On Wed, Sep 11, 2024, at 00:45, Charlie Jenkins wrote:

On Tue, Sep 10, 2024 at 03:08:14PM -0400, Liam R. Howlett wrote:

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."

This is also in the commit message of b569bab78d8d ("x86/mm: Prepare
to expose larger address space to userspace"), which introduced it.
However, I don't actually see the fallback to the full address space,
instead the actual behavior seems to be the same as arm64.

Am I missing something in the x86 implementation, or do we just
need to update the documentation?

      Arnd