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[221.241.217.81]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2d3aeb27e40sm9037135ad.41.2026.08.14.05.42.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 14 Aug 2026 05:42:30 -0700 (PDT) Date: Fri, 14 Aug 2026 21:42:26 +0900 Message-ID: From: Hajime Tazaki To: gregkh@linuxfoundation.org Cc: linux-mm@kvack.org, geert@linux-m68k.org, daniel@thingy.jp, arnd@arndb.de, willy@infradead.org, jack@suse.cz, akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, linux-fsdevel@vger.kernel.org Subject: Re: [RFC PATCH 3/6] mm: nommu: fix an issue on map request to /dev/zero In-Reply-To: <2026081355-remold-sterility-84f9@gregkh> References: <20260813063401.1786548-1-thehajime@gmail.com> <20260813063401.1786548-4-thehajime@gmail.com> <2026081355-remold-sterility-84f9@gregkh> User-Agent: Wanderlust/2.15.9 (Almost Unreal) Emacs/27.2 Mule/6.0 MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-Rspam-User: X-Stat-Signature: u8y1hbwouh3xktdmqxzs5ktt7xo5u44j X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 08FB11C0008 X-HE-Tag: 1786711351-945063 X-HE-Meta: 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 BskdaVDs aF58KCkIlgVdFkLj084s02xU75APij8Gypdzd4yOW9LjhoJlDh2jC5c7Q2fRqlBDCY1RUmW88nU/4jcQNn9J5xeeV1ikKsYfyaP+1KJeriU/GVdIUnYgSLIJZ18wJKjcPloirg1kfC9ar2PJvShFYqeX7OTvsr9+dJQWtZVy0TB/kVwikYzJ/qMV90gugvVyBtdhInLyZAlHdRRyCFUbwpE20GpVOfSaQLX/nwmj8niwSrcZzUKAbSMU3obC3hf+iqnyQRCSPdzEmy5WjJ37slULkaR/8iqbnCZYJLvRs33a3K+bYT3EdiWSHrwDVyBmknUTGSdioCb090OTxypOKoKGs87NAmukV91uH9i0TaQ2UDeEsx3EEEaIKcm5S5aDKEZu9JuXkrOfw55ZMRXakA8Jp+pT4OAuIi6PDEThboM22HwFNj4Iky3PKjFqnkDKH5nB4loIq33BArZOWcJx2QdKFQ2isrMJh8ti/Yx3D/QuyZCDYSbsWgpp4/AL8ZkgHhC4IZ5uT/VgNqhA9J0VD0UryCwx6lJNIDR0pHQuJKMR8w4Esoilve0+Ta+ezsXGpf41/gZQhsvbJtpZHWyQsDA0liOvOLUTNGBm2QKe8pLh45ZWWMD2k89O3Zx1zLSneEPDaFuGT6pozBq2dttC/mrlnV9osFJka54Y9cnYiPkC7ot/dCy2BeViPuL7eCI1gMfcQXorbTi3jtWTJqos2gtEX3dwXrwdh32aDAtDk7s6XR5Uf3KLceppNquOAR1mqQDoU4T1jwYUgTYZDdzMLH8AlXV8+JSctUfyRIb44+FhIIY2JnHQfJ+BeDkjBtrIQ8Iv/6JVJ3xBRPXA2bQIJbHb/deA/akc7Fphm/qNTkIo9w8wNwc43/Djglw2cB4E9EIiuq4A6niXsX3ZHK5yXWGDHEcApNyiYldAt Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hello Greg, thank you for your time to look at this patch. On Thu, 13 Aug 2026 21:19:47 +0900, Greg Kroah-Hartman wrote: > > On Thu, Aug 13, 2026 at 03:33:58PM +0900, Hajime Tazaki wrote: > > Upon a private file mapping request to /dev/zero, it calls > > kernel_read() in do_mmap_private(), getting a failure with the message > > like: "kernel reads not supported for file /dev/zero", which is because > > zero_fops defined in drivers/char/mem.c has both .read and .read_iter > > definitions. > > Do you actually use a no-mmu system? yes, and wish to use in future. I'm going to make a long story short; nommu kernel is originally for embedded devices but I wish to use for virtualization. I gave a talk to explain this motivation below (which you also had a keynote remotely). I'm not make a summary for this talk in this email, but am happy to explain again if needed. https://www.netdevconf.info/0x1A/sessions/talk/improving-debuggability-of-nommu-code-with-uml.html https://speakerdeck.com/thehajime/nommu-uml-netdev-0x1a/ > > Even fixing this issue, the map request to /dev/zero works fine without > > errors but the allocated vma isn't marked with anonymous because > > mmap_zero_prepare() isn't called under nommu platform, resulting > > vma_desc_set_anonymous() isn't called either. > > > > This commit fixes those issues by: > > 1) use vfs_iter_read() instead to avoid failure at kernel_read() > > 2) calls .mmap_prepare on private mapping in do_mmap() so that required > > preparations are done even in private mapping. > > > > Cc: Arnd Bergmann > > Cc: Greg Kroah-Hartman > > Cc: "Matthew Wilcox (Oracle)" > > Cc: Jan Kara > > Cc: Andrew Morton > > Cc: "Liam R. Howlett" > > Cc: Lorenzo Stoakes > > Cc: Vlastimil Babka > > Cc: Jann Horn > > Cc: Pedro Falcato > > Cc: linux-fsdevel@vger.kernel.org > > Cc: linux-mm@kvack.org (open list:PAGE CACHE) > > Fixes: 4d03e3cc5982 ("fs: don't allow kernel reads and writes without iter ops") > > Given the age of this issue, I don't think anyone uses no-mmu systems > anymore :( I tend to agree; nommu systems does have less users than others. but I believe this doesn't mean there are no users. I should also explain more, but I found this issue via a Sashiko review, which pointed me as a different comment, but when I tried to reproduce that case pointed by the review, which is about the use of check `vma->vm_file` v.s., `vma_is_anonymous(vma)`, I found that /dev/zero is not able to map on nommu kernel. https://sashiko.dev/#/patchset/20260710054648.924005-1-thehajime%40gmail.com https://sashiko.dev/#/message/m2cxwvti7z.wl-thehajime%40gmail.com The alternate path of open()=>read() of /dev/zero works fine as it doesn't use kernel_read(). I also understand what you feel from the age of this issue (almost 6 years old), and that is exactly why I start implementing test cases in kselftest, which is currently not able to run (even build) for nommu platforms. This series also includes the extension to kselftest ([4,5,6/6] patches are for that). > > - if (!file) { > > + /* private and file mapping will be marked anonymous later (do_mmap_private()). > > + * and /dev/zero is marked by them at .mmap_prepare, > > + * which should be _before_ this point. > > + */ > > Wrong coding style for the comment, which is very typical of > LLM-generated stuff. Always rewrite the output of these tools to > actually be sane. this is my fault. As I mentioned in the Assisted-by tag, I indeed used an AI review system (which is cubic.dev), but I wrote this comment by my hand. Because I thought this part is not easy to follow and felt that I need additional explanation rather than just showing diff. since checkpatch.pl doesn't complain this, I didn't carefully look at the style of this comment block. I will update this (as well as bunch of comments from Sashiko). thanks, -- Hajime