Re: [PATCH 00/16] mm: Introduce MAP_BELOW_HINT

Dave Hansen
2024-08-29T16:54:26Z

On 8/28/24 13:15, Charlie Jenkins wrote:
> A way to restrict mmap() to return LAM compliant addresses in an entire
> address space also doesn't have to be mutually exclusive with this flag.
> This flag allows for the greatest degree of control from applications.
> I don't believe there is additionally performance saving that could be
> achieved by having this be on a per address space basis.

I agree with you in general. The MAP_BELOW_HINT _is_ the most flexible.
But it's also rather complicated.

My _hope_ would be that a per-address-space property could share at
least some infrastructure with what x86/LAM and arm/TBI do to the
address space. Basically put the restrictions in place for purely
software reasons instead of the mostly hardware reasons for LAM/TBI.

Lorenzo also raised some very valid points about a having a generic
address-restriction ABI. I'm certainly not discounting those concerns.
It's not something that can be done lightly.