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

From: Charlie Jenkins <[email-masked]>
Date: Thu, 29 Aug 2024 10:33:22 -0700

On Thu, Aug 29, 2024 at 10:30:56AM +0200, Michal Hocko wrote:
> On Thu 29-08-24 00:15:57, 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.
> 
> IIRC this has been discussed at length when 5-level page tables support
> has been proposed for x86. Sorry I do not have a link handy but lore
> should help you. Linus was not really convinced and in the end vetoed it
> and prefer that those few applications that benefit from greater address
> space would do that explicitly than other way around.

I believe I found the conversation you were referring to. Ingo Molnar
recommended a flag similar to what I have proposed [1]. Catalin
recommended to make 52-bit opt-in on arm64 [2]. Dave Hansen brought up
MPX [3].

However these conversations are tangential to what I am proposing. arm64
and x86 decided to have the default address space be 48 bits. However
this was done on a per-architecture basis with no way for applications
to have guarantees between architectures. Even this behavior to restrict
to 48 bits does not even appear in the man pages, so would require
reading the kernel source code to understand that this feature is
available. Then to opt-in to larger address spaces, applications have to
know to provide a hint address that is greater than 47 bits, mmap() will
then return an address that contains up to 56 bits on x86 and 52 bits on
arm64. This difference of 4 bits causes inconsistency and is part of the
problem I am trying to solve with this flag.

I am not proposing to change x86 and arm64 away from using their opt-out
feature, I am instead proposing a standard ABI for applications that
need some guarantees of the bits used in pointers.

- Charlie

Link: https://lore.kernel.org/lkml/[email-masked]/ [1]
Link: https://lore.kernel.org/lkml/[email-masked]/
[2]
Link:
https://lore.kernel.org/lkml/[email-masked]/
[3]

> -- 
> Michal Hocko
> SUSE Labs