[PATCH] mm/huge_memory: set PG_has_hwpoisoned only after new folio head is established

[Rik van Riel <riel@surriel.com>]

__split_folio_to_order() copies the hwpoison state onto each new
sub-folio while splitting a folio to a non-zero order.  It did so via

	if (handle_hwpoison && page_range_has_hwpoisoned(new_head, new_nr_pages))
		folio_set_has_hwpoisoned(new_folio);

*before* clear_compound_head(new_head)/prep_compound_page(new_head, ...)
turn @new_head from a tail page into a proper folio head.

PG_has_hwpoisoned is a FOLIO_SECOND_PAGE flag, so folio_set_has_hwpoisoned()
resolves to folio_flags(folio, 1).  With the new compound_info-based
page-flags layout, folio_flags() asserts the page is not a tail:

	VM_BUG_ON_PGFLAGS(page->compound_info & 1, page);
	VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags.f), page);

At the original call site @new_head still has the tail marker
(compound_info bit 0 set, PG_head clear), so on CONFIG_DEBUG_VM kernels
this hits:

  kernel BUG at include/linux/page-flags.h:354
  folio_flags+0x82
  folio_set_has_hwpoisoned
  __split_folio_to_order
  __split_unmapped_folio
  __folio_split
  truncate_inode_partial_folio  (shmem hole-punch / MADV_REMOVE)

Reproduced by syzkaller: hwpoison-inject a few subpages of a large shmem
folio, then MADV_REMOVE (fallocate punch hole) on the same range, which
splits the partial folio to a non-zero order.

Move the folio_set_has_hwpoisoned() call to after
clear_compound_head()/prep_compound_page(), where @new_folio is a real
order-new_order head folio (handle_hwpoison implies new_order != 0, so a
second page always exists).  The flag still lands on the same struct page
(page[1] of the new folio); only the ordering relative to compound-head
setup changes, satisfying the FOLIO_SECOND_PAGE precondition.

Signed-off-by: Rik van Riel <riel@surriel.com>
Assisted-by: Claude:claude-opus-4-8
Fixes: fa5a06170036 ("mm/huge_memory: preserve PG_has_hwpoisoned if a folio is split to >0 order")
---
 mm/huge_memory.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 2bccb0a53a0a..ee7ecb3b45c6 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3587,10 +3587,6 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 				 (1L << PG_dropbehind) |
 				 LRU_GEN_MASK | LRU_REFS_MASK));
 
-		if (handle_hwpoison &&
-		    page_range_has_hwpoisoned(new_head, new_nr_pages))
-			folio_set_has_hwpoisoned(new_folio);
-
 		new_folio->mapping = folio->mapping;
 		new_folio->index = folio->index + i;
 
@@ -3612,6 +3608,18 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 			folio_set_large_rmappable(new_folio);
 		}
 
+		/*
+		 * PG_has_hwpoisoned is a FOLIO_SECOND_PAGE flag, so it can only
+		 * be set once @new_folio is a real (head) folio.  Defer setting
+		 * it until after clear_compound_head()/prep_compound_page() have
+		 * turned @new_head from a tail page into a proper folio head;
+		 * otherwise folio_flags() trips on (page->compound_info & 1).
+		 * handle_hwpoison implies new_order != 0.
+		 */
+		if (handle_hwpoison &&
+		    page_range_has_hwpoisoned(new_head, new_nr_pages))
+			folio_set_has_hwpoisoned(new_folio);
+
 		if (folio_test_young(folio))
 			folio_set_young(new_folio);
 		if (folio_test_idle(folio))
-- 
2.53.0-Meta