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[2003:cb:c72f:f700:104b:9184:1b45:1898]) by smtp.gmail.com with ESMTPSA id z9-20020a05600c114900b004128fa77216sm11858709wmz.1.2024.02.26.04.40.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 26 Feb 2024 04:40:55 -0800 (PST) Message-ID: Date: Mon, 26 Feb 2024 13:40:54 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] arm64/mm: Improve comment in contpte_ptep_get_lockless() To: Ryan Roberts , Andrew Morton , Catalin Marinas , Mark Rutland , John Hubbard Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org References: <20240226120321.1055731-1-ryan.roberts@arm.com> <20240226120321.1055731-3-ryan.roberts@arm.com> <4f215e3f-3395-4fab-b938-07a6d9e0a662@redhat.com> <8cda557a-cbf0-4926-be19-454cefdb4fdb@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: <8cda557a-cbf0-4926-be19-454cefdb4fdb@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-Server: rspam12 X-Rspamd-Queue-Id: 84E1014001B X-Stat-Signature: j88dc3z7zoi45p435p64g54hmtdqfhp6 X-HE-Tag: 1708951259-449707 X-HE-Meta: U2FsdGVkX1/tJTA29a3VmJqoZdZXw2vxjm3z33rxVRLR5kJc4T9gG4Db+68KFfGRNmPcv3G+CpkXEpMUj3C+sWcOS2YJUkIg4O9AaFfc1+TR4Ue4NYtCtSDoGB3o7Zx9Qy50lbEvTYTUbE9qvWTVnsZ0iBq4ssQkY5Kx98FKO1CCZ059ys4+zHW7V9S4JYgP0qzXat4253wb3XpKgtRPYzalAgEbD96OAxPJUCKlJMgfvhOTWMh3y6EheSn+Qp/8owCVVfESuI2nc7R8WmlGiAnJyMUABxJGrpXJsfLpgzDQxAxRsq/obJn3Ciw9vWN5ECw/DgEBmkkxIj/jjfOitTbYdgMjmN13moORZMzQHIPcG2Gn3OI1/R4VbfmtTsPFPxbhkqERkqyzzNuwif33yGcd835CcSrPq0p8ht6LYW07CPv90AudvBGwUjBTiGphMgRQpfKd5vVn4rcFwfzhNsX84oNhvllKDOL9E6/h58BivvONH0/sBWoCEHnlox5aOZEtYQmStayhVQWL7tejLy1N1QfSTdnp35Qh7zDxr0boOOOPug8syMG6PoXRgf3dDvz/GrzjexLLh9dZZe6OlbNkwT3+hnw33lnJ8VsRc8SSPrqCTLP127Ch0H9s0k/xdLHPqfMR7y/1ADlfFQclaqtx+QcBHjfn6WOm//7dnt6NU5AntEGU9FkwH32Yp5+UdTcL+MxTJMfjZHJoJWqTFRPKaXKKWQl7jlcNJiDuhMJtCKBYdK58h5jceXy89127D5f5rpKlyrzr5jhYy8kUPySY7XaFw2bhP1qQLbYmIfnqe5yRkmoQ9s5aLir5fiWIUBdRhkvtBJVq3SQRep8b7bOCIj2v6evZ1/NatyDx5lJra9WauzrwuuvMQBOcfQ0SjJm4cFLrkkznZVCeMAs8ttT1qZg0FSOq7zU3B0lXi4J7c+AnTbYnq4vJ6Gyx9hEvfBTWqqRFmlS12rvMLhv dtqEUS7O 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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 26.02.24 13:37, Ryan Roberts wrote: > On 26/02/2024 12:30, David Hildenbrand wrote: >> On 26.02.24 13:03, Ryan Roberts wrote: >>> Make clear the atmicity/consistency requirements of the API and how we >>> achieve them. >>> >>> Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/ >>> Signed-off-by: Ryan Roberts >>> --- >>>   arch/arm64/mm/contpte.c | 24 ++++++++++++++---------- >>>   1 file changed, 14 insertions(+), 10 deletions(-) >>> >>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c >>> index be0a226c4ff9..1b64b4c3f8bf 100644 >>> --- a/arch/arm64/mm/contpte.c >>> +++ b/arch/arm64/mm/contpte.c >>> @@ -183,16 +183,20 @@ EXPORT_SYMBOL_GPL(contpte_ptep_get); >>>   pte_t contpte_ptep_get_lockless(pte_t *orig_ptep) >>>   { >>>       /* >>> -     * Gather access/dirty bits, which may be populated in any of the ptes >>> -     * of the contig range. We may not be holding the PTL, so any contiguous >>> -     * range may be unfolded/modified/refolded under our feet. Therefore we >>> -     * ensure we read a _consistent_ contpte range by checking that all ptes >>> -     * in the range are valid and have CONT_PTE set, that all pfns are >>> -     * contiguous and that all pgprots are the same (ignoring access/dirty). >>> -     * If we find a pte that is not consistent, then we must be racing with >>> -     * an update so start again. If the target pte does not have CONT_PTE >>> -     * set then that is considered consistent on its own because it is not >>> -     * part of a contpte range. >>> +     * The ptep_get_lockless() API requires us to read and return *orig_ptep >>> +     * so that it is self-consistent, without the PTL held, so we may be >>> +     * racing with other threads modifying the pte. Usually a READ_ONCE() >>> +     * would suffice, but for the contpte case, we also need to gather the >>> +     * access and dirty bits from across all ptes in the contiguous block, >>> +     * and we can't read all of those neighbouring ptes atomically, so any >>> +     * contiguous range may be unfolded/modified/refolded under our feet. >>> +     * Therefore we ensure we read a _consistent_ contpte range by checking >>> +     * that all ptes in the range are valid and have CONT_PTE set, that all >>> +     * pfns are contiguous and that all pgprots are the same (ignoring >>> +     * access/dirty). If we find a pte that is not consistent, then we must >>> +     * be racing with an update so start again. If the target pte does not >>> +     * have CONT_PTE set then that is considered consistent on its own >>> +     * because it is not part of a contpte range. >>>        */ >>>         pgprot_t orig_prot; >> >> Reviewed-by: David Hildenbrand > > Thanks! > >> >> In an ideal world, we'd really not rely on any accessed/dirty on the lockless >> path and remove contpte_ptep_get_lockless() completely :) > > Not sure if you saw my RFC to do exactly that? (well, it doesn't actually remove > [contpte_]ptep_get_lockless() but it does remove all the callers). If you have > any feedback, we could get this moving... Yes, I saw it. Hoping we can get that in and then maybe remove the contpte_get_lockless() once all callers are gone :) ... on my todo list. -- Cheers, David / dhildenb