Date:   Wed, 20 Sep 2023 10:56:17 +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 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:
>>> 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.
>>> 
>>> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
>>> ---
>>>  mm/hugetlb.c         | 36 ++++++++++++++++++------------------
>>>  mm/hugetlb_vmemmap.c | 37 +++++++++++++++++++++++++++++++++++++
>>>  mm/hugetlb_vmemmap.h | 11 +++++++++++
>>>  3 files changed, 66 insertions(+), 18 deletions(-)
>>> 
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index d6f3db3c1313..814bb1982274 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -1836,36 +1836,36 @@ static void update_and_free_hugetlb_folio(struct hstate *h, struct folio *folio,
>>>  static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
>>>  {
>>> + int ret;
>>> + unsigned long restored;
>>>   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.
>>> +  * First allocate required vmemmmap (if necessary) for all folios.
>>>    */
>>> - list_for_each_entry_safe(folio, t_folio, list, lru) {
>>> - if (folio_test_hugetlb_vmemmap_optimized(folio)) {
>>> - if (hugetlb_vmemmap_restore(h, &folio->page)) {
>>> - spin_lock_irq(&hugetlb_lock);
>>> + ret = hugetlb_vmemmap_restore_folios(h, list, &restored);
>>> +
>>> + /*
>>> +  * If there was an error restoring vmemmap for ANY folios on the list,
>>> +  * add them back as surplus hugetlb pages.  add_hugetlb_folio() removes
>>> +  * the folio from THIS list.
>>> +  */
>>> + if (ret < 0) {
>>> + spin_lock_irq(&hugetlb_lock);
>>> + list_for_each_entry_safe(folio, t_folio, list, lru)
>>> + if (folio_test_hugetlb_vmemmap_optimized(folio))
>>>   add_hugetlb_folio(h, folio, true);
>>> - spin_unlock_irq(&hugetlb_lock);
>>> - } else
>>> - clear_dtor = true;
>>> - }
>>> + spin_unlock_irq(&hugetlb_lock);
>>>   }
>>>   /*
>>> -  * If vmemmmap allocation was performed on any folio above, take lock
>>> -  * to clear destructor of all folios on list.  This avoids the need to
>>> +  * If vmemmmap allocation was performed on ANY folio , 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.
>>>    */
>>> - if (clear_dtor) {
>>> + if (restored) {
>>>   spin_lock_irq(&hugetlb_lock);
>>>   list_for_each_entry(folio, list, lru)
>>>   __clear_hugetlb_destructor(h, folio);
>>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>>> index 4558b814ffab..463a4037ec6e 100644
>>> --- a/mm/hugetlb_vmemmap.c
>>> +++ b/mm/hugetlb_vmemmap.c
>>> @@ -480,6 +480,43 @@ 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: 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.
>>> + */
>>> +int hugetlb_vmemmap_restore_folios(const struct hstate *h,
>>> + struct list_head *folio_list,
>>> + unsigned long *restored)
>>> +{
>>> + unsigned long num_restored;
>>> + struct folio *folio;
>>> + int ret = 0, t_ret;
>>> +
>>> + num_restored = 0;
>>> + list_for_each_entry(folio, folio_list, lru) {
>>> + if (folio_test_hugetlb_vmemmap_optimized(folio)) {
>>> + t_ret = hugetlb_vmemmap_restore(h, &folio->page);
>> 
>> I still think we should free a non-optimized HugeTLB page if we
>> encounter an OOM situation instead of continue to restore
>> vemmmap pages. Restoring vmemmmap pages will only aggravate
>> the OOM situation. The suitable appraoch is to free a non-optimized
>> HugeTLB page to satisfy our allocation of vmemmap pages, what's
>> your opinion, Mike?
> 
> I agree.
> 
> As you mentioned previously, this may complicate this code path a bit.
> I will rewrite to make this happen.

Maybe we could introduced two list passed to update_and_free_pages_bulk (this
will be easy for the callers of it), one is for non-optimized huge page,
another is optimized one. In update_and_free_pages_bulk, we could first
free those non-optimized huge page, and then restore vemmmap pages for
those optimized ones, in which case, the code could be simple.
hugetlb_vmemmap_restore_folios() dose not need to add complexity, which
still continue to restore vmemmap pages and will stop once we encounter
an OOM situation.

Thanks.

> -- 
> Mike Kravetz

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