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 v1] mm: Fix access of uninitialized memmaps in
 fs/proc/page.c
To:     Michal Hocko <mhocko@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Qian Cai <cai@lca.pw>, Alexey Dobriyan <adobriyan@gmail.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Toshiki Fukasawa <t-fukasawa@vx.jp.nec.com>,
        Konstantin Khlebnikov <koct9i@gmail.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Anthony Yznaga <anthony.yznaga@oracle.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Dan Williams <dan.j.williams@intel.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Ira Weiny <ira.weiny@intel.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        linux-fsdevel@vger.kernel.org
References: <20191009091205.11753-1-david@redhat.com>
 <20191009093756.GV6681@dhcp22.suse.cz>
 <67aeaacc-d850-5c81-bd17-e95c7f7f75df@redhat.com>
 <20191009112424.GY6681@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: <a9659e39-3516-13c8-b9ab-d42bdaf4a488@redhat.com>
Date:   Wed, 9 Oct 2019 14:58:24 +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: <20191009112424.GY6681@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 09.10.19 13:24, Michal Hocko wrote:
> On Wed 09-10-19 12:19:59, David Hildenbrand wrote:
> [...]
>>> pfn_to_online_page makes sense because offline pages are not really in a
>>> defined state. This would be worth a patch of its own. I remember there
>>
>> The issue is, once I check for pfn_to_online_page(), these functions
>> can't handle ZONE_DEVICE at all anymore. Especially in regards to
>> memory_failure() I don't think this is acceptable.
> > Could you be more specific please? I am not sure I am following.

I wasn't quite clear, let me try to be more precise:

if (pfn_to_online_page(pfn)) {
	/* memmap initialized */
} else if (pfn_valid(pfn)) {
	/* ???
	 * a) offline memory. memmap garbage.
	 * b) memremap memory: memmap initialized to ZONE_DEVICE.
	 * c) memremap memory: reserved for driver. memmap garbage.
	 * (d) memremap memory: memmap currently initializing - garbage)
	 */
}

To distinguish between a) and b/c), we can currently only use
get_dev_pagemap(pfn, NULL). To distinguish between b) and c), we can
currently only use pfn_zone_device_reserved().

That implies, that - right now - if we want to fix what is described in
the patch without introducing more users of get_dev_pagemap(pfn, NULL),
we will not be able to support ZONE_DEVICE in:
- /proc/kpagecount
- /proc/kpageflags
- /proc/kpagecgroup

if (pfn_to_online_page(pfn)) {
	/* memmap initialized */
} else {
	/* skip */
}

Now, memory_failure() already contains a get_dev_pagemap(pfn, NULL)
check and adding pfn_to_online_page(pfn) would also work.

I would be fine with this, but it means that - for now - the three
/proc/ files won't be able to deal with ZONE_DEVICE memory.

> 
>> So while I
>> (personally) only care about adding pfn_to_online_page() checks, the
>> in-this-sense-fragile-subsection ZONE_DEVICE implementation requires me
>> to introduce a temporary check for initialized memmaps.
>>
>>> was a discussion about the uninitialized zone device memmaps. It would
>>> be really good to summarize this discussion in the changelog and
>>> conclude why the explicit check is really good and what were other
>>> alternatives considered.
>>
>> Yeah, I also expressed my feelings and the issues to be solved by
>> ZONE_DEVICE people in https://lkml.org/lkml/2019/9/6/114. However, the
>> discussion stalled and nobody really proposed a solution or followed up.
> 
> I will try to get back to that discussion but is there any technical
> reason that prevents any conclusion or it is just stuck on a lack of
> time of the participants?

I think it was both. People not responding to questions and not having
decided on a solution.


-- 

Thanks,

David / dhildenb