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Thu, 19 Sep 2024 01:04:16 -0700 (PDT) Received: from [10.131.4.59] ([83.68.141.146]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-378e73f62bfsm14404103f8f.51.2024.09.19.01.04.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 19 Sep 2024 01:04:14 -0700 (PDT) Message-ID: Date: Thu, 19 Sep 2024 10:04:15 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V2 3/7] mm: Use ptep_get() for accessing PTE entries To: Anshuman Khandual , linux-mm@kvack.org Cc: Andrew Morton , Ryan Roberts , "Mike Rapoport (IBM)" , Arnd Bergmann , x86@kernel.org, linux-m68k@lists.linux-m68k.org, linux-fsdevel@vger.kernel.org, kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org References: <20240917073117.1531207-1-anshuman.khandual@arm.com> <20240917073117.1531207-4-anshuman.khandual@arm.com> <8cafe140-35cf-4e9d-8218-dfbfc156ca69@arm.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: <8cafe140-35cf-4e9d-8218-dfbfc156ca69@arm.com> 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: 8bit X-Rspam-User: X-Rspamd-Queue-Id: 0DE8040003 X-Rspamd-Server: rspam01 X-Stat-Signature: kgrwdtt1q7tnud8zyy8rkqfg1jinw6ak X-HE-Tag: 1726733061-47023 X-HE-Meta: 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 o/MrfMti 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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 18.09.24 08:32, Anshuman Khandual wrote: > > > On 9/17/24 15:58, David Hildenbrand wrote: >> On 17.09.24 09:31, Anshuman Khandual wrote: >>> Convert PTE accesses via ptep_get() helper that defaults as READ_ONCE() but >>> also provides the platform an opportunity to override when required. This >>> stores read page table entry value in a local variable which can be used in >>> multiple instances there after. This helps in avoiding multiple memory load >>> operations as well possible race conditions. >>> >> >> Please make it clearer in the subject+description that this really only involves set_pte_safe(). > > I will update the commit message with some thing like this. > > mm: Use ptep_get() in set_pte_safe() > > This converts PTE accesses in set_pte_safe() via ptep_get() helper which > defaults as READ_ONCE() but also provides the platform an opportunity to > override when required. This stores read page table entry value in a local > variable which can be used in multiple instances there after. This helps > in avoiding multiple memory load operations as well as some possible race > conditions. > >> >> >>> Cc: Andrew Morton >>> Cc: David Hildenbrand >>> Cc: Ryan Roberts >>> Cc: "Mike Rapoport (IBM)" >>> Cc: linux-mm@kvack.org >>> Cc: linux-kernel@vger.kernel.org >>> Signed-off-by: Anshuman Khandual >>> --- >>>   include/linux/pgtable.h | 3 ++- >>>   1 file changed, 2 insertions(+), 1 deletion(-) >>> >>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >>> index 2a6a3cccfc36..547eeae8c43f 100644 >>> --- a/include/linux/pgtable.h >>> +++ b/include/linux/pgtable.h >>> @@ -1060,7 +1060,8 @@ static inline int pgd_same(pgd_t pgd_a, pgd_t pgd_b) >>>    */ >>>   #define set_pte_safe(ptep, pte) \ >>>   ({ \ >>> -    WARN_ON_ONCE(pte_present(*ptep) && !pte_same(*ptep, pte)); \ >>> +    pte_t __old = ptep_get(ptep); \ >>> +    WARN_ON_ONCE(pte_present(__old) && !pte_same(__old, pte)); \ >>>       set_pte(ptep, pte); \ >>>   }) >>> >> >> I don't think this is necessary. PTE present cannot flip concurrently, that's the whole reason of the "safe" part after all. > > Which is not necessary ? Converting de-references to ptep_get() OR caching > the page table read value in a local variable ? ptep_get() conversion also > serves the purpose providing an opportunity for platform to override. Which arch override are you thinking of where this change here would make a real difference? Would it even make a difference with cont-pte on arm? > >> >> Can we just move these weird set_pte/pmd_safe() stuff to x86 init code and be done with it? Then it's also clear *where* it is getting used and for which reason. >> > set_pte/pmd_safe() can be moved to x86 platform - as that is currently the > sole user for these helpers. But because set_pgd_safe() gets used in riscv > platform, just wondering would it be worth moving only the pte/pmd helpers > but not the pgd one ? My take would be just to move them where they are used, and possibly even inlining them. The point is that it's absolutely underdocumented what "_safe" is supposed to be here, and I don't really see the reason to have this in common code (making the common API more complicated). -- Cheers, David / dhildenb