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

[Vlastimil Babka]

Such a large recipient list and no linux-api. CC'd, please include it on
future postings.

On 8/29/24 09:15, 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].
> 
> 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.

I doubt it's desirable to have different behavior depending on architecture.
Also you could say it's a small class of applications that need more than 47
bits.

> 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