[PATCH RFC v2 0/4] mm: Introduce MAP_BELOW_HINT

[Charlie Jenkins <charlie@rivosinc.com>]

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@arndb.de>
To: Richard Henderson <richard.henderson@linaro.org>
To: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
To: Matt Turner <mattst88@gmail.com>
To: Vineet Gupta <vgupta@kernel.org>
To: Russell King <linux@armlinux.org.uk>
To: Guo Ren <guoren@kernel.org>
To: Huacai Chen <chenhuacai@kernel.org>
To: WANG Xuerui <kernel@xen0n.name>
To: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
To: James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
To: Helge Deller <deller@gmx.de>
To: Michael Ellerman <mpe@ellerman.id.au>
To: Nicholas Piggin <npiggin@gmail.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Naveen N Rao <naveen@kernel.org>
To: Alexander Gordeev <agordeev@linux.ibm.com>
To: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
To: Heiko Carstens <hca@linux.ibm.com>
To: Vasily Gorbik <gor@linux.ibm.com>
To: Christian Borntraeger <borntraeger@linux.ibm.com>
To: Sven Schnelle <svens@linux.ibm.com>
To: Yoshinori Sato <ysato@users.sourceforge.jp>
To: Rich Felker <dalias@libc.org>
To: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
To: David S. Miller <davem@davemloft.net>
To: Andreas Larsson <andreas@gaisler.com>
To: Thomas Gleixner <tglx@linutronix.de>
To: Ingo Molnar <mingo@redhat.com>
To: Borislav Petkov <bp@alien8.de>
To: Dave Hansen <dave.hansen@linux.intel.com>
To: x86@kernel.org
To: H. Peter Anvin <hpa@zytor.com>
To: Andy Lutomirski <luto@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
To: Muchun Song <muchun.song@linux.dev>
To: Andrew Morton <akpm@linux-foundation.org>
To: Liam R. Howlett <Liam.Howlett@oracle.com>
To: Vlastimil Babka <vbabka@suse.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
To: Shuah Khan <shuah@kernel.org>
Cc: linux-arch@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-alpha@vger.kernel.org
Cc: linux-snps-arc@lists.infradead.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-csky@vger.kernel.org
Cc: loongarch@lists.linux.dev
Cc: linux-mips@vger.kernel.org
Cc: linux-parisc@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-s390@vger.kernel.org
Cc: linux-sh@vger.kernel.org
Cc: sparclinux@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>

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@rivosinc.com

---
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