Date:   Fri, 22 Sep 2023 16:19:17 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        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 v4 4/8] hugetlb: perform vmemmap restoration on a list of
 pages



On 2023/9/22 05:58, Mike Kravetz wrote:
> On 09/21/23 17:31, Muchun Song wrote:
>>
>> On 2023/9/21 09:12, Mike Kravetz wrote:
>>> On 09/20/23 11:03, Muchun Song wrote:
>>>>> On Sep 20, 2023, at 10:56, Muchun Song <muchun.song@...ux.dev> wrote:
>>>>>> On Sep 20, 2023, at 04:57, Mike Kravetz <mike.kravetz@...cle.com> wrote:
>>>>>> On 09/19/23 17:52, Muchun Song wrote:
>>>>>>> On 2023/9/19 07:01, Mike Kravetz wrote:
>>> +/**
>>> + * hugetlb_vmemmap_restore_folios - restore vmemmap for every folio on the list.
>>> + * @h:		struct hstate.
>>> + * @folio_list:	list of folios.
>>> + * @restored:	Set to number of folios for which vmemmap was restored
>>> + *		successfully if caller passes a non-NULL pointer.
>>> + *
>>> + * Return: %0 if vmemmap exists for all folios on the list.  If an error is
>>> + *		encountered restoring vmemmap for ANY folio, an error code
>>> + *		will be returned to the caller.  It is then the responsibility
>>> + *		of the caller to check the hugetlb vmemmap optimized flag of
>>> + *		each folio to determine if vmemmap was actually restored.
>>> + *		Note that processing is stopped when first error is encountered.
>>> + */
>>> +int hugetlb_vmemmap_restore_folios(const struct hstate *h,
>>> +					struct list_head *folio_list,
>>> +					unsigned long *restored)
>> How about changing parameter of @restored to a list_head type which
>> returns the non-optimized (previously) or vmemmap-restored-sucessful huge
>> pages?
>> In which case, the caller could traverse this returned list to free
>> them first like you have implemented in bulk_vmemmap_restore_enomem(),
>> it will be more efficient. The meaning of returned value should also
>> be changed accordingly since update_and_free_pages_bulk() wants to
>> whether there is a vmemmap-optimized huge page being restored sucessfully
>> to determine if it should clear hugetlb flag. So
>> hugetlb_vmemmap_restore_folios()
>> could return how many huge pages being restored successful, if a negative
>> number is returned meaning there is some error in the process of restoring
>> of vmemmap.
>>
> I had similar thoughts.  An updated patch based on this approach is below.
> When creating the patch, I discovered that using the function return code
> for both number of vmemmap restored pages as well as error code was
> unnecessary.  Just checking !list_empty() of non-optimized pages tells us
> if any were restored or could be freed.

I also thought about this. But there is a little different. If HVO
is disabled, we will always clear the hugetlb flag twice since the
list couldn't be empty, I thought it is an optimization for HVO-disabled
case.

>
>  From b79f6eeb7a11644830bddfc43d2219c149d26405 Mon Sep 17 00:00:00 2001
> From: Mike Kravetz <mike.kravetz@...cle.com>
> Date: Sun, 10 Sep 2023 16:14:50 -0700
> Subject: [PATCH] hugetlb: perform vmemmap restoration on a list of pages
>
> 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@...cle.com>
> ---
>   mm/hugetlb.c         | 94 +++++++++++++++++++++++++++++++-------------
>   mm/hugetlb_vmemmap.c | 36 +++++++++++++++++
>   mm/hugetlb_vmemmap.h | 10 +++++
>   3 files changed, 112 insertions(+), 28 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 70fedf8682c4..11de3f885065 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1834,50 +1834,88 @@ 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 *list,
> +					struct list_head *non_op_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_op_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_op_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 vmemmap was not restored for ANY folios,
> +		 * we loop through them trying to restore 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 for one hugetlb page, we free
> +		 * it and quit processing the list to retry the bulk operation.
> +		 */
> +		list_for_each_entry_safe(folio, t_folio, list, lru)
>   			if (hugetlb_vmemmap_restore(h, &folio->page)) {
>   				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 *list)
> +{
> +	int ret;
> +	LIST_HEAD(non_op_folio);
> +	struct folio *folio, *t_folio;
>   
>   	/*
> -	 * 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, list, &non_op_folio);
> +	if (ret < 0) {
> +		bulk_vmemmap_restore_error(h, list, &non_op_folio);
> +		goto retry;
> +	}
> +
> +	/*
> +	 * At this point, list should be empty, and there should only be
> +	 * pages on the non_op_folio list.  free those entries.  Do note
> +	 * that the non_op_folio list could be empty.
> +	 */
> +	VM_WARN_ON(!list_empty(list));
> +	if (!list_empty(&non_op_folio)) {
>   		spin_lock_irq(&hugetlb_lock);
> -		list_for_each_entry(folio, list, lru)
> +		list_for_each_entry(folio, &non_op_folio, 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_op_folio, 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..f827d4efcf8e 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -480,6 +480,42 @@ 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_op_list:	Output list of folio for which vmemmap exists.
> + *
> + * Return: %0 if vmemmap exists for all folios on the list and all entries have
> + *		been moved to non_op_list.  If an error is encountered restoring
> + *		vmemmap for ANY folio, an error code will be returned to the
> + *		caller.  Processing en entries stops when the first error is
> + *		encountered.  folios processed before the error with vmemmap
> + *		will reside on the non_op_list.  The folio that experienced the
> + *		error and all non-processed folios will remain on folio_list.
> + */
> +int hugetlb_vmemmap_restore_folios(const struct hstate *h,
> +					struct list_head *folio_list,
> +					struct list_head *non_op_list)

non_optimized_list or vmemmap_intact_list? The abbreviation is not 
straightforward.

> +{
> +	struct folio *folio, *t_folio;
> +	int ret = 0;
> +
> +	list_for_each_entry_safe(folio, t_folio, folio_list, lru) {
> +		if (folio_test_hugetlb_vmemmap_optimized(folio)) {

hugetlb_vmemmap_restore() has this check as well, so it is unnecessary here.

> +			ret = hugetlb_vmemmap_restore(h, &folio->page);
> +			if (ret)
> +				goto out;

Maybe we could drop the label ("out") and just breaking or returning from
here is enough.

> +		}
> +
> +		/* Add non-optimized folios to output list */
> +		list_move(&folio->lru, non_op_list);
> +	}
> +
> +out:
> +	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..e6378ae5c5b6 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);
> +int hugetlb_vmemmap_restore_folios(const struct hstate *h,
> +					struct list_head *folio_list,
> +					struct list_head *non_op_folios);

It is better to keep 3rd name (non_op_folios) consistent with where it is
defined (it is non_op_list).

>   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 int hugetlb_vmemmap_restore_folios(const struct hstate *h,
> +					struct list_head *folio_list,
> +					struct list_head *non_op_folios)
> +{
> +	return 0;
> +}
> +
>   static inline void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
>   {
>   }

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