Subject: Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits

From: Charlie Jenkins <[email-masked]>
Date: Tue, 10 Sep 2024 17:45:07 -0700

On Tue, Sep 10, 2024 at 03:08:14PM -0400, Liam R. Howlett wrote:
> * Catalin Marinas <[email-masked]> [240906 07:44]:
> > On Fri, Sep 06, 2024 at 09:55:42AM +0000, Arnd Bergmann wrote:
> > > On Fri, Sep 6, 2024, at 09:14, Guo Ren wrote:
> > > > On Fri, Sep 6, 2024 at 3:18 PM Arnd Bergmann <[email-masked]> wrote:
> > > >> It's also unclear to me how we want this flag to interact with
> > > >> the existing logic in arch_get_mmap_end(), which attempts to
> > > >> limit the default mapping to a 47-bit address space already.
> > > >
> > > > To optimize RISC-V progress, I recommend:
> > > >
> > > > Step 1: Approve the patch.
> > > > Step 2: Update Go and OpenJDK's RISC-V backend to utilize it.
> > > > Step 3: Wait approximately several iterations for Go & OpenJDK
> > > > Step 4: Remove the 47-bit constraint in arch_get_mmap_end()
> > > 
> > > I really want to first see a plausible explanation about why
> > > RISC-V can't just implement this using a 47-bit DEFAULT_MAP_WINDOW
> > > like all the other major architectures (x86, arm64, powerpc64),
> > 
> > FWIW arm64 actually limits DEFAULT_MAP_WINDOW to 48-bit in the default
> > configuration. We end up with a 47-bit with 16K pages but for a
> > different reason that has to do with LPA2 support (I doubt we need this
> > for the user mapping but we need to untangle some of the macros there;
> > that's for a separate discussion).
> > 
> > That said, we haven't encountered any user space problems with a 48-bit
> > DEFAULT_MAP_WINDOW. So I also think RISC-V should follow a similar
> > approach (47 or 48 bit default limit). Better to have some ABI
> > consistency between architectures. One can still ask for addresses above
> > this default limit via mmap().
> 
> I think that is best as well.
> 
> Can we please just do what x86 and arm64 does?
> 
> Thanks,
> Liam

I responded to Arnd in the other thread, but I am still not convinced
that the solution that x86 and arm64 have selected is the best solution.
The solution of defaulting to 47 bits does allow applications the
ability to get addresses that are below 47 bits. However, due to
differences across architectures it doesn't seem possible to have all
architectures default to the same value. Additionally, this flag will be
able to help users avoid potential bugs where a hint address is passed
that causes upper bits of a VA to be used.

The other issue I have with this is that if there is not a hint address
specified to be greater than 47 bits on x86, then mmap() may return an
address that is greater than 47-bits. The documentation in
Documentation/arch/x86/x86_64/5level-paging.rst says:

"If hint address set above 47-bit, but MAP_FIXED is not specified, we try
to look for unmapped area by specified address. If it's already
occupied, we look for unmapped area in *full* address space, rather than
from 47-bit window."

arm64 on the other hand defines this as only being able to opt-into the
52-bit VA space with the hint address, and my understanding is that
mmap() will not fall back to the 52-bit address space. Please correct me
if I am wrong. From Documentation/arch/arm64/memory.rst:

"To maintain compatibility with software that relies on the ARMv8.0
VA space maximum size of 48-bits, the kernel will, by default,
return virtual addresses to userspace from a 48-bit range.

"Software can "opt-in" to receiving VAs from a 52-bit space by
specifying an mmap hint parameter that is larger than 48-bit."

This is an inconsistency I am trying to solve with this personality
flag.

- Charlie