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

[Peter Zijlstra]

On Fri, Nov 20, 2020 at 03:15:24PM +0000, Will Deacon wrote:
> On Fri, Nov 20, 2020 at 04:00:23PM +0100, Peter Zijlstra wrote:
> > 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().
> 
> Ah, so you mean ditch the mmu_gather altogether?

Yes. Alternatively, if we decide mmu_gather is 'right', then we should
probably look at converting mprotect().

That is, I see no reason why this and mprotect should differ on this
point.

> > 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.
> 
> Hmm, I didn't spot that, although ptep_modify_prot_start() does actually
> clear the pte so we could just move this up a few lines.

Yes, but hash-entry != pte. If I'm not mistaken (and I could very well
be, it's Friday and Power-MMUs being the maze they are), the end result
here is an updated PTE but an empty hash-entry.