Re: [PATCH] arm64: mm: drop tlb flush operation when clearing the access bit

[Will Deacon <will@kernel.org>]

On Wed, Oct 25, 2023 at 09:39:19AM +0800, Yin, Fengwei wrote:
> 
> >> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> >> index 0bd18de9fd97..2979d796ba9d 100644
> >> --- a/arch/arm64/include/asm/pgtable.h
> >> +++ b/arch/arm64/include/asm/pgtable.h
> >> @@ -905,21 +905,22 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
> >>  static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
> >>                                          unsigned long address, pte_t *ptep)
> >>  {
> >> -       int young = ptep_test_and_clear_young(vma, address, ptep);
> >> -
> >> -       if (young) {
> >> -               /*
> >> -                * We can elide the trailing DSB here since the worst that can
> >> -                * happen is that a CPU continues to use the young entry in its
> >> -                * TLB and we mistakenly reclaim the associated page. The
> >> -                * window for such an event is bounded by the next
> >> -                * context-switch, which provides a DSB to complete the TLB
> >> -                * invalidation.
> >> -                */
> >> -               flush_tlb_page_nosync(vma, address);
> >> -       }
> >> -
> >> -       return young;
> >> +       /*
> >> +        * This comment is borrowed from x86, but applies equally to ARM64:
> >> +        *
> >> +        * Clearing the accessed bit without a TLB flush doesn't cause
> >> +        * data corruption. [ It could cause incorrect page aging and
> >> +        * the (mistaken) reclaim of hot pages, but the chance of that
> >> +        * should be relatively low. ]
> >> +        *
> >> +        * So as a performance optimization don't flush the TLB when
> >> +        * clearing the accessed bit, it will eventually be flushed by
> >> +        * a context switch or a VM operation anyway. [ In the rare
> >> +        * event of it not getting flushed for a long time the delay
> >> +        * shouldn't really matter because there's no real memory
> >> +        * pressure for swapout to react to. ]
> >> +        */
> >> +       return ptep_test_and_clear_young(vma, address, ptep);
> >>  }
> From https://lore.kernel.org/lkml/20181029105515.GD14127@arm.com/:
> 
> This is blindly copied from x86 and isn't true for us: we don't invalidate
> the TLB on context switch. That means our window for keeping the stale
> entries around is potentially much bigger and might not be a great idea.

I completely agree.

> My understanding is that arm64 doesn't do invalidate the TLB during
> context switch. The flush_tlb_page_nosync() here + DSB during context
> switch make sure the TLB is invalidated during context switch.
> So we can't remove flush_tlb_page_nosync() here? Or something was changed
> for arm64 (I have zero knowledge to TLB on arm64. So some obvious thing
> may be missed)? Thanks.

As you point out, we already elide the DSB here but I don't think we should
remove the TLB invalidation entirely because then we lose the guarantee
that the update ever becomes visible to the page-table walker.

I'm surprised that the TLBI is showing up as a performance issue without
the DSB present. Is it because we're walking over a large VA range and
invalidating on a per-page basis? If so, we'd be better off batching them
up and doing the invalidation at the end (which will be upgraded to a
full-mm invalidation if the range is large enough).

Will