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

[Isaac Manjarres <isaacmanjarres@google.com>]

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.
> 
> Here are some "new" things I've wanted to do with pagetables, which are not
> currently supported by any library:
> 
> - Have a second kernel pagetable (for ASI's "nonsensitive address space")
> 
> - Modify pagetables safely from a context where allocation is not possible
> 
> - Modify the kernel's pagetables while accounting pagetable allocations to the
>   current process
> 
> I think it's time to discuss if there's a way to scope out a "library" that:
> 
> a) Reduces the overall amount of code in the kernel, while
> 
> b) Serving the needs of the incoming guest_memfd and ASI features.
> 
> In this session I'd first like to do a quick survey of the pagetable
> manipulation systems already in the kernel (that I know about), what purposes
> they serve and what capabilities they have. Then I'd like to discuss some ideas
> for the scope of a new "library" and which of these components it might replace.
> 
> Mike Rapoport has shared a prototype that he wrote for a generic higher-level
> PGD abstraction, so I will be using that as inspiration.
> 
> This is mostly about looking for feedback and input from maintainers and
> experts: what opportunities for refactoring might I be missing? What challenges
> might I be forgetting about for sharing code?
> 
> [0] https://lpc.events/event/19/contributions/2029/
> [1] https://lore.kernel.org/all/20260126164445.11867-1-kalyazin@amazon.com/

Hello Brendan,

Thanks for sharing this! I this it's a great idea to introduce a library
like this for the kernel page tables. I'm interested in participating
in this discussion as well.

Thanks,
Isaac