[PATCH 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge_page

[Muchun Song <songmuchun@xxxxxxxxxxxxx>]

When dissolve_free_huge_page() races with __free_huge_page(), we can
do a retry. Because the race window is small.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/hugetlb.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 72608008f8b4..db00ae375d2a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1763,10 +1763,11 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
  * nothing for in-use hugepages and non-hugepages.
  * This function returns values like below:
  *
- *  -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
- *          (allocated or reserved.)
- *       0: successfully dissolved free hugepages or the page is not a
- *          hugepage (considered as already dissolved)
+ *  -EAGAIN: race with __free_huge_page() and can do a retry
+ *  -EBUSY:  failed to dissolved free hugepages or the hugepage is in-use
+ *           (allocated or reserved.)
+ *       0:  successfully dissolved free hugepages or the page is not a
+ *           hugepage (considered as already dissolved)
  */
 int dissolve_free_huge_page(struct page *page)
 {
@@ -1815,8 +1816,10 @@ 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)))
+		if (unlikely(!PageHugeFreed(head))) {
+			rc = -EAGAIN;
 			goto out;
+		}
 
 		/*
 		 * Move PageHWPoison flag from head page to the raw error page,
@@ -1857,7 +1860,10 @@ int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
 
 	for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << minimum_order) {
 		page = pfn_to_page(pfn);
+retry:
 		rc = dissolve_free_huge_page(page);
+		if (rc == -EAGAIN)
+			goto retry;
 		if (rc)
 			break;
 	}
-- 
2.11.0