Re: [RFC PATCH 4/8] hugetlb: call update_and_free_page without hugetlb_lock

Michal Hocko
2021-03-22T14:20:28Z

On Fri 19-03-21 15:42:05, Mike Kravetz 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@oracle.com>

Looks good to me. I will not ack it right now though. I am still
crawling through the series and want to get a full picture. So far it
looks promising ;).

> ---
> mm/hugetlb.c | 21 +++++++++++++--------
> 1 file changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ae185d3315e0..3028cf10d504 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1362,14 +1362,8 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> 1 << PG_writeback);
> }
> if (hstate_is_gigantic(h)) {
> - /*
> - * Temporarily drop the hugetlb_lock, because
> - * we might block in free_gigantic_page().
> - */
> - spin_unlock(&hugetlb_lock);
> destroy_compound_gigantic_page(page, huge_page_order(h));
> free_gigantic_page(page, huge_page_order(h));
> - spin_lock(&hugetlb_lock);
> } else {
> __free_pages(page, huge_page_order(h));
> }
> @@ -1435,16 +1429,18 @@ 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);
> }
>
> /*
> @@ -1725,7 +1721,13 @@ 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;
> }
> @@ -1794,8 +1796,9 @@ 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);
> @@ -2572,7 +2575,9 @@ static void try_to_free_low(struct hstate *h, unsigned long count,
> remove_hugetlb_page(h, page, false);
> h->free_huge_pages--;
> h->free_huge_pages_node[page_to_nid(page)]--;
> + spin_unlock(&hugetlb_lock);
> update_and_free_page(h, page);
> + spin_lock(&hugetlb_lock);
>
> /*
> * update_and_free_page could have dropped lock so
> --
> 2.30.2
>

--
Michal Hocko
SUSE Labs