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[2003:cb:c706:4e00:31ad:5274:e21c:b59]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4280574b0e8sm231690805e9.26.2024.07.30.12.49.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 30 Jul 2024 12:49:55 -0700 (PDT) Message-ID: <96ae32c7-8e49-4802-a9ed-c5428b3a1f14@redhat.com> Date: Tue, 30 Jul 2024 21:49:54 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 1/2] mm: let pte_lockptr() consume a pte_t pointer To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Muchun Song , Oscar Salvador , Qi Zheng , Hugh Dickins References: <20240725183955.2268884-1-david@redhat.com> <20240725183955.2268884-2-david@redhat.com> <9e671388-a5c6-4de1-8c85-b7af8aee7f44@redhat.com> 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-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 58DF2C0027 X-Stat-Signature: fymmyog3sw4rfku1j4zgar9f3ue479k4 X-HE-Tag: 1722369000-853245 X-HE-Meta: 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 tmgw16Oq nRGJlBZVhdw/Xe4F8FStF5lTSbw1eJ5gy3sIifdjezm5G8C2PQugsnlYosN/64aL2WPFIiHszRI7wUnhLgrdqtXs18DLudy6flml2+KDyDLDFzu7NE3fuQR9zqq11+V0he3WnshxH2jHkhR7w7yXWgnw8XMKMf3TvYAgWhbTJXyh8umA4lAQy4keqHBNbVNSinYC0AqW1zRw3skYsibr9SbDAnTLXmqt2r6xC6uDYts3QKtQb3+5yQVdXvc5556tq+SIEbnLPb7UyZmFtGRurfgoh5XeIh8oPGtyfgkFwI/3jEn1VYp3qeOWCnsQQkdv3wFaCPhyAvxxonrUEd3FuwuTfjgjC06IkOsA/pV63ESTTG/U3L5qfTq9FTYp2NfOXcUdgVvK3n7JpmxVCjh3/0tT5fnAZZxeevarW X-Bogosity: Ham, tests=bogofilter, spamicity=0.000016, 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 30.07.24 20:44, Peter Xu wrote: > On Mon, Jul 29, 2024 at 07:46:26PM +0200, David Hildenbrand wrote: >> I see what you mean but this is a very similar pattern as used in >> collapse_pte_mapped_thp(), no? There we have >> >> start_pte = pte_offset_map_nolock(mm, pmd, haddr, &ptl); >> ... >> if (!pml) >> spin_lock(ptl); >> ... >> pte_unmap(start_pte); >> if (!pml) >> spin_unlock(ptl); >> >> >> Again, I don't have a strong opinion on this, but doing it more similar to >> collapse_pte_mapped_thp() to obtain locks makes it clearer to me. But if I >> am missing something obvious please shout and I'll change it. > > Right.. I don't think that path can change the pte pgtable either, and > there is even the line Hugh left showing it's impossible: > > if (!start_pte) /* mmap_lock + page lock should prevent this */ > goto abort; > > I was thinking maybe the page lock is the critical one, irrelevant of mmap > lock. > > No strong opinion either. Not sure whether Hugh has some thoughts. But > maybe if we stick with pte_offset_map_nolock() and if there'll be a repost > anyway, we could add a similar comment like this one showing that the pte > pgtable should be actually as stable as the ptlock. I think this all deserves some future cleanups :) ... for the time being I'll have to drop this patch completely. I should have known that virt_to_page() does not work on kmap'ed pages, but part of me didn't want to believe it. CONFIG_HIGHPTE wants to make my life challenging :) -- Cheers, David / dhildenb