Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized
 memmaps in memory_failure()
To:     Michal Hocko <mhocko@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20191009142435.3975-1-david@redhat.com>
 <20191009142435.3975-3-david@redhat.com>
 <20191009144323.GH6681@dhcp22.suse.cz>
 <5a626821-77e9-e26b-c2ee-219670283bf0@redhat.com>
 <20191010073526.GC18412@dhcp22.suse.cz>
From:   David Hildenbrand <david@redhat.com>
Openpgp: preference=signencrypt
Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata=
 xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ
 dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL
 QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp
 XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK
 Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9
 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt
 WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc
 UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv
 jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwX4EEwECACgFAljj9eoCGwMFCQlmAYAGCwkI
 BwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEE3eEPcA/4Na5IIP/3T/FIQMxIfNzZshIq687qgG
 8UbspuE/YSUDdv7r5szYTK6KPTlqN8NAcSfheywbuYD9A4ZeSBWD3/NAVUdrCaRP2IvFyELj
 xoMvfJccbq45BxzgEspg/bVahNbyuBpLBVjVWwRtFCUEXkyazksSv8pdTMAs9IucChvFmmq3
 jJ2vlaz9lYt/lxN246fIVceckPMiUveimngvXZw21VOAhfQ+/sofXF8JCFv2mFcBDoa7eYob
 s0FLpmqFaeNRHAlzMWgSsP80qx5nWWEvRLdKWi533N2vC/EyunN3HcBwVrXH4hxRBMco3jvM
 m8VKLKao9wKj82qSivUnkPIwsAGNPdFoPbgghCQiBjBe6A75Z2xHFrzo7t1jg7nQfIyNC7ez
 MZBJ59sqA9EDMEJPlLNIeJmqslXPjmMFnE7Mby/+335WJYDulsRybN+W5rLT5aMvhC6x6POK
 z55fMNKrMASCzBJum2Fwjf/VnuGRYkhKCqqZ8gJ3OvmR50tInDV2jZ1DQgc3i550T5JDpToh
 dPBxZocIhzg+MBSRDXcJmHOx/7nQm3iQ6iLuwmXsRC6f5FbFefk9EjuTKcLMvBsEx+2DEx0E
 UnmJ4hVg7u1PQ+2Oy+Lh/opK/BDiqlQ8Pz2jiXv5xkECvr/3Sv59hlOCZMOaiLTTjtOIU7Tq
 7ut6OL64oAq+zsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCghCj/CA/lc/LMthqQ773ga
 uB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseBfDXHA6m4B3mUTWo13nid
 0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts6TZ+IrPOwT1hfB4WNC+X
 2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiuQmt3yqrmN63V9wzaPhC+
 xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKBTccu2AXJXWAE1Xjh6GOC
 8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvFFFyAS0Nk1q/7EChPcbRb
 hJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh2YmnmLRTro6eZ/qYwWkC
 u8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRkF3TwgucpyPtcpmQtTkWS
 gDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0LLH63+BrrHasfJzxKXzqg
 rW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4vq7oFCPsOgwARAQABwsFl
 BBgBAgAPBQJVy5+RAhsMBQkJZgGAAAoJEE3eEPcA/4NagOsP/jPoIBb/iXVbM+fmSHOjEshl
 KMwEl/m5iLj3iHnHPVLBUWrXPdS7iQijJA/VLxjnFknhaS60hkUNWexDMxVVP/6lbOrs4bDZ
 NEWDMktAeqJaFtxackPszlcpRVkAs6Msn9tu8hlvB517pyUgvuD7ZS9gGOMmYwFQDyytpepo
 YApVV00P0u3AaE0Cj/o71STqGJKZxcVhPaZ+LR+UCBZOyKfEyq+ZN311VpOJZ1IvTExf+S/5
 lqnciDtbO3I4Wq0ArLX1gs1q1XlXLaVaA3yVqeC8E7kOchDNinD3hJS4OX0e1gdsx/e6COvy
 qNg5aL5n0Kl4fcVqM0LdIhsubVs4eiNCa5XMSYpXmVi3HAuFyg9dN+x8thSwI836FoMASwOl
 C7tHsTjnSGufB+D7F7ZBT61BffNBBIm1KdMxcxqLUVXpBQHHlGkbwI+3Ye+nE6HmZH7IwLwV
 W+Ajl7oYF+jeKaH4DZFtgLYGLtZ1LDwKPjX7VAsa4Yx7S5+EBAaZGxK510MjIx6SGrZWBrrV
 TEvdV00F2MnQoeXKzD7O4WFbL55hhyGgfWTHwZ457iN9SgYi1JLPqWkZB0JRXIEtjd4JEQcx
 +8Umfre0Xt4713VxMygW0PnQt5aSQdMD58jHFxTk092mU+yIHj5LeYgvwSgZN4airXk5yRXl
 SE+xAvmumFBY
Organization: Red Hat GmbH
Message-ID: <18383432-c74a-9ce5-a3c6-1e57d54cb629@redhat.com>
Date:   Thu, 10 Oct 2019 09:52:20 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
MIME-Version: 1.0
In-Reply-To: <20191010073526.GC18412@dhcp22.suse.cz>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 10.10.19 09:35, Michal Hocko wrote:
> On Thu 10-10-19 09:27:32, David Hildenbrand wrote:
>> On 09.10.19 16:43, Michal Hocko wrote:
>>> On Wed 09-10-19 16:24:35, David Hildenbrand wrote:
>>>> We should check for pfn_to_online_page() to not access uninitialized
>>>> memmaps. Reshuffle the code so we don't have to duplicate the error
>>>> message.
>>>>
>>>> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>> Cc: Michal Hocko <mhocko@kernel.org>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>>  mm/memory-failure.c | 14 ++++++++------
>>>>  1 file changed, 8 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>>> index 7ef849da8278..e866e6e5660b 100644
>>>> --- a/mm/memory-failure.c
>>>> +++ b/mm/memory-failure.c
>>>> @@ -1253,17 +1253,19 @@ int memory_failure(unsigned long pfn, int flags)
>>>>  	if (!sysctl_memory_failure_recovery)
>>>>  		panic("Memory failure on page %lx", pfn);
>>>>  
>>>> -	if (!pfn_valid(pfn)) {
>>>> +	p = pfn_to_online_page(pfn);
>>>> +	if (!p) {
>>>> +		if (pfn_valid(pfn)) {
>>>> +			pgmap = get_dev_pagemap(pfn, NULL);
>>>> +			if (pgmap)
>>>> +				return memory_failure_dev_pagemap(pfn, flags,
>>>> +								  pgmap);
>>>> +		}
>>>>  		pr_err("Memory failure: %#lx: memory outside kernel control\n",
>>>>  			pfn);
>>>>  		return -ENXIO;
>>>
>>> Don't we need that earlier at hwpoison_inject level?
>>>
>>
>> Theoretically yes, this is another instance. But pfn_to_online_page(pfn)
>> alone would not be sufficient as discussed. We would, again, have to
>> special-case ZONE_DEVICE via things like get_dev_pagemap() ...
>>
>> But mm/hwpoison-inject.c:hwpoison_inject() is a pure debug feature either way:
>>
>> 	/*
>> 	 * Note that the below poison/unpoison interfaces do not involve
>> 	 * hardware status change, hence do not require hardware support.
>> 	 * They are mainly for testing hwpoison in software level.
>> 	 */
>>
>> So it's not that bad compared to memory_failure() called from real HW or
>> drivers/base/memory.c:soft_offline_page_store()/hard_offline_page_store()
> 
> Yes, this is just a toy. And yes we need to handle zone device pages
> here because a) people likely want to test MCE behavior even on these
> pages and b) HW can really trigger MCEs there as well. I was just
> pointing that the patch is likely incomplete.
> 

I rather think this deserves a separate patch as it is a separate
interface :)

I do wonder why hwpoison_inject() has to perform so much extra work
compared to other memory_failure() users. This smells like legacy
leftovers to me, but I might be wrong. The interface is fairly old,
though. Does anybody know why we need this magic? I can spot quite some
duplicate checks/things getting performed.

Naiive me would just make the interface perform the same as
hard_offline_page_store(). But most probably I am not getting the real
purpose of both different interfaces.

HWPOISON_INJECT is only selected for DEBUG_KERNEL, so I would have
guessed that fixing this is not urgent.

BTW: mm/memory-failure.c:soft_offline_page() also looks wrong and needs
fixing to make sure we access initialized memmaps.

-- 

Thanks,

David / dhildenb