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[2003:cb:c72f:f700:104b:9184:1b45:1898]) by smtp.gmail.com with ESMTPSA id z14-20020a05600c0a0e00b00412a37130ccsm4368870wmp.48.2024.02.26.04.30.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 26 Feb 2024 04:30:11 -0800 (PST) Message-ID: <4f215e3f-3395-4fab-b938-07a6d9e0a662@redhat.com> Date: Mon, 26 Feb 2024 13:30:10 +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> 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: <20240226120321.1055731-3-ryan.roberts@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: 7bit X-Rspamd-Queue-Id: BC90AC001D X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: mrytnq8xetmobn6uqyrbntbboxxkreom X-HE-Tag: 1708950615-970948 X-HE-Meta: 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 FOhxUew3 bLVJjzD2FBOY4yBLzpd32h9Uxno8ft5plS/nv2bnpjw1VUGfwKIOUMNMfRuXDkkNxpxT1nYd10g3VpavDDfrpZ35N0TGhKNOi711eaXKeJK87re9cflzxD+ciIzc3cKPW3fk8EcTXrOvszVn8R7ICwf3dXIzM+pRcrM1lWCQqdsDvudgFovilMLAFX2DaCZg67a/sFihFW+EQb0lK2CIAdWCJ3A== 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 13:03, 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 In an ideal world, we'd really not rely on any accessed/dirty on the lockless path and remove contpte_ptep_get_lockless() completely :) -- Cheers, David / dhildenb