Re: [PATCH 4/6] mm: proc: Invalidate TLB after clearing soft-dirty page state

[Peter Zijlstra]

On Fri, Nov 20, 2020 at 02:35:55PM +0000, Will Deacon wrote:
> Since commit 0758cd830494 ("asm-generic/tlb: avoid potential double flush"),
> TLB invalidation is elided in tlb_finish_mmu() if no entries were batched
> via the tlb_remove_*() functions. Consequently, the page-table modifications
> performed by clear_refs_write() in response to a write to
> /proc/<pid>/clear_refs do not perform TLB invalidation. Although this is
> fine when simply aging the ptes, in the case of clearing the "soft-dirty"
> state we can end up with entries where pte_write() is false, yet a
> writable mapping remains in the TLB.
> 
> Fix this by calling tlb_remove_tlb_entry() for each entry being
> write-protected when cleating soft-dirty.
> 

> @@ -1053,6 +1054,7 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma,
>  		ptent = pte_wrprotect(old_pte);
>  		ptent = pte_clear_soft_dirty(ptent);
>  		ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent);
> +		tlb_remove_tlb_entry(tlb, pte, addr);
>  	} else if (is_swap_pte(ptent)) {
>  		ptent = pte_swp_clear_soft_dirty(ptent);
>  		set_pte_at(vma->vm_mm, addr, pte, ptent);

Oh!

Yesterday when you had me look at this code; I figured the sane thing
to do was to make it look more like mprotect().

Why did you chose to make it work with mmu_gather instead? I'll grant
you that it's probably the smaller patch, but I still think it's weird
to use mmu_gather here.

Also, is tlb_remote_tlb_entry() actually correct? If you look at
__tlb_remove_tlb_entry() you'll find that Power-Hash-32 will clear the
entry, which might not be what we want here, we want to update the
entrty.