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

[Kirill A. Shutemov]

On Thu, Aug 29, 2024 at 12:15:57AM -0700, 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.

This argument looks broken to me.

The "small class of applications" is going to be broken unless they got
patched to use your new mmap() flag. You are asking for bugs.

Consider the case when you write, compile and validate a piece of software
on machine that has <=47bit VA. The binary got shipped to customers.
Later, customer gets a new shiny machine that supports larger address
space and your previously working software is broken. Such binaries might
exist today.

It is bad idea to use >47bit VA by default. Most of software got tested on
x86 with 47bit VA.

We can consider more options to opt-in into wider address space like
personality or prctl() handle. But opt-out is no-go from what I see.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov