Re: [PATCH v5 4/8] hugetlb: perform vmemmap restoration on a list of pages

[Muchun Song <muchun.song@linux.dev>]

On 2023/9/25 08:39, Mike Kravetz wrote:
> The routine update_and_free_pages_bulk already performs vmemmap
> restoration on the list of hugetlb pages in a separate step.  In
> preparation for more functionality to be added in this step, create a
> new routine hugetlb_vmemmap_restore_folios() that will restore
> vmemmap for a list of folios.
>
> This new routine must provide sufficient feedback about errors and
> actual restoration performed so that update_and_free_pages_bulk can
> perform optimally.
>
> Special care must be taken when encountering an error from
> hugetlb_vmemmap_restore_folios.  We want to continue making as much
> forward progress as possible.  A new routine bulk_vmemmap_restore_error
> handles this specific situation.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>   mm/hugetlb.c         | 98 +++++++++++++++++++++++++++++++-------------
>   mm/hugetlb_vmemmap.c | 38 +++++++++++++++++
>   mm/hugetlb_vmemmap.h | 10 +++++
>   3 files changed, 118 insertions(+), 28 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index da0ebd370b5f..53df35fbc3f2 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1834,50 +1834,92 @@ static void update_and_free_hugetlb_folio(struct hstate *h, struct folio *folio,
>   		schedule_work(&free_hpage_work);
>   }
>   
> -static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
> +static void bulk_vmemmap_restore_error(struct hstate *h,
> +					struct list_head *folio_list,
> +					struct list_head *non_hvo_folios)
>   {
>   	struct folio *folio, *t_folio;
> -	bool clear_dtor = false;
>   
> -	/*
> -	 * First allocate required vmemmmap (if necessary) for all folios on
> -	 * list.  If vmemmap can not be allocated, we can not free folio to
> -	 * lower level allocator, so add back as hugetlb surplus page.
> -	 * add_hugetlb_folio() removes the page from THIS list.
> -	 * Use clear_dtor to note if vmemmap was successfully allocated for
> -	 * ANY page on the list.
> -	 */
> -	list_for_each_entry_safe(folio, t_folio, list, lru) {
> -		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
> +	if (!list_empty(non_hvo_folios)) {
> +		/*
> +		 * Free any restored hugetlb pages so that restore of the
> +		 * entire list can be retried.
> +		 * The idea is that in the common case of ENOMEM errors freeing
> +		 * hugetlb pages with vmemmap we will free up memory so that we
> +		 * can allocate vmemmap for more hugetlb pages.
> +		 */
> +		list_for_each_entry_safe(folio, t_folio, non_hvo_folios, lru) {
> +			list_del(&folio->lru);
> +			spin_lock_irq(&hugetlb_lock);
> +			__clear_hugetlb_destructor(h, folio);
> +			spin_unlock_irq(&hugetlb_lock);
> +			update_and_free_hugetlb_folio(h, folio, false);
> +			cond_resched();
> +		}
> +	} else {
> +		/*
> +		 * In the case where there are no folios which can be
> +		 * immediately freed, we loop through the list trying to restore
> +		 * vmemmap individually in the hope that someone elsewhere may
> +		 * have done something to cause success (such as freeing some
> +		 * memory).  If unable to restore a hugetlb page, the hugetlb
> +		 * page is made a surplus page and removed from the list.
> +		 * If are able to restore vmemmap and free one hugetlb page, we
> +		 * quit processing the list to retry the bulk operation.
> +		 */
> +		list_for_each_entry_safe(folio, t_folio, folio_list, lru)
>   			if (hugetlb_vmemmap_restore(h, &folio->page)) {

IIUC, the folio should be deleted from the folio list since this
huge page will be added to the free list (the list is corrupted),
right?

>   				spin_lock_irq(&hugetlb_lock);
>   				add_hugetlb_folio(h, folio, true);
>   				spin_unlock_irq(&hugetlb_lock);
> -			} else
> -				clear_dtor = true;
> -		}
> +			} else {
> +				list_del(&folio->lru);
> +				spin_lock_irq(&hugetlb_lock);
> +				__clear_hugetlb_destructor(h, folio);
> +				spin_unlock_irq(&hugetlb_lock);
> +				update_and_free_hugetlb_folio(h, folio, false);
> +				cond_resched();
> +				break;
> +			}
>   	}
> +}
> +
> +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();
>   	}
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 4558b814ffab..77f44b81ff01 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -480,6 +480,44 @@ int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head)
>   	return ret;
>   }
>   
> +/**
> + * hugetlb_vmemmap_restore_folios - restore vmemmap for every folio on the list.
> + * @h:			hstate.
> + * @folio_list:		list of folios.
> + * @non_hvo_folios:	Output list of folios for which vmemmap exists.
> + *
> + * Return: number of folios for which vmemmap was restored, or an error code
> + *		if an error was encountered restoring vmemmap for a folio.
> + *		Folios that have vmemmap are moved to the non_hvo_folios
> + *		list.  Processing of entries stops when the first error is
> + *		encountered. The folio that experienced the error and all
> + *		non-processed folios will remain on folio_list.
> + */
> +long hugetlb_vmemmap_restore_folios(const struct hstate *h,
> +					struct list_head *folio_list,
> +					struct list_head *non_hvo_folios)
> +{
> +	struct folio *folio, *t_folio;
> +	long restored = 0;
> +	long ret = 0;
> +
> +	list_for_each_entry_safe(folio, t_folio, folio_list, lru) {
> +		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
> +			ret = hugetlb_vmemmap_restore(h, &folio->page);
> +			if (ret)
> +				break;
> +			restored++;
> +		}
> +
> +		/* Add non-optimized folios to output list */
> +		list_move(&folio->lru, non_hvo_folios);
> +	}
> +
> +	if (!ret)
> +		ret = restored;
> +	return ret;
> +}
> +
>   /* Return true iff a HugeTLB whose vmemmap should and can be optimized. */
>   static bool vmemmap_should_optimize(const struct hstate *h, const struct page *head)
>   {
> 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;
> +}
> +
>   static inline void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
>   {
>   }