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[2003:cb:c70c:de00:1200:8636:b63b:f43]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38254910706sm5190673f8f.46.2024.11.21.05.40.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 21 Nov 2024 05:40:45 -0800 (PST) Message-ID: <7f9c4c98-86bf-40a0-817a-858767946cb0@redhat.com> Date: Thu, 21 Nov 2024 14:40:44 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/mempolicy: Fix redundant check and refine lock protection scope in init_nodemask_of_mempolicy To: Zhen Ni , akpm@linux-foundation.org Cc: linux-mm@kvack.org References: <20241121122409.277927-1-zhen.ni@easystack.cn> 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: <20241121122409.277927-1-zhen.ni@easystack.cn> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: SdAxspDsttATvlHqPDrHGilokCxIDntE3EKGrIop69Q_1732196446 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: rspam12 X-Rspamd-Queue-Id: 0324F2000C X-Stat-Signature: a9g69rx17xog1s5tbexjryot4gcbr78k X-HE-Tag: 1732196391-920367 X-HE-Meta: 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 RHU73OLY JpRnNgcd5FO8GqUhwyMHYfdhcPBHj0P+RaH3k0bPzYZBkO/of9P74GMqmHIdBMMAveWLJAimAHiSsJ5Cl2eQdrrSKCQNIoNjH0trxPuGcepwC6ql6zitqQGPWKad3AZJOBrxj9VwEEBhEAWSa+JYdSOytizmAUWrp5x5OH96j6bj/X+By64SOoJtTH563IRogPG74MgzvZo6xmB/XxAVM8LAQjNjynRlrfDKY0WCZZJwlYkPQM4fMV7Bx3x/NZ5VStYyPm6P+MM3xqd2orwxlqFOhIhdw0eJyUyjCfrhdgw5ru+8aM9tqfsxU6fXZkjKSLaVzdp2rbkPbttzkDVaIIo6Vj4Tl3wo9KiBr5uDqkE+vEeKq2olRkPeVOtKqAx50p09879BDHdSSsiCHxrdr0swkTLzWddgUbtDsq+NRqHGXlomRsWHFP2dMG1stNLt30IhU 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 21.11.24 13:24, Zhen Ni wrote: > 1.Removing redundant checks for current->mempolicy, with a more concise > check order. > > 2.Using READ_ONCE(current->mempolicy) for safe, single access to > current->mempolicy to prevent potential race conditions. > > 3.Optimizing the scope of task_lock(current). The lock now only protects > the critical section where mempolicy is accessed, reducing the duration > the lock is held. This enhances performance by limiting the scope of the > lock to only what is necessary. > > Signed-off-by: Zhen Ni > --- > mm/mempolicy.c | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-) > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index b646fab3e45e..8bff8830b7e6 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -2132,11 +2132,14 @@ bool init_nodemask_of_mempolicy(nodemask_t *mask) > { > struct mempolicy *mempolicy; > > - if (!(mask && current->mempolicy)) > + if (!mask)> + return false; > + > + mempolicy = READ_ONCE(current->mempolicy); > + if (!mempolicy) > return false; > > task_lock(current); > - mempolicy = current->mempolicy; > switch (mempolicy->mode) { > case MPOL_PREFERRED: > case MPOL_PREFERRED_MANY: (a) current->mempolicy can only change under the task_lock(), see do_set_mempolicy(). (b) current->mempolicy cannot usually NULL once it was none-NULL. There are two exceptions: copy_process() might set it to NULL when creating the process, before the task gets scheduled. do_exit() might set it to NULL via mpol_put_task_policy(). I don't think init_nodemask_of_mempolicy() can be called while our task is exiting ... Now, if you read current->mempolicy outside of task_lock() to the *dereference* it when it might already have changed+was freed, that's a BUG you're introducing. So, regarding your (1): which redundant "check" ? There is a single "not NULL" chech. Reagrding your (2), I think you are introducing a BUG and regrding your (3) please share performance numbers, or realize that youa re making something up ;) The only improvement we could make here is converting: if (!(mask && current->mempolicy)) into a more readable if (!mask || !current->mapping) And maybe adding a comment that while we can race with someone changing current->mapping, we cannot race with someone changing it to NULL. -- Cheers, David / dhildenb