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[2003:cb:c709:d500:4044:6d4c:f060:f677]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-354f276a49bsm722171f8f.96.2024.05.22.07.05.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 22 May 2024 07:05:13 -0700 (PDT) Message-ID: <9c23d824-f2c7-4f9a-ade2-e8dd3a0d30af@redhat.com> Date: Wed, 22 May 2024 16:05:12 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] mm,memory_hotplug: {READ,WRITE}_ONCE unsynchronized zone data To: Brendan Jackman , Oscar Salvador , Andrew Morton , Mike Rapoport Cc: Michal Hocko , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240521-mm-hotplug-sync-v1-0-6d53706c1ba8@google.com> <20240521-mm-hotplug-sync-v1-2-6d53706c1ba8@google.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: <20240521-mm-hotplug-sync-v1-2-6d53706c1ba8@google.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: B284A10001C X-Stat-Signature: 1dshxr3pa7q7du8qj97b4qtbcu17f71u X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1716386719-385436 X-HE-Meta: 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 Uk1tE+0Q WhKLAj/J+Jm+0kUWfAsnpW7Vzochn5Jl+qKOrwjVJ+iSZiBymhk9jT82sIah3rCcw99C12F0zvoym9rmiJBp2rTEvfMEKv7XZmVHrGOYSg2d8dPrkADmUDoTseHdP7xINozfCOFfNXUL/qP0zn/hgHA6axQTmLVBy+w9MZsZaUBgD06pNbvFZ6owsa8ZeqRJUs5rX52r5fEIvjoTJF8YQUU8U9g== 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 21.05.24 14:57, Brendan Jackman wrote: > These fields are written by memory hotplug under mem_hotplug_lock but > read without any lock. It seems like reader code is robust against the > value being stale or "from the future", but we also need to account > for: > > 1. Load/store tearing (according to Linus[1], this really happens, > even when everything is aligned as you would hope). > > 2. Invented loads[2] - the compiler can spill and re-read these fields > ([2] calls this "invented loads") and assume that they have not > changed. > > Note we don't need READ_ONCE in paths that have the mem_hotplug_lock > for write, but we still need WRITE_ONCE to prevent store-tearing. > > [1] https://lore.kernel.org/all/CAHk-=wj2t+GK+DGQ7Xy6U7zMf72e7Jkxn4_-kGyfH3WFEoH+YQ@mail.gmail.com/T/#u > As discovered via the original big-bad article[2] > [2] https://lwn.net/Articles/793253/ > > Signed-off-by: Brendan Jackman > --- > include/linux/mmzone.h | 14 ++++++++++---- > mm/compaction.c | 2 +- > mm/memory_hotplug.c | 20 ++++++++++++-------- > mm/mm_init.c | 2 +- > mm/page_alloc.c | 2 +- > mm/show_mem.c | 8 ++++---- > mm/vmstat.c | 4 ++-- > 7 files changed, 31 insertions(+), 21 deletions(-) > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index 194ef7fed9d6..bdb3be76d10c 100644 > --- a/include/linux/mmzone.h > +++ b/include/linux/mmzone.h > @@ -1018,11 +1018,13 @@ static inline unsigned long zone_cma_pages(struct zone *zone) > #endif > } > > +/* This is unstable unless you hold mem_hotplug_lock. */ > static inline unsigned long zone_end_pfn(const struct zone *zone) > { > - return zone->zone_start_pfn + zone->spanned_pages; > + return zone->zone_start_pfn + READ_ONCE(zone->spanned_pages); It's weird to apply that logic only to spanned_pages, whereby zone_start_pfn can (and will) similarly change when onlining/offlining memory. -- Cheers, David / dhildenb