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

From: Miaohe Lin <linmiaohe@huawei.com>
To: "HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>,
	David Hildenbrand <david@redhat.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Muchun Song <songmuchun@bytedance.com>,
	Matthew Wilcox <willy@infradead.org>,
	Michal Hocko <mhocko@suse.com>, Yang Shi <shy828301@gmail.com>
Subject: Re: [PATCH] mm/huge_memory: use pfn_to_online_page() in split_huge_pages_all()
Date: Thu, 8 Sep 2022 11:25:54 +0800	[thread overview]
Message-ID: <fc4fa44d-de55-803d-5c33-1a6c7770caeb@huawei.com> (raw)
In-Reply-To: <20220908030652.GA1492282@hori.linux.bs1.fc.nec.co.jp>

On 2022/9/8 11:06, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Thu, Sep 08, 2022 at 10:19:03AM +0800, Miaohe Lin wrote:
>> On 2022/9/7 20:11, Naoya Horiguchi wrote:
> ...
>>> >From 8a5c284df732943065d23838090d15c94cd10395 Mon Sep 17 00:00:00 2001
>>> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
>>> Date: Wed, 7 Sep 2022 20:58:44 +0900
>>> Subject: [PATCH] mm/huge_memory: use pfn_to_online_page() in
>>>  split_huge_pages_all()
>>>
>>> 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.
>>
>> With changes proposed by David, this patch looks good to me.
>>
>> Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
> 
> Thank you.
> 
>>
>> BTW: IMHO, there might be many similar code places need to take care of memory hotremove where
>> *pfn is operated directly* while it's not protected against memory hotremove.
> 
> I had the similar concern, but there seems many place doing PFN walk,
> so checking them one-by-one that offlined memory can be walked over
> requires much effort.

Yes, that will be a heavy work. We could fix them one by one if they ever occur. ;)

Thanks,
Miaohe Lin

> 
> Thanks,
> Naoya Horiguchi
> 