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[221.241.217.81]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-c9e8f5c175dsm6341551a12.11.2026.07.06.05.57.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 06 Jul 2026 05:57:54 -0700 (PDT) Date: Mon, 06 Jul 2026 21:57:51 +0900 Message-ID: From: Hajime Tazaki To: ljs@kernel.org Cc: geert@linux-m68k.org, akpm@linux-foundation.org, willy@infradead.org, liam@infradead.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, gerg@uclinux.org Subject: Re: [PATCH] mm: nommu: point to the write iterator upon split_vma In-Reply-To: References: <20260702012546.665383-1-thehajime@gmail.com> <20260705152708.582fd8d87561f600da23c0a7@linux-foundation.org> <20260705222343.5d5c78cbb54839c5efcaabf5@linux-foundation.org> User-Agent: Wanderlust/2.15.9 (Almost Unreal) Emacs/27.2 Mule/6.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=ISO-8859-7 Content-Transfer-Encoding: quoted-printable On Mon, 06 Jul 2026 20:51:36 +0900, Lorenzo Stoakes wrote: >=20 > On Mon, Jul 06, 2026 at 10:27:43AM +0200, Geert Uytterhoeven wrote: > > Hi Andrew, > > > > On Mon, 6 Jul 2026 at 07:23, Andrew Morton = wrote: > > > On Mon, 6 Jul 2026 04:58:28 +0100 Matthew Wilcox wrote: > > > > On Sun, Jul 05, 2026 at 03:27:08PM -0700, Andrew Morton wrote: > > > > > Thanks. Unfortunately we aren't very diligent about the nommu co= de > > > > > (are we?). Perhaps appropriately - clearly this code doesn't ge= t used a > > > > > lot. > > > > > > > > Should we delete support for NOMMU? As you say, it doesn't get much > > > > testing, or presumably usage. I have this quote from #m68k a coupl= e of > > > > years ago... > > > > > > Greg is busily developing kernel code. > > > > > > > : I looked at nommu very briefly when I had my 68000 board on the b= ench > > > > : but didn=A2t get anywhere. Also didn=A2t try particularly hard. I= think > > > > : you are better off with fuzix or OS9 if you want something unixy. > > > > > > > > (i suspect Zephyr would also be good, but it doesn't support m68k, = just > > > > arc, arm, arm64, mips, openrisc, renesas rx, riscv, sparc, x86 and = xtensa) > > > > > > Greg, how mush use is NOMMU Linux seeing nowadays? > > > > Nommu (SH-compatible J-Core ASIC) is used in actual products, running > > modern kernels. The rationale is low interrupt latency without using > > a much more costly app CPU core + RT CPU core split. > > > > Please watch the recording of the "32bit and-or noMMU Linux BoF"[1] > > at LPC2025 for more info. >=20 > This crops up now and again and people point to uses, but fails to addres= s the > real issue here. >=20 > Nobody (aside from Hajime - thank you!) who uses nommu maintains or contr= ibutes > to it, or _even tests_ it as far as I can tell. >=20 > We broken nommu for a year I think it was? And there wasn't one report. >=20 > The last time I raised this, I was admonished and told there are critical= arm32 > nommu devices that absolutely _must_ have the latest kernel and this is > perfectly working code. >=20 > And riscv also introduced (and I'm so very surprised Linus allowed it) a = brand > new nommu architecture (!!) >=20 > And yet... >=20 > If this is really so important, can those who care perhaps help out a bit= ? At > least with testing? >=20 > I'm a bit fed up of it really. >=20 > As far as hobbyist retro stuff and upstream goes, I am sympathetic, I cam= e from > being a hobbyist (though not retro), and would be one again if I couldn't= do it > as a job. >=20 > But retro people do NOT need the latest kernel. As far as I'm concerned, = museum > piece support should be ripped out, and those who care run downstream ker= nels. >=20 > As for the 'critical products' that must use nommu - PLEASE start > contributing. Please start testing. Anything. >=20 > Again thank you Hajime for doing the one thing that really counts here - > contributing to the nommu code. >=20 > Anyway, as to what Matthew said - 100% I am for us ripping it out. >=20 > But it's not up to me, and unfortunately I think we'll get a clammer of v= oices > saying how important it is, and silence again when it comes to do any of = the > work. as nommu started for embedded devices which are not easy to test, I agree that it is also not easy (in my opinion) to maintain. indeed, buildroot images for nommu targets (with qemu) are available, but running/testing each of commit to the kernel tree give me more than 1 hour to rebuild the images (including userspace build with long flat binary conversion, etc) (which I'm playing under github actions with riscv-nommu). # yes, should use ccache :) one of the goal which I am with nommu UML (*1) is to provide an environment which can be tested in a simple way; runs on x86 machines. An extension to UML means that you can also use KUnit infrastructure. LTP (Linux Test Project) dropped the nommu tests 2 years ago (*2) (which is pity), but the reason is similar: no maintainers appeared when the project asked. I wish to be here to improve this situation, both mm/nommu subsystem and LTP, as well as any !CONFIG_MMU code in kernel tree. This patch is actually discovered during LTP (re-)integration with nommu kernel, which I'm going to propose to LTP. I hope this helps a bit. *1 https://lore.kernel.org/all/cover.1770170302.git.thehajime@gmail.com/ *2 https://lore.kernel.org/ltp/20240105131135.GA1484621@pevik/ -- Hajime