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[2003:cb:c738:3100:8133:26cf:7877:94aa]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38a8e383654sm15658291f8f.30.2025.01.14.09.02.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Jan 2025 09:02:05 -0800 (PST) Message-ID: <7c1087c5-570d-4380-850b-ac26f01d325a@redhat.com> Date: Tue, 14 Jan 2025 18:02:03 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] /dev/zero: make private mapping full anonymous mapping From: David Hildenbrand To: Lorenzo Stoakes Cc: Yang Shi , arnd@arndb.de, gregkh@linuxfoundation.org, Liam.Howlett@oracle.com, vbabka@suse.cz, jannh@google.com, willy@infradead.org, liushixin2@huawei.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250113223033.4054534-1-yang@os.amperecomputing.com> <08e7f845-6b86-4fdc-830d-23f15ad90475@redhat.com> <8e88bce5-3be3-4e84-a327-841c91906a79@lucifer.local> <75dbebfa-d13c-436e-a9ac-87f98b140cc6@redhat.com> 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: <75dbebfa-d13c-436e-a9ac-87f98b140cc6@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: L6p9n3aQxJooFHmUOeDUaL7Vz9gTv6OdboalYbhvy48_1736874128 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 3EE2D40015 X-Stat-Signature: t5ptht8hiihj6updcpcbq6eb5anmzu8g X-Rspam-User: X-HE-Tag: 1736874131-175104 X-HE-Meta: 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 kLdVIOdC 58oYImpy7Z5zYFHXmYNvLYKsE4F86UL+DLZo+sW1Yb0KBNaTf9mIhpjFyAOWdf3vBKtVmewRqtE+0ZxHGkPVf1o5ck82gRFbP3fKbEq7T332DGVJWNnIH/1c6RXw0IkbV4qQiKHyJTd9ugCaWz/gBEavhMy1UBJqwe26cIyEHGCUkEC2gdMiLWb20hmTozNL4JJwLmRPVs5IEra5wqkAdjkbcdFTrx3sGIvdPfjLWWtLeX4YV+/4CvF9dVJGMNAgVf9oCOi+bdQGCOakzz4hfQBUkonWSmronuXxGwbG7Vi6Yyplh7S8GRkTcnqowCZ3P17ytGcIoQfa7L5Ja6f6HQnDK+vN5X5KZ3jtHD371MWhxKLoeotTzIDb5k7Ynxyv03TgK1zdSMPuB6c+jho85/40Mfk/LP0Gx0YSlqubH0DnyMhA= 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 14.01.25 16:06, David Hildenbrand wrote: > On 14.01.25 15:52, Lorenzo Stoakes wrote: >> On Tue, Jan 14, 2025 at 02:01:32PM +0100, David Hildenbrand wrote: >>> On 13.01.25 23:30, Yang Shi wrote: >>>> When creating private mapping for /dev/zero, the driver makes it an >>>> anonymous mapping by calling set_vma_anonymous(). But it just sets >>>> vm_ops to NULL, vm_file is still valid and vm_pgoff is also file offset. >>>> >>>> This is a special case and the VMA doesn't look like either anonymous VMA >>>> or file VMA. It confused other kernel subsystem, for example, khugepaged [1]. >>>> >>>> It seems pointless to keep such special case. Making private /dev/zero> >>> mapping a full anonymous mapping doesn't change the semantic of >>>> /dev/zero either. >>>> >>>> The user visible effect is the mapping entry shown in /proc//smaps >>>> and /proc//maps. >>>> >>>> Before the change: >>>> ffffb7190000-ffffb7590000 rw-p 00001000 00:06 8 /dev/zero >>>> >>>> After the change: >>>> ffffb6130000-ffffb6530000 rw-p 00000000 00:00 0 >>>> >>> >>> Hm, not sure about this. It's actually quite consistent to have that output >>> in smaps the way it is. You mapped a file at an offset, and it behaves like >>> an anonymous mapping apart from that. >>> >>> Not sure if the buggy khugepaged thing is a good indicator to warrant this >>> change. >> >> Yeah, this is a user-facing fundamental change that hides information and >> defies expectation so I mean - it's a no go really isn't it? >> >> I'd rather we _not_ make this anon though, because isn't life confusing >> enough David? I thought it was bad enough with 'anon, file and lol shmem' >> but 'lol lol also /dev/zero' is enough to make me want to frolick in the >> fields... > > I recall there are users that rely on this memory to get the shared > zeropage on reads etc (in comparison to shmem!), so I better not ... > mess with this *at all* :) Heh, and I recall reading something about odd behavior of /dev/zero and some interesting history [1]. " Unlike /dev/null, /dev/zero may be used as a source, not only as a sink for data. All write operations to /dev/zero succeed with no other effects. However, /dev/null is more commonly used for this purpose. When /dev/zero is memory-mapped, e.g., with mmap, to the virtual address space, it is equivalent to using anonymous memory; i.e. memory not connected to any file. " "equivalent to using anonymous memory" is interesting. Also, /dev/zero was there before MAP_ANONYMOUS was invented according to [1], which is quite interesting. ... so this is anonymous memory as "real" as it can get :) [1] https://en.wikipedia.org/wiki//dev/zero -- Cheers, David / dhildenb