On Wed, Sep 11, 2024 at 07:25:08AM +0000, Arnd Bergmann wrote:
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
Yeah I guess it is incorrect documentation then? It seems more reasonable to me to have a hint address fall back onto the larger address space because otherwise the "hint" address can cause allocations to fail even if there is space above the 47-bit limit. This is another reason I wanted to avoid having this default behavior on riscv, to not have this abuse of the hint address.
- Charlie