Re: [PATCH 3/6] alpha: fix the local TLB invalidate in flush_tlb_page()

[Matt Turner]

On Sun, Aug 9, 2026 at 4:55 AM Magnus Lindholm <linmag7@xxxxxxxxx> wrote:
>
> flush_tlb_page() invalidates the calling CPU itself before asking the
> others, and gates that on current->active_mm. For a non-executable vma
> that means a targeted tbi(2, addr), which acts on the context currently
> loaded, so as in the IPI handler it reaches nothing when only active_mm
> names the mm, and nothing forces the old ASN to be retired afterwards.
>
> Test current->mm instead. When no thread of the mm is current, clear
> mm->context[cpu] so a fresh ASN is taken at the next switch. That also
> covers the case where the mm is not this CPU's active_mm at all: the CPU
> may still hold translations for it, and smp_call_function() does not call
> back into the caller.
>
> Reached in practice by folio_mkclean() from the writeback flusher
> kworker, which has no mm of its own: about half the calls during
> writeback of a shared mapping, and none at all on anonymous memory.
>
> flush_tlb_mm() does not need the same active_mm to current->mm change,
> because its active_mm path loads a new context rather than issuing a
> targeted tbi(). It does have the separate caller-CPU omission when the
> target mm is not active_mm; that is fixed in the following patch.
>
> Signed-off-by: Magnus Lindholm <linmag7@xxxxxxxxx>
> ---
>  arch/alpha/kernel/smp.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
> index 0dfe29b59039..d167b3ba1303 100644
> --- a/arch/alpha/kernel/smp.c
> +++ b/arch/alpha/kernel/smp.c
> @@ -696,7 +696,15 @@ flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
>
>         preempt_disable();
>
> -       if (mm == current->active_mm) {
> +       /*
> +        * As in ipi_flush_tlb_page(): the targeted tbi() reaches MM's
> +        * translations only when a thread of MM is current, so test
> +        * current->mm.  Otherwise - lazily borrowing MM, or not running it
> +        * at all - clear mm->context[cpu] so a fresh ASN is taken at the
> +        * next switch.  smp_call_function() below does not call back into
> +        * this CPU, so this is the only chance to retire what it holds.
> +        */
> +       if (mm == current->mm) {
>                 flush_tlb_current_page(mm, vma, addr);
>                 if (atomic_read(&mm->mm_users) <= 1) {
>                         int cpu, this_cpu = smp_processor_id();
> @@ -709,6 +717,8 @@ flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
>                         preempt_enable();
>                         return;
>                 }
> +       } else {
> +               flush_tlb_other(mm);
>         }
>
>         data.vma = vma;
> --
> 2.53.0
>

I think we could split this patch, like how patches 2 (tests change)
and 5 (missing else) are split.

No strong preference.