Re: [External] [PATCH v2 5/8] hugetlb: call update_and_free_page without hugetlb_lock

[Mike Kravetz]

On 3/29/21 7:21 PM, Muchun Song wrote:
> On Tue, Mar 30, 2021 at 7:24 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>>
>> With the introduction of remove_hugetlb_page(), there is no need for
>> update_and_free_page to hold the hugetlb lock.  Change all callers to
>> drop the lock before calling.
>>
>> With additional code modifications, this will allow loops which decrease
>> the huge page pool to drop the hugetlb_lock with each page to reduce
>> long hold times.
>>
>> The ugly unlock/lock cycle in free_pool_huge_page will be removed in
>> a subsequent patch which restructures free_pool_huge_page.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>> Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>> ---
>>  mm/hugetlb.c | 32 +++++++++++++++++++++++++++-----
>>  1 file changed, 27 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 16beabbbbe49..dec7bd0dc63d 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -1451,16 +1451,18 @@ static void __free_huge_page(struct page *page)
>>
>>         if (HPageTemporary(page)) {
>>                 remove_hugetlb_page(h, page, false);
>> +               spin_unlock(&hugetlb_lock);
>>                 update_and_free_page(h, page);
>>         } else if (h->surplus_huge_pages_node[nid]) {
>>                 /* remove the page from active list */
>>                 remove_hugetlb_page(h, page, true);
>> +               spin_unlock(&hugetlb_lock);
>>                 update_and_free_page(h, page);
>>         } else {
>>                 arch_clear_hugepage_flags(page);
>>                 enqueue_huge_page(h, page);
>> +               spin_unlock(&hugetlb_lock);
>>         }
>> -       spin_unlock(&hugetlb_lock);
>>  }
>>
>>  /*
>> @@ -1741,7 +1743,13 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
>>                                 list_entry(h->hugepage_freelists[node].next,
>>                                           struct page, lru);
>>                         remove_hugetlb_page(h, page, acct_surplus);
>> +                       /*
>> +                        * unlock/lock around update_and_free_page is temporary
>> +                        * and will be removed with subsequent patch.
>> +                        */
>> +                       spin_unlock(&hugetlb_lock);
>>                         update_and_free_page(h, page);
>> +                       spin_lock(&hugetlb_lock);
>>                         ret = 1;
>>                         break;
>>                 }
>> @@ -1810,8 +1818,9 @@ int dissolve_free_huge_page(struct page *page)
>>                 }
>>                 remove_hugetlb_page(h, page, false);
>>                 h->max_huge_pages--;
>> +               spin_unlock(&hugetlb_lock);
>>                 update_and_free_page(h, head);
>> -               rc = 0;
>> +               return 0;
>>         }
>>  out:
>>         spin_unlock(&hugetlb_lock);
>> @@ -2674,22 +2683,35 @@ static void try_to_free_low(struct hstate *h, unsigned long count,
>>                                                 nodemask_t *nodes_allowed)
>>  {
>>         int i;
>> +       struct page *page, *next;
>> +       LIST_HEAD(page_list);
>>
>>         if (hstate_is_gigantic(h))
>>                 return;
>>
>> +       /*
>> +        * Collect pages to be freed on a list, and free after dropping lock
>> +        */
>> +       INIT_LIST_HEAD(&page_list);
> 
> INIT_LIST_HEAD is unnecessary. Because the macro of
> LIST_HEAD already initializes the list_head structure.
> 

Thanks.
I will fix here and the same issue in patch 6.
-- 
Mike Kravetz