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(p200300cbc7452a00d74aa8c520b63ec3.dip0.t-ipconnect.de. [2003:cb:c745:2a00:d74a:a8c5:20b6:3ec3]) by smtp.gmail.com with ESMTPSA id j25-20020a05600c1c1900b004076f522058sm15112542wms.0.2023.11.27.08.22.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 Nov 2023 08:22:15 -0800 (PST) Message-ID: <90c1b3c7-cdf7-40fa-ae06-5565c1d760ee@redhat.com> Date: Mon, 27 Nov 2023 17:22:14 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/gup: Fix follow_devmap_p[mu]d() on page==NULL handling To: Peter Xu , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Dan Williams , Mel Gorman , Matthew Wilcox , "Aneesh Kumar K . V" , Christoph Hellwig References: <20231123180222.1048297-1-peterx@redhat.com> <20231124112059.3519d6fdfe71f846f8bf726f@linux-foundation.org> From: David Hildenbrand 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: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: jrt3j4c35rsokzmsmbc4wgf5utp7ob3t X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 6B99840014 X-HE-Tag: 1701102141-938349 X-HE-Meta: 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 9EiGQJg1 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 26.11.23 22:55, Peter Xu wrote: > On Fri, Nov 24, 2023 at 11:20:59AM -0800, Andrew Morton wrote: >> On Thu, 23 Nov 2023 13:02:22 -0500 Peter Xu wrote: >> >>> This is a bug found not by any report but only by code observations. >>> >>> When GUP sees a devpmd/devpud and if page==NULL is returned, it means a >>> fault is probably required. Here falling through when page==NULL can cause >>> unexpected behavior. >>> >> >> Well this is worrisome. We aren't able to construct a test case to >> demonstrate this bug? Why is that? Is it perhaps just dead code? > > IIUC it's not dead code. Take the example of follow_devmap_pmd(), it can > return page==NULL at least when seeing write bit missing: > > if (flags & FOLL_WRITE && !pmd_write(*pmd)) > return NULL; > > AFAICT it can happen if someone does "echo 4 > /proc/$PID/clear_refs" when > the mm contains the devmap pmd. Same to pud. > > It'll be nice if someone that works with dax would like to verify it. In > my series (refactor hugetlb gup, part 2) IIUC some hugetlb selftest can > start to trigger this path, but I'll need to check. So far it's dax-only. It certainly looks weird to continue there. Triggering it by mmaping some devdax device might be possible (e.g., using devdax emulation). We know the PMD is present and the PMD is devmap. We take the pmd lock, and in follow_devmap_pmd() we recheck both. I suspect the original idea was: if it's suddenly no longer present or no longer devmap, it was replaced by a PTE table. So we know a deeper level is there and can simply continue instead of triggering a fault. But that does not seem to be the case, because I suspect the PMD could have been zapped (MADV_DONTNEED?) in the meantime, and the "writability" check is similarly weird. So I assume the patch from Peter is ok: even if the PMD got replaced by a PTE table, we'd trigger a fault and simply retry. Acked-by: David Hildenbrand -- Cheers, David / dhildenb