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

From: Charlie Jenkins <[email-masked]>
Date: Thu, 29 Aug 2024 18:00:25 -0700

On Thu, Aug 29, 2024 at 09:54:08AM -0700, Dave Hansen wrote:
> 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.

Can you expand upon what you mean by it being complicated? Complicated
for the kernel or complicated for a user?

> 
> 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.

That is a good point, perhaps that would be a way to hook this into LAM,
TBI, and any other architecture's specific address masking feature.

- Charlie

> 
> 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.