On Tue, 27 Jan 2026 at 11:21, Ryan Roberts <[email protected]> wrote: > > On 26/01/2026 09:26, Ard Biesheuvel wrote: > > From: Ard Biesheuvel <[email protected]> > > > > Now that the DRAM mapping routines respect existing table mappings and > > contiguous block and page mappings, it is no longer needed to fiddle > > with the memblock tables to set and clear the NOMAP attribute. Instead, > > map the kernel text and rodata alias first, avoiding contiguous > > mappings, so that they will not be added later when mapping the > > memblocks. > > Should we do something similar for kfence? Currently we have > arm64_kfence_alloc_pool() which marks some memory NOMAP then > arm64_kfence_map_pool() which PTE-maps it and clears NOMAP. Presumably we > could > rationalize into a single function that does it all, prior to mapping the bulk > of the linear map? >
Yeah, good point - I did not spot that but I will address it in the next revision. > > > > Signed-off-by: Ard Biesheuvel <[email protected]> > > --- > > arch/arm64/mm/mmu.c | 27 ++++++++------------ > > 1 file changed, 10 insertions(+), 17 deletions(-) > > > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > > index 80587cd47ce7..18415d4743bf 100644 > > --- a/arch/arm64/mm/mmu.c > > +++ b/arch/arm64/mm/mmu.c > > @@ -1149,12 +1149,17 @@ static void __init map_mem(void) > > flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS; > > > > /* > > - * Take care not to create a writable alias for the > > - * read-only text and rodata sections of the kernel image. > > - * So temporarily mark them as NOMAP to skip mappings in > > - * the following for-loop > > + * Map the linear alias of the [_text, __init_begin) interval > > + * as non-executable now, and remove the write permission in > > + * mark_linear_text_alias_ro() above (which will be called after > > + * alternative patching has completed). This makes the contents > > + * of the region accessible to subsystems such as hibernate, > > + * but protects it from inadvertent modification or execution. > > + * Note that contiguous mappings cannot be remapped in this way, > > + * so we should avoid them here. > > */ > > - memblock_mark_nomap(kernel_start, kernel_end - kernel_start); > > + __map_memblock(kernel_start, kernel_end, PAGE_KERNEL, > > + flags | NO_CONT_MAPPINGS); > > So the reason to disallow cont mappings is because we need to modify the > permissions later? It _is_ safe to change permissions on a live contiguous > mapping in this way. That was clarified in the architecture a couple of years > back and we rely on it for contpte_wrprotect_ptes(); see comment there. > OK, good to know - I was hoping to get your take on this ... > I think we could relax this? > OK, I suppose that means that we can drop the NO_CONT_MAPPINGS here, but we still need to map the kernel text/rodata alias initially to ensure that no block mappings are created that would need to broken down, right?
