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Mon, 9 Sep 2024 05:46:47 -0400 (EDT) Date: Mon, 9 Sep 2024 12:46:42 +0300 From: "Kirill A. Shutemov" To: Charlie Jenkins Cc: Arnd Bergmann , Richard Henderson , Ivan Kokshaysky , Matt Turner , Vineet Gupta , Russell King , Guo Ren , Huacai Chen , WANG Xuerui , Thomas Bogendoerfer , "James E.J. Bottomley" , Helge Deller , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Naveen N Rao , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Sven Schnelle , Yoshinori Sato , Rich Felker , John Paul Adrian Glaubitz , "David S. Miller" , Andreas Larsson , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Muchun Song , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Lorenzo Stoakes , Shuah Khan , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linux-snps-arc@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-csky@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH RFC v2 0/4] mm: Introduce MAP_BELOW_HINT Message-ID: References: <20240829-patches-below_hint_mmap-v2-0-638a28d9eae0@rivosinc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 0192816001D X-Stat-Signature: prueaka39k1int4sw4romxzbrdxj7zdz X-HE-Tag: 1725875228-313063 X-HE-Meta: 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 EKpkiLvc h8Ct3n3qnhfihEYg23/TvVkCmqKtoZN7ElQ7fvpDbT1tNQ+asTwVVMIzOZP0J9jXOGwS0lASLBWkQJJF0X7X7TK8An6RoV1stJbHJlPjV1CjKTfJiOFhYcmC1+4tY0CPZhHS5Dr8uloYsNRLl7Pac5HHgIjahEpqdWUILViMGiTb5UbmwLLKF7mFMqIjHK/kzX5qr6xw7NHKDxB/WqNDH0C0da6j2R7Vui7ZwTmhfxba9FFWBsEGmVGhgoJUAfSh6l/ZU34O5IO40o1pJJ+6WpZE3KUL3B3ut5FUQWVr4PPr6bOujnUNbA1IXdh9PncJOe+//PLyMO+A2gpRByO5VNVJPEysE4YRoExkxyw6Y7noFew7JOSsMuKmw5qy1Y9nYjF2/Jv9rE+vJ7OtTKLNY3bH5T+0XAfSLueY3fM6rG/K2BM9G+DIxMbc2yw== 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 Thu, Sep 05, 2024 at 10:26:52AM -0700, Charlie Jenkins wrote: > On Thu, Sep 05, 2024 at 09:47:47AM +0300, Kirill A. Shutemov wrote: > > On Thu, Aug 29, 2024 at 12:15:57AM -0700, Charlie Jenkins wrote: > > > Some applications rely on placing data in free bits addresses allocated > > > by mmap. Various architectures (eg. x86, arm64, powerpc) restrict the > > > address returned by mmap to be less than the 48-bit address space, > > > unless the hint address uses more than 47 bits (the 48th bit is reserved > > > for the kernel address space). > > > > > > The riscv architecture needs a way to similarly restrict the virtual > > > address space. On the riscv port of OpenJDK an error is thrown if > > > attempted to run on the 57-bit address space, called sv57 [1]. golang > > > has a comment that sv57 support is not complete, but there are some > > > workarounds to get it to mostly work [2]. I also saw libmozjs crashing with 57-bit address space on x86. > > > These applications work on x86 because x86 does an implicit 47-bit > > > restriction of mmap() address that contain a hint address that is less > > > than 48 bits. > > > > > > Instead of implicitly restricting the address space on riscv (or any > > > current/future architecture), a flag would allow users to opt-in to this > > > behavior rather than opt-out as is done on other architectures. This is > > > desirable because it is a small class of applications that do pointer > > > masking. You reiterate the argument about "small class of applications". But it makes no sense to me. With full address space by default, this small class of applications is going to *broken* unless they would handle RISC-V case specifically. On other hand, if you limit VA to 128TiB by default (like many architectures do[1]) everything would work without intervention. And if an app needs wider address space it would get it with hint opt-in, because it is required on x86-64 anyway. Again, no RISC-V-specific code. I see no upside with your approach. Just worse user experience. [1] See va_high_addr_switch test case in https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/mm/Makefile#n115 -- Kiryl Shutsemau / Kirill A. Shutemov