[PATCH v4 1/8] 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

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Acked-by: James Houghton <jthoughton@xxxxxxxxxx>
---
 mm/hugetlb.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e2123d1bb4a2..1ca0377c62ab 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1837,7 +1837,46 @@ 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)
 {
 	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.
+	 */
+	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);
+				add_hugetlb_folio(h, folio, true);
+				spin_unlock_irq(&hugetlb_lock);
+			} else
+				clear_dtor = true;
+		}
+	}
+
+	/*
+	 * If vmemmmap allocation was performed on any folio above, 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) {
+		spin_lock_irq(&hugetlb_lock);
+		list_for_each_entry(folio, list, lru)
+			__clear_hugetlb_destructor(h, folio);
+		spin_unlock_irq(&hugetlb_lock);
+	}
+
+	/*
+	 * Free folios 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(folio, t_folio, list, lru) {
 		update_and_free_hugetlb_folio(h, folio, false);
 		cond_resched();
-- 
2.41.0