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

[Brendan Jackman <jackmanb@google.com>]

On Mon Feb 23, 2026 at 11:28 AM UTC, Mike Rapoport wrote:
> 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.

That makes sense. 

I've also just posted an RFC that does more awkward ad-hoc manipulation: 

https://lore.kernel.org/all/20260225-page_alloc-unmapped-v1-4-e8808a03cd66@google.com/

This might help illustrate the kinda thing that we could benefit from
with a more general library, besides just deduplicating code.