[PATCH v3 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge_page

[Muchun Song]

There is a race between dissolve_free_huge_page() and put_page(),
and the race window is quite small. 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.

If we free a HugeTLB page from a non-task context, it is deferred
through a workqueue. In this case, we need to flush the work.

The dissolve_free_huge_page() can be called from memory hotplug,
the caller aims to free the HugeTLB page to the buddy allocator
so that the caller can unplug the page successfully.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/hugetlb.c | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 4a9011e12175..a176ceed55f1 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)
 {
@@ -1793,8 +1794,10 @@ 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,
@@ -1813,6 +1816,14 @@ int dissolve_free_huge_page(struct page *page)
 	}
 out:
 	spin_unlock(&hugetlb_lock);
+
+	/*
+	 * If the freeing of the HugeTLB page is put on a work queue, we should
+	 * flush the work before retrying.
+	 */
+	if (unlikely(rc == -EAGAIN))
+		flush_work(&free_hpage_work);
+
 	return rc;
 }
 
@@ -1835,7 +1846,12 @@ 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) {
+			cpu_relax();
+			goto retry;
+		}
 		if (rc)
 			break;
 	}
-- 
2.11.0