[PATCH 2/2] hugetlb: optimize update_and_free_pages_bulk to avoid lock cycles

[Mike Kravetz]

update_and_free_pages_bulk is designed to free a list of hugetlb pages
back to their associated lower level allocators.  This may require
allocating vmemmmap pages associated with each hugetlb page.  The
hugetlb page destructor must be changed before pages are freed to lower
level allocators.  However, the destructor must be changed under the
hugetlb lock.  This means there is potentially one lock cycle per page.

Minimize the number of lock cycles in update_and_free_pages_bulk by:
1) allocating necessary vmemmap for all hugetlb pages on the list
2) take hugetlb lock and clear destructor for all pages on the list
3) free all pages on list back to low level allocators

Fixes: ad2fa3717b74 ("mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/hugetlb.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 1b67bf341c32..e751fced870a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1856,11 +1856,44 @@ static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
 {
 	struct page *page, *t_page;
 	struct folio *folio;
+	bool clear_dtor = false;
 
+	/*
+	 * First allocate required vmemmmap for all pages on list.  If vmemmap
+	 * can not be allocated, we can not free page to lower level allocator,
+	 * so add back as hugetlb surplus page.
+	 */
+	list_for_each_entry_safe(page, t_page, list, lru) {
+		if (HPageVmemmapOptimized(page)) {
+			clear_dtor = true;
+			if (hugetlb_vmemmap_restore(h, page)) {
+				spin_lock_irq(&hugetlb_lock);
+				add_hugetlb_folio(h, folio, true);
+				spin_unlock_irq(&hugetlb_lock);
+			}
+			cond_resched();
+		}
+	}
+
+	/*
+	 * If vmemmmap allocation performed above, then take lock * to clear
+	 * destructor of all pages on list.
+	 */
+	if (clear_dtor) {
+		spin_lock_irq(&hugetlb_lock);
+		list_for_each_entry(page, list, lru)
+			__clear_hugetlb_destructor(h, page_folio(page));
+		spin_unlock_irq(&hugetlb_lock);
+	}
+
+	/*
+	 * Free pages 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(page, t_page, list, lru) {
 		folio = page_folio(page);
 		update_and_free_hugetlb_folio(h, folio, false);
-		cond_resched();
 	}
 }
 
-- 
2.41.0