Re: [PATCH -v2 2/2] arm64, tlbflush: don't TLBI broadcast if page reused in write fault

[Barry Song <21cnbao@gmail.com>]

On Wed, Oct 22, 2025 at 10:02 PM Huang, Ying
<ying.huang@linux.alibaba.com> wrote:
>
> Barry Song <21cnbao@gmail.com> writes:
>
> >> >
> >> > static inline void __flush_tlb_page_nosync(struct mm_struct *mm,
> >> >                                            unsigned long uaddr)
> >> > {
> >> >         unsigned long addr;
> >> >
> >> >         dsb(ishst);
> >> >         addr = __TLBI_VADDR(uaddr, ASID(mm));
> >> >         __tlbi(vale1is, addr);
> >> >         __tlbi_user(vale1is, addr);
> >> >         mmu_notifier_arch_invalidate_secondary_tlbs(mm, uaddr & PAGE_MASK,
> >> >                                                 (uaddr & PAGE_MASK) +
> >> > PAGE_SIZE);
> >> > }
> >>
> >> IIUC, _nosync() here means doesn't synchronize with the following code.
> >> It still synchronizes with the previous code, mainly the page table
> >> changing.  And, Yes.  There may be room to improve this.
> >>
> >> > On the other hand, __ptep_set_access_flags() doesn’t seem to use
> >> > set_ptes(), so there’s no guarantee the updated PTEs are visible to all
> >> > cores. If a remote CPU later encounters a page fault and performs a TLB
> >> > invalidation, will it still see a stable PTE?
> >>
> >> I don't think so.  We just flush local TLB in local_flush_tlb_page()
> >> family functions.  So, we only needs to guarantee the page table changes
> >> are available for the local page table walking.  If a page fault occurs
> >> on a remote CPU, we will call local_flush_tlb_page() on the remote CPU.
> >>
> >
> > My concern is that:
> >
> > We don’t have a dsb(ish) to ensure the PTE page table is visible to remote
> > CPUs, since you’re using dsb(nsh). So even if a remote CPU performs
> > local_flush_tlb_page(), the memory may not be synchronized yet, and it could
> > still see the old PTE.
>
> So, do you think that after the load/store unit of the remote CPU have
> seen the new PTE, the page table walker could still see the old PTE?  I

Without a barrier in the ish domain, remote CPUs’ load/store units may not
see the new PTE written by the first CPU performing the reuse.

That’s why we need a barrier in the ish domain to ensure the PTE is
actually visible across the SMP domain. A store instruction doesn’t guarantee
that the data is immediately visible to other CPUs — at least not for load
instructions.

Though, I’m not entirely sure about the page table walker case.

> doubt it.  Even if so, the worse case is one extra spurious page fault?
> If the possibility of the worst case is low enough, that should be OK.

CPU0:                    CPU1:

write pte;

do local tlbi;

                                       page fault;
                                       do local tlbi; -> still old PTE

pte visible to CPU1

>
> Additionally, the page table lock is held when writing PTE on this CPU
> and re-reading PTE on the remote CPU.  That provides some memory order
> guarantee too.

Right, the PTL might take care of it automatically.

Thanks
Barry