[RFC PATCH v1 0/6] Lazy mmu mode fixes and improvements

[Lorenzo Stoakes]

+cc Jann who is a specialist in all things page table-y and especially scary
edge cases :)

On Fri, May 30, 2025 at 03:04:38PM +0100, Ryan Roberts wrote:
> Hi All,
>
> I recently added support for lazy mmu mode on arm64. The series is now in
> Linus's tree so should be in v6.16-rc1. But during testing in linux-next we
> found some ugly corners (unexpected nesting). I was able to fix those issues by
> making the arm64 implementation more permissive (like the other arches). But
> this is quite fragile IMHO. So I'd rather fix the root cause and ensure that
> lazy mmu mode never nests, and more importantly, that code never makes pgtable
> modifications expecting them to be immediate, not knowing that it's actually in
> lazy mmu mode so the changes get deferred.

When you say fragile, are you confident it _works_ but perhaps not quite as well
as you want? Or are you concerned this might be broken upstream in any way?

I am thinking specifically about the proposed use in Dev's new series [0] and
obviously hoping (and assuming in fact) that it's the former :)

[0]: https://lore.kernel.org/linux-mm/20250530090407.19237-1-dev.jain@arm.com/

>
> The first 2 patches are unrelated, very obvious bug fixes. They don't affect
> arm64 because arm64 only uses lazy mmu for kernel mappings. But I noticed them
> during code review and think they should be fixed.
>
> The next 3 patches are aimed at solving the nesting issue.
>
> And the final patch is reverting the "permissive" fix I did for arm64, which is
> no longer needed after the previous 3 patches.
>
> I've labelled this RFC for now because it depends on the arm64 lazy mmu patches
> in Linus's master, so it won't apply to mm-unstable. But I'm keen to get review
> and siince I'm touching various arches and modifying some core mm stuff, I
> thought that might take a while so thought I'd beat the rush and get a first
> version out early.
>
> I've build-tested all the affected arches. And I've run mm selftests for the
> arm64 build, with no issues (with DEBUG_PAGEALLOC and KFENCE enabled).
>
> Applies against Linus's master branch (f66bc387efbe).
>
> Thanks,
> Ryan
>
>
> Ryan Roberts (6):
>   fs/proc/task_mmu: Fix pte update and tlb maintenance ordering in
>     pagemap_scan_pmd_entry()
>   mm: Fix pte update and tlb maintenance ordering in
>     migrate_vma_collect_pmd()
>   mm: Avoid calling page allocator from apply_to_page_range()
>   mm: Introduce arch_in_lazy_mmu_mode()
>   mm: Avoid calling page allocator while in lazy mmu mode
>   Revert "arm64/mm: Permit lazy_mmu_mode to be nested"
>
>  arch/arm64/include/asm/pgtable.h              | 22 ++++----
>  .../include/asm/book3s/64/tlbflush-hash.h     | 15 ++++++
>  arch/sparc/include/asm/tlbflush_64.h          |  1 +
>  arch/sparc/mm/tlb.c                           | 12 +++++
>  arch/x86/include/asm/paravirt.h               |  5 ++
>  arch/x86/include/asm/paravirt_types.h         |  1 +
>  arch/x86/kernel/paravirt.c                    |  6 +++
>  arch/x86/xen/mmu_pv.c                         |  6 +++
>  fs/proc/task_mmu.c                            |  3 +-
>  include/asm-generic/tlb.h                     |  2 +
>  include/linux/mm.h                            |  6 +++
>  include/linux/pgtable.h                       |  1 +
>  kernel/bpf/arena.c                            |  6 +--
>  mm/kasan/shadow.c                             |  2 +-
>  mm/memory.c                                   | 54 ++++++++++++++-----
>  mm/migrate_device.c                           |  3 +-
>  mm/mmu_gather.c                               | 15 ++++++
>  17 files changed, 128 insertions(+), 32 deletions(-)
>
> --
> 2.43.0
>