Re: [PATCH 0/4] arch, mm: improve robustness of direct map manipulation

On 25.10.20 11:15, Mike Rapoport wrote:

From: Mike Rapoport <redacted>

Hi,

During recent discussion about KVM protected memory, David raised a concern
about usage of __kernel_map_pages() outside of DEBUG_PAGEALLOC scope [1].

Indeed, for architectures that define CONFIG_ARCH_HAS_SET_DIRECT_MAP it is
possible that __kernel_map_pages() would fail, but since this function is
void, the failure will go unnoticed.

Moreover, there's lack of consistency of __kernel_map_pages() semantics
across architectures as some guard this function with
#ifdef DEBUG_PAGEALLOC, some refuse to update the direct map if page
allocation debugging is disabled at run time and some allow modifying the
direct map regardless of DEBUG_PAGEALLOC settings.

This set straightens this out by restoring dependency of
__kernel_map_pages() on DEBUG_PAGEALLOC and updating the call sites
accordingly.

So, I was primarily wondering if we really have to touch direct mappings 
in hibernation code, or if we can avoid doing that. I was wondering if 
we cannot simply do something like kmap() when trying to access a 
!mapped page. Similar to reading old-os memory after kexec when in 
kdump. Just a thought.

-- 
Thanks,

David / dhildenb