
> +static void update_and_free_pages_bulk(struct hstate *h,
> +						struct list_head *folio_list)
> +{
> +	long ret;
> +	struct folio *folio, *t_folio;
> +	LIST_HEAD(non_hvo_folios);
>  
>  	/*
> -	 * If vmemmmap allocation was performed on any folio above, take lock
> -	 * to clear destructor of all folios on list.  This avoids the need to
> -	 * lock/unlock for each individual folio.
> -	 * The assumption is vmemmap allocation was performed on all or none
> -	 * of the folios on the list.  This is true expect in VERY rare cases.
> +	 * First allocate required vmemmmap (if necessary) for all folios.
> +	 * Carefully handle errors and free up any available hugetlb pages
> +	 * in an effort to make forward progress.
>  	 */
> -	if (clear_dtor) {
> +retry:
> +	ret = hugetlb_vmemmap_restore_folios(h, folio_list, &non_hvo_folios);
> +	if (ret < 0) {
> +		bulk_vmemmap_restore_error(h, folio_list, &non_hvo_folios);
> +		goto retry;
> +	}
> +
> +	/*
> +	 * At this point, list should be empty, ret should be >= 0 and there
> +	 * should only be pages on the non_hvo_folios list.
> +	 * Do note that the non_hvo_folios list could be empty.
> +	 * Without HVO enabled, ret will be 0 and there is no need to call
> +	 * __clear_hugetlb_destructor as this was done previously.
> +	 */
> +	VM_WARN_ON(!list_empty(folio_list));
> +	VM_WARN_ON(ret < 0);
> +	if (!list_empty(&non_hvo_folios) && ret) {
>  		spin_lock_irq(&hugetlb_lock);
> -		list_for_each_entry(folio, list, lru)
> +		list_for_each_entry(folio, &non_hvo_folios, lru)
>  			__clear_hugetlb_destructor(h, folio);
>  		spin_unlock_irq(&hugetlb_lock);
>  	}
>  
> -	/*
> -	 * Free folios back to low level allocators.  vmemmap and destructors
> -	 * were taken care of above, so update_and_free_hugetlb_folio will
> -	 * not need to take hugetlb lock.
> -	 */
> -	list_for_each_entry_safe(folio, t_folio, list, lru) {
> +	list_for_each_entry_safe(folio, t_folio, &non_hvo_folios, lru) {
>  		update_and_free_hugetlb_folio(h, folio, false);
>  		cond_resched();
>  	}
<snip>
> diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> index c512e388dbb4..0b7710f90e38 100644
> --- a/mm/hugetlb_vmemmap.h
> +++ b/mm/hugetlb_vmemmap.h
> @@ -19,6 +19,9 @@
>  
>  #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
>  int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head);
> +long hugetlb_vmemmap_restore_folios(const struct hstate *h,
> +					struct list_head *folio_list,
> +					struct list_head *non_hvo_folios);
>  void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head);
>  void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_list);
>  
> @@ -45,6 +48,13 @@ static inline int hugetlb_vmemmap_restore(const struct hstate *h, struct page *h
>  	return 0;
>  }
>  
> +static long hugetlb_vmemmap_restore_folios(const struct hstate *h,
> +					struct list_head *folio_list,
> +					struct list_head *non_hvo_folios)
> +{
> +	return 0;
> +}

update_and_free_pages_bulk depends on pages with complete vmemmap being
moved from folio_list to non_hvo_folios.  In the case where we return 0,
it expects ALL pages to be moved.  Therefore, in the case where
!CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP the stub above must perform

	list_splice_init(folio_list, non_hvo_folios);

before returning 0.

I will update and send a new version along with any changes needed to
address the arm64 boot issue reported with patch 2.
-- 
Mike Kravetz