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

From: Miaohe Lin <linmiaohe@huawei.com>
To: Michal Hocko <mhocko@suse.com>
Cc: "HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>,
	"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>,
	"Yang Shi" <shy828301@gmail.com>
Subject: Re: [PATCH] mm/huge_memory: use pfn_to_online_page() in split_huge_pages_all()
Date: Fri, 9 Sep 2022 08:27:48 +0800	[thread overview]
Message-ID: <2d0792c0-88e1-710c-c8bd-c126cc375e88@huawei.com> (raw)
In-Reply-To: <YxmUwNYDrOK11Rum@dhcp22.suse.cz>

On 2022/9/8 15:07, Michal Hocko wrote:
> On Thu 08-09-22 11:25:54, Miaohe Lin wrote:
>> 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. ;)
> 
> Most of those whic are directly triggerable should be taken care of. It
> would be still good to go through `git grep -w pfn_to_page' and evaluate 
> all callers. Still more than 400 callsites so not a trivial task.

Agree. Even if pfn_to_online_page() is used, it might still races with the memory hotremove if
there's no way to guard against it, e.g. holding an extra page refcnt. So code audit should also
apply to pfn_to_online_page().

Thanks,
Miaohe Lin