linux-kernel - Re: [RFC PATCH 2/2] zap_pte_range: fix partial TLB flushing in response to a dirty pte

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Date:	Tue, 28 Oct 2014 08:18:39 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH 2/2] zap_pte_range: fix partial TLB flushing in
 response to a dirty pte

On Tue, Oct 28, 2014 at 4:44 AM, Will Deacon <will.deacon@....com> wrote:
> @@ -1194,11 +1194,10 @@ again:
>                  * then update the range to be the remaining
>                  * TLB range.
>                  */
> -               old_end = tlb->end;
> -               tlb->end = addr;
> +               tlb->end = old_end = min(tlb->end, addr);
>                 tlb_flush_mmu_tlbonly(tlb);
> -               tlb->start = addr;
> -               tlb->end = old_end;
> +               tlb->start = old_end;
> +               tlb->end = end;

I don't think this is right. Setting "tlb->end = end" looks very wrong
indeed, because "end" here inside zap_pte_range() is *not* the final
end of the zap range, it is just the end of the current set of pte's.

There's a reason the old code *saved* the old end value. You've now
ripped that out, and use the "old_end" for something else entirely.

Your arm64 version of tlb_add_flush() then hides the bug you just
introduced by updating the end range for each page you encounter. But
quite frankly, I think your problems are all fundamental to that very
issue. You're playing games with start/end during the TLB flush
itself, which is not how those things were designed to work.

So now you break everything that *doesn't* do your arm games.

                   Linus
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