Subject: Re: [PATCH v2 1/3] arm64: mm: Fix rodata=full block mapping support for realm guests

From: Catalin Marinas <[email-masked]>
Date: Thu, 9 Apr 2026 16:20:01 +0100

On Thu, Apr 09, 2026 at 11:53:41AM +0200, Kevin Brodsky wrote:
> On 07/04/2026 12:52, Catalin Marinas wrote:
> >> if we have forced pte mapping then the value of
> >> can_set_direct_map() is irrelevant - we will never need to split because we are
> >> already pte-mapped.
> >
> > can_set_direct_map() is used in other places, so its value is
> > relevant, e.g. sys_memfd_secret() is rejected if this function returns
> > false.
> 
> Indeed, I have noticed this before: currently set_direct_map_*_noflush()
> and other functions will either fail or do nothing if none of the
> features (rodata=full, etc.) is enabled, even if we would be able to
> split the linear map using BBML2-noabort.

That's what I have been trying to say to Ryan ;), can_set_direct_map()
has different meanings depending on the caller: hint that it might split
or asking whether splitting is permitted. The latter is not captured.
Ignoring realms, if we have BBML2_NOABORT the kernel won't force pte
mappings under the assumption that split_kernel_leaf_mapping() is safe.
However set_direct_map_*_noflush() won't even reach the split function
because the "can" part says "no, you can't".

> What would make more sense to me is to enable the use of BBML2-noabort
> unconditionally if !force_pte_mapping(). We can then have
> can_set_direct_map() return true if we have BBML2-noabort, and we no
> longer need to check it in map_mem().

Indeed.

> This is a functional change that doesn't have anything to do with realms
> so it should probably be a separate series - happy to take care of it
> once the dust settles on the realm handling.

I think it can be done in parallel, it shouldn't interfere with realms.
The realm part should just affect force_pte_mapping() and
can_set_direct_map() should return just what's possible, not what may
need to set the direct map.

-- 
Catalin