Re: [RFC PATCH v1 1/6] fs/proc/task_mmu: Fix pte update and tlb maintenance ordering in pagemap_scan_pmd_entry()

[Jann Horn <jannh@xxxxxxxxxx>]

On Fri, May 30, 2025 at 4:04 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
> pagemap_scan_pmd_entry() was previously modifying ptes while in lazy mmu
> mode, then performing tlb maintenance for the modified ptes, then
> leaving lazy mmu mode. But any pte modifications during lazy mmu mode
> may be deferred until arch_leave_lazy_mmu_mode(), inverting the required
> ordering between pte modificaiton and tlb maintenance.
>
> Let's fix that by leaving mmu mode, forcing all the pte updates to be
> actioned, before doing the tlb maintenance.
>
> This is a theorectical bug discovered during code review.
>
> Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally 
> clear info about PTEs")

Hmm... isn't lazy mmu mode supposed to also delay TLB flushes, and
preserve the ordering of PTE modifications and TLB flushes?

Looking at the existing implementations of lazy MMU:

 - In Xen PV implementation of lazy MMU, I see that TLB flush
hypercalls are delayed as well (xen_flush_tlb(),
xen_flush_tlb_one_user() and xen_flush_tlb_multi() all use
xen_mc_issue(XEN_LAZY_MMU) which delays issuing if lazymmu is active).
 - The sparc version also seems to delay TLB flushes, and sparc's
arch_leave_lazy_mmu_mode() seems to do TLB flushes via
flush_tlb_pending() if necessary.
 - powerpc's arch_leave_lazy_mmu_mode() also seems to do TLB flushes.

Am I missing something?

If arm64 requires different semantics compared to all existing
implementations and doesn't delay TLB flushes for lazy mmu mode, I
think the "Fixes" tag should point to your addition of lazy mmu
support for arm64.

> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
>  fs/proc/task_mmu.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 994cde10e3f4..361f3ffd9a0c 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -2557,10 +2557,9 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned 
> long start,
>         }
>
>  flush_and_return:
> +       arch_leave_lazy_mmu_mode();
>         if (flush_end)
>                 flush_tlb_range(vma, start, addr);
> -
> -       arch_leave_lazy_mmu_mode();

I think this ordering was probably intentional, because doing it this
way around allows Xen PV to avoid one more hypercall, because the TLB
flush can be batched together with the page table changes?


>         pte_unmap_unlock(start_pte, ptl);
>
>         cond_resched();
> --
> 2.43.0
>