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

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Message-ID: <20201120151523.GA6861@willie-the-truck>
Date:   Fri, 20 Nov 2020 15:15:24 +0000
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        Catalin Marinas <catalin.marinas@....com>,
        Yu Zhao <yuzhao@...gle.com>, Minchan Kim <minchan@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 4/6] mm: proc: Invalidate TLB after clearing soft-dirty
 page state

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?

> 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.

Will

> 
> 
>