[PATCH v2 2/2] mm: hugetlb: soft-offline: dissolve_free_huge_page() return zero on !PageHuge

[Naoya Horiguchi]

madvise(MADV_SOFT_OFFLINE) often returns -EBUSY when calling soft offline
for hugepages with overcommitting enabled. That was caused by the suboptimal
code in current soft-offline code. See the following part:

    ret = migrate_pages(&pagelist, new_page, NULL, MPOL_MF_MOVE_ALL,
                            MIGRATE_SYNC, MR_MEMORY_FAILURE);
    if (ret) {
            ...
    } else {
            /*
             * We set PG_hwpoison only when the migration source hugepage
             * was successfully dissolved, because otherwise hwpoisoned
             * hugepage remains on free hugepage list, then userspace will
             * find it as SIGBUS by allocation failure. That's not expected
             * in soft-offlining.
             */
            ret = dissolve_free_huge_page(page);
            if (!ret) {
                    if (set_hwpoison_free_buddy_page(page))
                            num_poisoned_pages_inc();
            }
    }
    return ret;

Here dissolve_free_huge_page() returns -EBUSY if the migration source page
was freed into buddy in migrate_pages(), but even in that case we actually
has a chance that set_hwpoison_free_buddy_page() succeeds. So that means
current code gives up offlining too early now.

dissolve_free_huge_page() checks that a given hugepage is suitable for
dissolving, where we should return success for !PageHuge() case because
the given hugepage is considered as already dissolved.

This change also affects other callers of dissolve_free_huge_page(),
which are cleaned up together.

Reported-by: Chen, Jerry T <jerry.t.chen@xxxxxxxxx>
Tested-by: Chen, Jerry T <jerry.t.chen@xxxxxxxxx>
Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Fixes: 6bc9b56433b76 ("mm: fix race on soft-offlining")
Cc: <stable@xxxxxxxxxxxxxxx> # v4.19+
---
 mm/hugetlb.c        | 15 +++++++++------
 mm/memory-failure.c |  5 +----
 2 files changed, 10 insertions(+), 10 deletions(-)

diff --git v5.2-rc3/mm/hugetlb.c v5.2-rc3_patched/mm/hugetlb.c
index ac843d3..048d071 100644
--- v5.2-rc3/mm/hugetlb.c
+++ v5.2-rc3_patched/mm/hugetlb.c
@@ -1519,7 +1519,12 @@ int dissolve_free_huge_page(struct page *page)
 	int rc = -EBUSY;
 
 	spin_lock(&hugetlb_lock);
-	if (PageHuge(page) && !page_count(page)) {
+	if (!PageHuge(page)) {
+		rc = 0;
+		goto out;
+	}
+
+	if (!page_count(page)) {
 		struct page *head = compound_head(page);
 		struct hstate *h = page_hstate(head);
 		int nid = page_to_nid(head);
@@ -1564,11 +1569,9 @@ 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);
-		if (PageHuge(page) && !page_count(page)) {
-			rc = dissolve_free_huge_page(page);
-			if (rc)
-				break;
-		}
+		rc = dissolve_free_huge_page(page);
+		if (rc)
+			break;
 	}
 
 	return rc;
diff --git v5.2-rc3/mm/memory-failure.c v5.2-rc3_patched/mm/memory-failure.c
index 7ea485e..3a83e27 100644
--- v5.2-rc3/mm/memory-failure.c
+++ v5.2-rc3_patched/mm/memory-failure.c
@@ -1859,11 +1859,8 @@ static int soft_offline_in_use_page(struct page *page, int flags)
 
 static int soft_offline_free_page(struct page *page)
 {
-	int rc = 0;
-	struct page *head = compound_head(page);
+	int rc = dissolve_free_huge_page(page);
 
-	if (PageHuge(head))
-		rc = dissolve_free_huge_page(page);
 	if (!rc) {
 		if (set_hwpoison_free_buddy_page(page))
 			num_poisoned_pages_inc();
-- 
2.7.0