[PATCH v2 01/20] hugetlb: Don't restore vmemmap of non-HVOed folios on bulk restore error

[James Houghton <jthoughton@google.com>]

When hugetlb_vmemmap_restore_folios() fails, it stops processing the
list, so folio_list may still contain folios that were never vmemmap
optimized, e.g. because HVO failed when they were allocated. Their
hugetlb flag has already been cleared by remove_hugetlb_folio().

If no folios could be restored, bulk_vmemmap_restore_error() then calls
hugetlb_vmemmap_restore_folio() on every folio left on folio_list,
including those, which trips the !folio_test_hugetlb() check in
__hugetlb_vmemmap_restore_folio():

  page dumped because: VM_WARN_ON_ONCE_FOLIO(!folio_test_hugetlb(folio))
  WARNING: mm/hugetlb_vmemmap.c:503 at __hugetlb_vmemmap_restore_folio+0x270/0x300
  ...
   __hugetlb_vmemmap_restore_folio+0x270/0x300 (P)
   hugetlb_vmemmap_restore_folio+0x1c/0x30
   update_and_free_pages_bulk+0x2c0/0x368
   __nr_hugepages_store_common+0x2e8/0x3f0

Otherwise this is benign: the restore is a no-op returning success for
non-optimized folios, which are then freed as intended.

Skip the restore for folios that are not vmemmap optimized; they can be
freed right away.

This is very unlikely to happen without fault injection, as it requires
both HVO and a subsequent vmemmap restore to fail.

Fixes: cfb8c75099db ("hugetlb: perform vmemmap restoration on a list of pages")
Assisted-by: LLM
Signed-off-by: James Houghton <jthoughton@google.com>
---
 mm/hugetlb.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a1b51251103b..4dac7ed1df57 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1613,9 +1613,14 @@ static void bulk_vmemmap_restore_error(struct hstate *h,
 		 * page is made a surplus page and removed from the list.
 		 * If are able to restore vmemmap and free one hugetlb page, we
 		 * quit processing the list to retry the bulk operation.
+		 *
+		 * Folios whose vmemmap was never optimized (e.g. because HVO
+		 * failed when they were allocated) can be freed right away;
+		 * remove_hugetlb_folio() has already cleared their hugetlb flag.
 		 */
 		list_for_each_entry_safe(folio, t_folio, folio_list, lru)
-			if (hugetlb_vmemmap_restore_folio(h, folio)) {
+			if (folio_test_hugetlb_vmemmap_optimized(folio) &&
+			    hugetlb_vmemmap_restore_folio(h, folio)) {
 				list_del(&folio->lru);
 				spin_lock_irq(&hugetlb_lock);
 				add_hugetlb_folio(h, folio, true);
-- 
2.56.0.rc1.315.gc6ed9934b7-goog