Re: [PATCH v1] riscv: mm: Define PAGE_KERNEL_RO

[Paul Walmsley <pjw@kernel.org>]

On Sat, 17 Oct 2026, Nickolai Zeldovich wrote:

> Kernel mappings that generic code creates read-only are writable on
> riscv. With HugeTLB vmemmap optimization (selected by riscv and on by
> default with CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON), the tail
> vmemmap pages of every optimized huge page are remapped to one shared
> page with PAGE_KERNEL_RO so that a stray write to a tail struct page
> faults. On riscv that remap keeps _PAGE_WRITE, so such a write silently
> changes the struct pages of every optimized huge page instead.
> early_memremap_ro() (EFI system and configuration tables), the
> firmware loader's vmap() of the loaded image, and acpi_os_ioremap() for
> EFI runtime code and NOMAP regions, which asks for a read-only mapping
> precisely to avoid a writable mapping of executable code, are affected
> the same way.
> 
> riscv defines PAGE_KERNEL_READ but not PAGE_KERNEL_RO, so
> include/linux/pgtable.h falls back to "#define PAGE_KERNEL_RO
> PAGE_KERNEL". Define PAGE_KERNEL_RO as PAGE_KERNEL_READ. FIXMAP_PAGE_RO
> then follows from asm-generic/fixmap.h: early_memremap_ro() now passes
> 0xe3 instead of 0xe7 to __early_ioremap().
> 
> Tested on QEMU virt (riscv64, defconfig with
> CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON and CONFIG_PTDUMP_DEBUGFS,
> booted with hugepages=8): before, every vmemmap entry in
> /sys/kernel/debug/kernel_page_tables has W set; after, the 28K of tail
> vmemmap of each huge page is mapped R without W.

Thanks, queued for v7.4-rc.


- Paul