linux-kernel - Re: [PATCH v2] mm/huge_memory: use pfn_to_online_page() in split_huge_pages

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Message-ID: <20220908122303.7pofcdcmbuq4ou6u@box.shutemov.name>
Date:   Thu, 8 Sep 2022 15:23:03 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Naoya Horiguchi <naoya.horiguchi@...ux.dev>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>, Yang Shi <shy828301@...il.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/huge_memory: use pfn_to_online_page() in
 split_huge_pages_all()

On Thu, Sep 08, 2022 at 01:11:50PM +0900, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@....com>
> 
> 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@....com>
> Co-developed-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Yang Shi <shy828301@...il.com>
> Acked-by: Michal Hocko <mhocko@...e.com>
> Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> Cc: <stable@...r.kernel.org> # 5.10+

Looks good:

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

But it makes me think if there's other similar cases. "page is offline" is
rather obscure case that rarely covered by routine testing. Otherwise the
bug would not survive for 6 years.

After quick look, kvm_pfn_to_refcounted_page() looks suspicious.
kdb_getphys() too.

Maybe we should make pfn_valid() false for offline pages and introduce
other check that allows offline pages which can be used in codepaths that
deal with offline pages explicitly.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov