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[2003:cb:c718:1300:9860:66a2:fe4d:c379]) by smtp.gmail.com with ESMTPSA id fc9-20020a05600c524900b004162a9f03a6sm15534185wmb.7.2024.04.08.00.41.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Apr 2024 00:41:31 -0700 (PDT) Message-ID: Date: Mon, 8 Apr 2024 09:41:30 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm,swap: add document about RCU read lock and swapoff interaction To: Huang Ying , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ryan Roberts , Miaohe Lin , Hugh Dickins , Minchan Kim References: <20240407065450.498821-1-ying.huang@intel.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: <20240407065450.498821-1-ying.huang@intel.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-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: D41D6100005 X-Stat-Signature: wop4d5iq8cp65t3gjzyymyao5z6cezhi X-HE-Tag: 1712562097-176188 X-HE-Meta: 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 KtXfhFpi 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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 07.04.24 08:54, Huang Ying wrote: > During reviewing a patch to fix the race condition between > free_swap_and_cache() and swapoff() [1], it was found that the > document about how to prevent racing with swapoff isn't clear enough. > Especially RCU read lock can prevent swapoff from freeing data > structures. So, the document is added as comments. > > [1] https://lore.kernel.org/linux-mm/c8fe62d0-78b8-527a-5bef-ee663ccdc37a@huawei.com/ > > Signed-off-by: "Huang, Ying" > Cc: Ryan Roberts > Cc: David Hildenbrand > Cc: Miaohe Lin > Cc: Hugh Dickins > Cc: Minchan Kim > --- > mm/swapfile.c | 26 +++++++++++++------------- > 1 file changed, 13 insertions(+), 13 deletions(-) > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 4919423cce76..6925462406fa 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -1226,16 +1226,15 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p, > > /* > * When we get a swap entry, if there aren't some other ways to > - * prevent swapoff, such as the folio in swap cache is locked, page > - * table lock is held, etc., the swap entry may become invalid because > - * of swapoff. Then, we need to enclose all swap related functions > - * with get_swap_device() and put_swap_device(), unless the swap > - * functions call get/put_swap_device() by themselves. > + * prevent swapoff, such as the folio in swap cache is locked, RCU > + * reader side is locked, etc., the swap entry may become invalid > + * because of swapoff. Then, we need to enclose all swap related > + * functions with get_swap_device() and put_swap_device(), unless the > + * swap functions call get/put_swap_device() by themselves. > * > - * Note that when only holding the PTL, swapoff might succeed immediately > - * after freeing a swap entry. Therefore, immediately after > - * __swap_entry_free(), the swap info might become stale and should not > - * be touched without a prior get_swap_device(). > + * RCU reader side lock (including any spinlock) is sufficient to > + * prevent swapoff, because synchronize_rcu() is called in swapoff() > + * before freeing data structures. > * > * Check whether swap entry is valid in the swap device. If so, > * return pointer to swap_info_struct, and keep the swap entry valid > @@ -2495,10 +2494,11 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile) > > /* > * Wait for swap operations protected by get/put_swap_device() > - * to complete. > - * > - * We need synchronize_rcu() here to protect the accessing to > - * the swap cache data structure. > + * to complete. Because of synchronize_rcu() here, all swap > + * operations protected by RCU reader side lock (including any > + * spinlock) will be waited too. This makes it easy to > + * prevent folio_test_swapcache() and the following swap cache > + * operations from racing with swapoff. > */ > percpu_ref_kill(&p->users); > synchronize_rcu(); Reviewed-by: David Hildenbrand -- Cheers, David / dhildenb