Re: [PATCH RFC v3 0/2] mm: Introduce ADDR_LIMIT_47BIT personality flag

["Jiaxun Yang" <jiaxun.yang@flygoat.com>]

在2024年9月5日九月 下午10:15，Charlie Jenkins写道：
> 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. 

Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>

Tested on MIPS VA 48 system, fixed pointer tagging on mozjs!

Thanks!

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- Jiaxun