[PATCH 1/2] hugetlb: fix update_and_free_page contig page struct assumption

[Mike Kravetz]

page structs are not guaranteed to be contiguous for gigantic pages.  The
routine update_and_free_page can encounter a gigantic page, yet it assumes
page structs are contiguous when setting page flags in subpages.

If update_and_free_page encounters non-contiguous page structs, we can
see “BUG: Bad page state in process …” errors.

Non-contiguous page structs are generally not an issue.  However, they can
exist with a specific kernel configuration and hotplug operations.  For
example: Configure the kernel with CONFIG_SPARSEMEM and
!CONFIG_SPARSEMEM_VMEMMAP.  Then, hotplug add memory for the area where the
gigantic page will be allocated.
Zi Yan outlined steps to reproduce here [1].

[1] https://lore.kernel.org/linux-mm/16F7C58B-4D79-41C5-9B64-A1A1628F4AF2@xxxxxxxxxx/

Fixes: 944d9fec8d7a ("hugetlb: add support for gigantic page allocation at runtime")
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
 mm/hugetlb.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 4bdb58ab14cb..94e9fa803294 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1312,14 +1312,16 @@ static inline void destroy_compound_gigantic_page(struct page *page,
 static void update_and_free_page(struct hstate *h, struct page *page)
 {
 	int i;
+	struct page *subpage = page;
 
 	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
 		return;
 
 	h->nr_huge_pages--;
 	h->nr_huge_pages_node[page_to_nid(page)]--;
-	for (i = 0; i < pages_per_huge_page(h); i++) {
-		page[i].flags &= ~(1 << PG_locked | 1 << PG_error |
+	for (i = 0; i < pages_per_huge_page(h);
+	     i++, subpage = mem_map_next(subpage, page, i)) {
+		subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
 				1 << PG_referenced | 1 << PG_dirty |
 				1 << PG_active | 1 << PG_private |
 				1 << PG_writeback);
-- 
2.29.2