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), 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. 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 <arnd@xxxxxxxx> To: Richard Henderson <richard.henderson@xxxxxxxxxx> To: Ivan Kokshaysky <ink@xxxxxxxxxxxxxxxxxxxx> To: Matt Turner <mattst88@xxxxxxxxx> To: Vineet Gupta <vgupta@xxxxxxxxxx> To: Russell King <linux@xxxxxxxxxxxxxxx> To: Guo Ren <guoren@xxxxxxxxxx> To: Huacai Chen <chenhuacai@xxxxxxxxxx> To: WANG Xuerui <kernel@xxxxxxxxxx> To: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx> To: James E.J. Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> To: Helge Deller <deller@xxxxxx> To: Michael Ellerman <mpe@xxxxxxxxxxxxxx> To: Nicholas Piggin <npiggin@xxxxxxxxx> To: Christophe Leroy <christophe.leroy@xxxxxxxxxx> To: Naveen N Rao <naveen@xxxxxxxxxx> To: Alexander Gordeev <agordeev@xxxxxxxxxxxxx> To: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx> To: Heiko Carstens <hca@xxxxxxxxxxxxx> To: Vasily Gorbik <gor@xxxxxxxxxxxxx> To: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx> To: Sven Schnelle <svens@xxxxxxxxxxxxx> To: Yoshinori Sato <ysato@xxxxxxxxxxxxxxxxxxxx> To: Rich Felker <dalias@xxxxxxxx> To: John Paul Adrian Glaubitz <glaubitz@xxxxxxxxxxxxxxxxxxx> To: David S. Miller <davem@xxxxxxxxxxxxx> To: Andreas Larsson <andreas@xxxxxxxxxxx> To: Thomas Gleixner <tglx@xxxxxxxxxxxxx> To: Ingo Molnar <mingo@xxxxxxxxxx> To: Borislav Petkov <bp@xxxxxxxxx> To: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> To: x86@xxxxxxxxxx To: H. Peter Anvin <hpa@xxxxxxxxx> To: Andy Lutomirski <luto@xxxxxxxxxx> To: Peter Zijlstra <peterz@xxxxxxxxxxxxx> To: Muchun Song <muchun.song@xxxxxxxxx> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> To: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> To: Vlastimil Babka <vbabka@xxxxxxx> To: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> To: Shuah Khan <shuah@xxxxxxxxxx> Cc: linux-arch@xxxxxxxxxxxxxxx Cc: linux-kernel@xxxxxxxxxxxxxxx Cc: linux-alpha@xxxxxxxxxxxxxxx Cc: linux-snps-arc@xxxxxxxxxxxxxxxxxxx Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx Cc: linux-csky@xxxxxxxxxxxxxxx Cc: loongarch@xxxxxxxxxxxxxxx Cc: linux-mips@xxxxxxxxxxxxxxx Cc: linux-parisc@xxxxxxxxxxxxxxx Cc: linuxppc-dev@xxxxxxxxxxxxxxxx Cc: linux-s390@xxxxxxxxxxxxxxx Cc: linux-sh@xxxxxxxxxxxxxxx Cc: sparclinux@xxxxxxxxxxxxxxx Cc: linux-mm@xxxxxxxxx Cc: linux-kselftest@xxxxxxxxxxxxxxx Signed-off-by: Charlie Jenkins <charlie@xxxxxxxxxxxx> 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/20240827-patches-below_hint_mmap-v1-0-46ff2eb9022d@xxxxxxxxxxxx --- Charlie Jenkins (4): mm: Add MAP_BELOW_HINT mm: Add hint and mmap_flags to struct vm_unmapped_area_info mm: Support MAP_BELOW_HINT in vm_unmapped_area() selftests/mm: Create MAP_BELOW_HINT test arch/alpha/kernel/osf_sys.c | 2 ++ arch/arc/mm/mmap.c | 3 +++ arch/arm/mm/mmap.c | 7 ++++++ arch/csky/abiv1/mmap.c | 3 +++ arch/loongarch/mm/mmap.c | 3 +++ arch/mips/mm/mmap.c | 3 +++ arch/parisc/kernel/sys_parisc.c | 3 +++ arch/powerpc/mm/book3s64/slice.c | 7 ++++++ arch/s390/mm/hugetlbpage.c | 4 ++++ arch/s390/mm/mmap.c | 6 ++++++ arch/sh/mm/mmap.c | 6 ++++++ arch/sparc/kernel/sys_sparc_32.c | 3 +++ arch/sparc/kernel/sys_sparc_64.c | 6 ++++++ arch/sparc/mm/hugetlbpage.c | 4 ++++ arch/x86/kernel/sys_x86_64.c | 6 ++++++ arch/x86/mm/hugetlbpage.c | 4 ++++ fs/hugetlbfs/inode.c | 4 ++++ include/linux/mm.h | 2 ++ include/uapi/asm-generic/mman-common.h | 1 + mm/mmap.c | 9 ++++++++ tools/include/uapi/asm-generic/mman-common.h | 1 + tools/testing/selftests/mm/Makefile | 1 + tools/testing/selftests/mm/map_below_hint.c | 32 ++++++++++++++++++++++++++++ 23 files changed, 120 insertions(+) --- base-commit: 5be63fc19fcaa4c236b307420483578a56986a37 change-id: 20240827-patches-below_hint_mmap-b13d79ae1c55 -- - Charlie
