Received-SPF: pass (google.com: domain of linux-kernel+bounces-90292-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Message-ID: <d0b19fb4-ee05-4109-9b06-5cdeabc0fcff@redhat.com>
Date: Mon, 4 Mar 2024 10:17:09 +0100
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [Question] CoW on VM_PFNMAP vma during write fault
Content-Language: en-US
To: mawupeng <mawupeng1@huawei.com>, akpm@linux-foundation.org,
 khlebnikov@openvz.org, jaredeh@gmail.com, linmiaohe@huawei.com,
 hpa@zytor.com, tglx@linutronix.de, mingo@redhat.com,
 dave.hansen@linux.intel.com, cotte@de.ibm.com, npiggin@suse.de
Cc: luto@kernel.org, peterz@infradead.org, linux-kernel@vger.kernel.org,
 x86@kernel.org, rdunlap@infradead.org, bhelgaas@google.com,
 linux-mm@kvack.org
References: <20240227122814.3781907-1-mawupeng1@huawei.com>
 <a74c8b96-3fc5-44ff-949c-6e5c5e05e122@redhat.com>
 <de0cc6e0-dd15-40b6-89c2-c8e83fd6f587@redhat.com>
 <fa8f9f3b-e01e-4662-a2d5-77caeeb7b9f5@huawei.com>
 <234a5423-8d6d-444a-a27c-c772a71c9871@huawei.com>
 <6e948123-df3a-4450-8fd3-76b9131a35a0@redhat.com>
 <fefd4a04-0af3-4135-9e98-c983ee321e52@huawei.com>
From: David Hildenbrand <david@redhat.com>
Autocrypt: addr=david@redhat.com; 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
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW
 AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q
 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp
 rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf
 wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4
 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l
 pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd
 KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE
 BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs
 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF
 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9
 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz
 Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb
 T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A
 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk
 CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G
 NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75
 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx
 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS
 lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv
 AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa
 N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3
 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB
 boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq
 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f
 XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ
 a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq
 Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6
 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8
 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E
 th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr
 jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt
 WNyWQQ==
Organization: Red Hat
In-Reply-To: <fefd4a04-0af3-4135-9e98-c983ee321e52@huawei.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

On 04.03.24 10:04, mawupeng wrote:
> 
> 
> On 2024/3/4 16:57, David Hildenbrand wrote:
>> On 04.03.24 09:47, mawupeng wrote:
>>> Hi Maintainers, kindly ping...
>>>
>>> On 2024/2/28 9:55, mawupeng wrote:
>>>>
>>>>
>>>> On 2024/2/27 21:15, David Hildenbrand wrote:
>>>>> On 27.02.24 14:00, David Hildenbrand wrote:
>>>>>> On 27.02.24 13:28, Wupeng Ma wrote:
>>>>>>> We find that a warn will be produced during our test, the detail log is
>>>>>>> shown in the end.
>>>>>>>
>>>>>>> The core problem of this warn is that the first pfn of this pfnmap vma is
>>>>>>> cleared during memory-failure. Digging into the source we find that this
>>>>>>> problem can be triggered as following:
>>>>>>>
>>>>>>> // mmap with MAP_PRIVATE and specific fd which hook mmap
>>>>>>> mmap(MAP_PRIVATE, fd)
>>>>>>>       __mmap_region
>>>>>>>         remap_pfn_range
>>>>>>>         // set vma with pfnmap and the prot of pte is read only
>>>>>>>       
>>>>>>
>>>>>> Okay, so we get a MAP_PRIVATE VM_PFNMAP I assume.
>>>>>>
>>>>>> What fd is that exactly? Often, we disallow private mappings in the
>>>>>> mmap() callback (for a good reason).
>>>
>>> We found this problem in 5.10, Commit 9f78bf330a66 ("xsk: support use vaddr as ring") Fix this
>>> problem during supporting vaddr by remap VM_PFNMAP by VM_MIXEDMAP. But other modules which
>>> use remap_pfn_range may still have this problem.
>>
>> I wrote a simple reproducer using MAP_PRIVATE of iouring queues on Friday.
>>
>>>
>>> It do seems wired for private mappings, What is the good reason?
>>
>> I'm sure there are some use cases that require MAP_PRIVATE of such areas, and usually there is nothing wrong with that.
> 
> So MAP_PRIVATE for VM_PFNMAP area with write access is ok? What is the user case for this situation?
I recall that MAP_PRIVATE /dev/mem mappings were required for some use 
cases. No details/ideas about other users, though.

Likely sufficient use case that people really thought about ways to get 
it working -- see vm_normal_page() :)

-- 
Cheers,

David / dhildenb