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[2003:cb:c721:b900:4f34:b2b7:739d:a650]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-429ed6586f7sm17211215e9.23.2024.08.16.01.59.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 16 Aug 2024 01:59:02 -0700 (PDT) Message-ID: Date: Fri, 16 Aug 2024 10:59:01 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v2 1/7] mm: pgtable: make pte_offset_map_nolock() return pmdval To: Qi Zheng Cc: hughd@google.com, willy@infradead.org, mgorman@suse.de, muchun.song@linux.dev, vbabka@kernel.org, akpm@linux-foundation.org, zokeefe@google.com, rientjes@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, the arch/x86 maintainers References: <0e8e0503-5796-4b61-bb5b-249e285f5d21@redhat.com> <39281a4d-d896-46fd-80a5-8cd547d1625f@bytedance.com> <0f467510-a0d0-4a98-8517-43813fa4c131@redhat.com> <3e8253c4-9181-4027-84ee-28e1fc488f61@bytedance.com> From: David Hildenbrand Content-Language: en-US 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: <3e8253c4-9181-4027-84ee-28e1fc488f61@bytedance.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 12.08.24 08:21, Qi Zheng wrote: > Hi David, > > On 2024/8/10 00:54, David Hildenbrand wrote: >> On 07.08.24 05:08, Qi Zheng wrote: >>> Hi David, >>> >>> On 2024/8/6 22:16, David Hildenbrand wrote: >>>> On 06.08.24 04:40, Qi Zheng wrote: >>>>> Hi David, >>>>> >>>>> On 2024/8/5 22:43, David Hildenbrand wrote: >>>>>> On 05.08.24 14:55, Qi Zheng wrote: >>>>>>> Make pte_offset_map_nolock() return pmdval so that we can recheck the >>>>>>> *pmd once the lock is taken. This is a preparation for freeing empty >>>>>>> PTE pages, no functional changes are expected. >>>>>> >>>>>> Skimming the patches, only patch #4 updates one of the callsites >>>>>> (collapse_pte_mapped_thp). >>>>> >>>>> In addition, retract_page_tables() and reclaim_pgtables_pmd_entry() >>>>> also used the pmdval returned by pte_offset_map_nolock(). >>>> >>>> Right, and I am questioning if only touching these two is sufficient, >>>> and how we can make it clearer when someone actually has to recheck the >>>> PMD. >>>> >>>>> >>>>>> >>>>>> Wouldn't we have to recheck if the PMD val changed in more cases after >>>>>> taking the PTL? >>>>>> >>>>>> If not, would it make sense to have a separate function that >>>>>> returns the >>>>>> pmdval and we won't have to update each and every callsite? >>>>> >>>>> pte_offset_map_nolock() had already obtained the pmdval previously, >>>>> just >>>>> hadn't returned it. And updating those callsite is simple, so I think >>>>> there may not be a need to add a separate function. >>>> >>>> Let me ask this way: why is retract_page_tables() and >>>> reclaim_pgtables_pmd_entry() different to the other ones, and how would >>>> someone using pte_offset_map_nolock() know what's to do here? >>> >>> If we acuqire the PTL (PTE or PMD lock) after calling >>> pte_offset_map_nolock(), it means we may be modifying the corresponding >>> pte or pmd entry. In that case, we need to perform a pmd_same() check >>> after holding the PTL, just like in pte_offset_map_lock(), to prevent >>> the possibility of the PTE page being reclaimed at that time. >> >> Okay, what I thought. >> >>> >>> If we call pte_offset_map_nolock() and do not need to acquire the PTL >>> afterwards, it means we are only reading the PTE page. In this case, the >>> rcu_read_lock() in pte_offset_map_nolock() will ensure that the PTE page >>> cannot be reclaimed. >>> >>>> >>>> IIUC, we must check the PMDVAL after taking the PTL in case >>>> >>>> (a) we want to modify the page table to turn pte_none() entries to >>>>       !pte_none(). Because it could be that the page table was >>>> removed and >>>>       now is all pte_none() >>>> >>>> (b) we want to remove the page table ourselves and want to check if it >>>>       has already been removed. >>>> >>>> Is that it? >>> >>> Yes. >>> >>>> >>>> So my thinking is if another function variant can make that clearer. >>> >>> OK, how about naming it pte_offset_map_before_lock? >> >> That's the issue with some of the code: for example in >> filemap_fault_recheck_pte_none() we'll call pte_offset_map_nolock() and >> conditionally take the PTL. But we won't be modifying the pages tables. >> >> Maybe something like: >> >> pte_offset_map_readonly_nolock() >> >> and >> >> pte_offset_map_maywrite_nolock() >> >> The latter would require you to pass the PMD pointer such that you have >> to really mess up to ignore what to do with it (check PMD same or not >> check PMD same if you really know what you are douing). >> >> The first would not take a PMD pointer at all, because there is no need to. > > These two function names LGTM. Will do in the next version. That is probably something you can send as a separate patch independent of this full series. Then we might also get more review+thoughts from other folks! -- Cheers, David / dhildenb