[PATCH] mm/hugetlb: fix missing migratable flag on same-node hugetlb migration

[Wupeng Ma]

Commit ba23f58de896 ("mm/migrate: don't call
folio_putback_active_hugetlb() on dst hugetlb folio") moved setting of
the migratable flag and active-list placement from
folio_putback_active_hugetlb(dst) into move_hugetlb_state(), so that
the freshly allocated destination folio is handled where allocation is
known to have succeeded.

Unfortunately, the new code was appended after the existing
temporary-folio block in move_hugetlb_state(), which contains an early
return added earlier by commit 5af1ab1d24e08 ("mm/hugetlb: optimize
the surplus state transfer code in move_hugetlb_state()"):

  if (folio_test_hugetlb_temporary(new_folio)) {
      ...
      if (new_nid == old_nid)
          return;                       <-- skips the new code
      ...
  }

  /* added by ba23f58 */
  folio_set_hugetlb_migratable(new_folio);
  list_move_tail(&new_folio->lru, ...&h->hugepage_activelist);

When the destination folio is temporary (i.e. the hugetlb pool was
exhausted and the migration callback fell back to
alloc_migrate_hugetlb_folio()) and the migration does not cross a
node -- the common case, and always true on a single-NUMA system --
move_hugetlb_state() returns before setting the migratable flag or
adding the new folio to the active list. The destination folio is
then installed in the page table but cannot be isolated afterwards,
since folio_isolate_hugetlb() rejects folios without the migratable
flag; a subsequent soft-offline, hard-offline or memory-hotplug
offline of that folio fails with -EBUSY.

This was reproduced on a single-NUMA arm64 VM: a second
MADV_SOFT_OFFLINE on an already-migrated hugetlb page returned EBUSY
and logged "hugepage isolation failed".

Keep the surplus adjustment, which is the only part that depends on
the node crossing, guarded by `if (new_nid != old_nid)', while making
the migratable flag and active-list placement unconditional. This
preserves the cleanup intent of ba23f58 and closes the early-return
hole.

Fixes: ba23f58de896 ("mm/migrate: don't call folio_putback_active_hugetlb() on dst hugetlb folio")
Signed-off-by: Wupeng Ma <mawupeng1@xxxxxxxxxx>
---
 mm/hugetlb.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 571212b80835..cafadfdb63c0 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -7211,14 +7211,14 @@ void move_hugetlb_state(struct folio *old_folio, struct folio *new_folio, int re
 		 * There is no need to transfer the per-node surplus state
 		 * when we do not cross the node.
 		 */
-		if (new_nid == old_nid)
-			return;
-		spin_lock_irq(&hugetlb_lock);
-		if (h->surplus_huge_pages_node[old_nid]) {
-			h->surplus_huge_pages_node[old_nid]--;
-			h->surplus_huge_pages_node[new_nid]++;
+		if (new_nid != old_nid) {
+			spin_lock_irq(&hugetlb_lock);
+			if (h->surplus_huge_pages_node[old_nid]) {
+				h->surplus_huge_pages_node[old_nid]--;
+				h->surplus_huge_pages_node[new_nid]++;
+			}
+			spin_unlock_irq(&hugetlb_lock);
 		}
-		spin_unlock_irq(&hugetlb_lock);
 	}
 
 	/*
-- 
2.43.0