Re: [LSF/MM/BPF TOPIC] A pagetable library for the kernel?

[Mike Rapoport <rppt@kernel.org>]

On Thu, Feb 19, 2026 at 05:51:09PM +0000, Brendan Jackman wrote:
> As work on Address Space Isolation [0] trudges slowly along (next series coming
> soon™... I promise... some details of the plan are in [0]) I've been running
> into a common issue whenever I try to do new stuff with the kernel address
> space: We have too many sets of pagetable manipulation routines, and yet we
> don't have one that suits ASI's needs.
> 
> Similarly, I'm currently working on support for efficiently unmapping
> guest_memfd pages from the physmap (an extension to [1]) - in this case I've run
> into very much the same issues as with ASI.
> 
> Here are some areas of the kernel that manipulate pagetables:
> 
> 1. The collection of APIs that are specific to userspace pagetables: mmu_gather,
>    mm/pagewalk.c, some vm_fault logic, all that good stuff.
> 
> 2. The set_memory_* and set_direct_map_* APIs. (Which are implemented per-arch).
> 
> 3. Some non-userspace-specific APIs in mm/memory.c, such as
>    apply_to_page_range().
> 
> 4. mm/vmalloc.c
> 
> 5. Highmem logic such as kmap_local_*
> 
> 6. Boot and memory-hotplug support code (your architecture's version of
>    arch/x86/mm/init_64.c).
> 
> 7. x86's KPTI
> 
> 8. x86's LDT logic
> 
> (At LPC I started enumerating these off the top of my head and multiple people
> spoke out with more examples I hadn't thought of - please join in if you can see
> more!)
> 
> By and large, these components are designed completely independently from one
> another. This is made possible by the smart design of the low-level helper API
> (pte_present() and friends), and it does lead to nice explicit coding style.

By and large, lots of functionality that deals with kernel page tables was
added ad-hoc, like e.g. adopting set_memory() designed for DEBUG_PAGE_ALLOC
for protecting kernel and modules code.

I think it's a good idea to have a generic abstraction that can deal with
kernel page tables, like the one we have for the userspace page tables.

-- 
Sincerely yours,
Mike.