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
