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]. 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), provide a flag to the personality syscall that can be used to ensure an application works in any arbitrary VA space. A similar feature has already been implemented by the personality syscall in ADDR_LIMIT_32BIT. This flag will also allow seemless compatibility between all architectures, so applications like Go and OpenJDK that use bits in a virtual address can request the exact number of bits they need in a generic way. The flag can be checked inside of vm_unmapped_area() so that this flag does not have to be handled individually by each architecture. Link: https://github.com/openjdk/jdk/blob/f080b4bb8a75284db1b6037f8c00ef3b1ef1add1/src/hotspot/cpu/riscv/vm_version_riscv.cpp#L79 [1] Link: https://github.com/golang/go/blob/9e8ea567c838574a0f14538c0bbbd83c3215aa55/src/runtime/tagptr_64bit.go#L47 [2] To: Arnd Bergmann <[email protected]> To: Richard Henderson <[email protected]> To: Ivan Kokshaysky <[email protected]> To: Matt Turner <[email protected]> To: Vineet Gupta <[email protected]> To: Russell King <[email protected]> To: Guo Ren <[email protected]> To: Huacai Chen <[email protected]> To: WANG Xuerui <[email protected]> To: Thomas Bogendoerfer <[email protected]> To: James E.J. Bottomley <[email protected]> To: Helge Deller <[email protected]> To: Michael Ellerman <[email protected]> To: Nicholas Piggin <[email protected]> To: Christophe Leroy <[email protected]> To: Naveen N Rao <[email protected]> To: Alexander Gordeev <[email protected]> To: Gerald Schaefer <[email protected]> To: Heiko Carstens <[email protected]> To: Vasily Gorbik <[email protected]> To: Christian Borntraeger <[email protected]> To: Sven Schnelle <[email protected]> To: Yoshinori Sato <[email protected]> To: Rich Felker <[email protected]> To: John Paul Adrian Glaubitz <[email protected]> To: David S. Miller <[email protected]> To: Andreas Larsson <[email protected]> To: Thomas Gleixner <[email protected]> To: Ingo Molnar <[email protected]> To: Borislav Petkov <[email protected]> To: Dave Hansen <[email protected]> To: [email protected] To: H. Peter Anvin <[email protected]> To: Andy Lutomirski <[email protected]> To: Peter Zijlstra <[email protected]> To: Muchun Song <[email protected]> To: Andrew Morton <[email protected]> To: Liam R. Howlett <[email protected]> To: Vlastimil Babka <[email protected]> To: Lorenzo Stoakes <[email protected]> To: Shuah Khan <[email protected]> To: Christoph Hellwig <[email protected]> To: Michal Hocko <[email protected]> To: "Kirill A. Shutemov" <[email protected]> To: Chris Torek <[email protected]> Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Signed-off-by: Charlie Jenkins <[email protected]> Changes in v2: - Added much greater detail to cover letter - Removed all code that touched architecture specific code and was able to factor this out into all generic functions, except for flags that needed to be added to vm_unmapped_area_info - Made this an RFC since I have only tested it on riscv and x86 - Link to v1: https://lore.kernel.org/r/[email protected] Changes in v3: - Use a personality flag instead of an mmap flag - Link to v2: https://lore.kernel.org/r/[email protected] --- Charlie Jenkins (2): mm: Add personality flag to limit address to 47 bits selftests/mm: Create ADDR_LIMIT_47BIT test include/uapi/linux/personality.h | 1 + mm/mmap.c | 3 ++ tools/testing/selftests/mm/.gitignore | 1 + tools/testing/selftests/mm/Makefile | 1 + tools/testing/selftests/mm/map_47bit_personality.c | 34 ++++++++++++++++++++++ 5 files changed, 40 insertions(+) --- base-commit: 5be63fc19fcaa4c236b307420483578a56986a37 change-id: 20240827-patches-below_hint_mmap-b13d79ae1c55 -- - Charlie
