[PATCH v4 4/6] mm: hugetlb: retry dissolve page when hitting race

[Muchun Song]

There is a race between dissolve_free_huge_page() and put_page().
Theoretically, we should return -EBUSY when we encounter this race.
In fact, we have a chance to successfully dissolve the page if we
do a retry. Because the race window is quite small. If we seize
this opportunity, it is an optimization for increasing the success
rate of dissolving page.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/hugetlb.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 4a9011e12175..898e4ea43e13 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1772,6 +1772,7 @@ int dissolve_free_huge_page(struct page *page)
 {
 	int rc = -EBUSY;
 
+retry:
 	/* Not to disrupt normal path by vainly holding hugetlb_lock */
 	if (!PageHuge(page))
 		return 0;
@@ -1793,8 +1794,23 @@ int dissolve_free_huge_page(struct page *page)
 		 * We should make sure that the page is already on the free list
 		 * when it is dissolved.
 		 */
-		if (unlikely(!PageHugeFreed(head)))
-			goto out;
+		if (unlikely(!PageHugeFreed(head))) {
+			spin_unlock(&hugetlb_lock);
+
+			/*
+			 * Theoretically, we should return -EBUSY when we
+			 * encounter this race. In fact, we have a chance
+			 * to successfully dissolve the page if we do a
+			 * retry. Because the race window is quite small.
+			 * If we seize this opportunity, it is an optimization
+			 * for increasing the success rate of dissolving page.
+			 */
+			while (PageHeadHuge(head) && !PageHugeFreed(head)) {
+				cond_resched();
+				cpu_relax();
+			}
+			goto retry;
+		}
 
 		/*
 		 * Move PageHWPoison flag from head page to the raw error page,
-- 
2.11.0