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

__split_folio_to_order() copies the hwpoison state onto each new sub-folio
while splitting a folio to a non-zero order.  It does 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, ...)
turns @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 current 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.

memory_failure() tries to split the poisoned folio to order 0 first, but
that split is best-effort; when it fails the folio is left large with
PG_has_hwpoisoned set, the case fa5a061700 added this hwpoison copying
for.

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.

Link: https://lore.kernel.org/20260701174235.3173401-1-riel@surriel.com
Fixes: fa5a061700 ("mm/huge_memory: preserve PG_has_hwpoisoned if a folio is split to >0 order")
Signed-off-by: Rik van Riel <riel@surriel.com>
Assisted-by: Claude:claude-opus-4-8
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Tested-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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 +3608,14 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 			folio_set_large_rmappable(new_folio);
 		}
 
+		/*
+		 * PG_has_hwpoisoned is on the 2nd page, so set it after
+		 * the compound head is prepped.
+		 */
+		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))