[PATCH v2] mm/huge_memory: use pfn_to_online_page() in split_huge_pages_all()

From: Naoya Horiguchi
Date: Thu Sep 08 2022 - 00:12:42 EST

From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>

NULL pointer dereference is triggered when calling thp split via debugfs
on the system with offlined memory blocks. With debug option enabled,
the following kernel messages are printed out:

page:00000000467f4890 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x121c000
flags: 0x17fffc00000000(node=0|zone=2|lastcpupid=0x1ffff)
raw: 0017fffc00000000 0000000000000000 dead000000000122 0000000000000000
raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
page dumped because: unmovable page
page:000000007d7ab72e is uninitialized and poisoned
page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
------------[ cut here ]------------
kernel BUG at include/linux/mm.h:1248!
invalid opcode: 0000 [#1] PREEMPT SMP PTI
CPU: 16 PID: 20964 Comm: bash Tainted: G I 6.0.0-rc3-foll-numa+ #41
RIP: 0010:split_huge_pages_write+0xcf4/0xe30

This shows that page_to_nid() in page_zone() is unexpectedly called for an
offlined memmap.

Use pfn_to_online_page() to get struct page in PFN walker.

Fixes: 49071d436b51 ("thp: add debugfs handle to split all huge pages")
Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
Co-developed-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 5.10+
mm/huge_memory.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 5fa2ba86dae4..730eb6d6836b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2894,11 +2894,9 @@ static void split_huge_pages_all(void)
max_zone_pfn = zone_end_pfn(zone);
for (pfn = zone->zone_start_pfn; pfn < max_zone_pfn; pfn++) {
int nr_pages;
- if (!pfn_valid(pfn))
- continue;

- page = pfn_to_page(pfn);
- if (!get_page_unless_zero(page))
+ page = pfn_to_online_page(pfn);
+ if (!page || !get_page_unless_zero(page))

if (zone != page_zone(page))