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

From: "HORIGUCHI NAOYA(堀口　直也)" <naoya.horiguchi@nec.com>
To: Miaohe Lin <linmiaohe@huawei.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 03:06:52 +0000	[thread overview]
Message-ID: <20220908030652.GA1492282@hori.linux.bs1.fc.nec.co.jp> (raw)
In-Reply-To: <9cb0b01e-c80d-667c-47f1-ad30e329784b@huawei.com>

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.

Thanks,
Naoya Horiguchi