Re: [PATCH v4 4/8] hugetlb: perform vmemmap restoration on a list of pages

[Muchun Song]

> On Sep 20, 2023, at 10:56, Muchun Song <muchun.song@xxxxxxxxx> wrote:
> 
> 
> 
>> On Sep 20, 2023, at 04:57, Mike Kravetz <mike.kravetz@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>> ---
>>>> 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.

BTW, maybe we should try again iff there are some non-optimized huge page
whose vmemmap pages are restored successfully previously and could be freed
first, then continue to restore the vmemmap pages of the remaining huge pages.
I think the retry code could be done in update_and_free_pages_bulk() as well.

> 
> Thanks.
> 
>> -- 
>> Mike Kravetz