On Wed, 7 Sep 2022 21:11:57 +0900 Naoya Horiguchi <naoya.horiguchi@xxxxxxxxx> wrote:
>
> 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")
January of 2016! Or was this a long-term bug which was recently
exposed by some other change?
