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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 22 Aug 2018 22:11:41 -0700
Message-ID: <CA+55aFzM77G9-Q6LboPLJ=5gHma66ZQKiMGCMqXoKABirdF98w@mail.gmail.com>
Subject: Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE
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To: Benjamin Herrenschmidt <benh@au1.ibm.com>
Cc: Nick Piggin <npiggin@gmail.com>, Peter Zijlstra <peterz@infradead.org>, Andrew Lutomirski <luto@kernel.org>, the arch/x86 maintainers <x86@kernel.org>, Borislav Petkov <bp@alien8.de>, Will Deacon <will.deacon@arm.com>, Rik van Riel <riel@surriel.com>, Jann Horn <jannh@google.com>, Adin Scannell <ascannell@google.com>, Dave Hansen <dave.hansen@intel.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, David Miller <davem@davemloft.net>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Michael Ellerman <mpe@ellerman.id.au>

On Wed, Aug 22, 2018 at 9:54 PM Benjamin Herrenschmidt <benh@au1.ibm.com> wrote:
>
>
> So we do need a different flush instruction for the page tables vs. the
> normal TLB pages.

Right. ARM wants it too. x86 is odd in that a regular "invlpg" already
invalidates all the internal tlb cache nodes.

So the "new world order" is exactly that patch that PeterZ sent you, that adds a

+       unsigned int            freed_tables : 1;

to the 'struct mmu_gather', and then makes all those
pte/pmd/pud/p4d_free_tlb() functions set that bit.

So I'm referring to the email PeterZ sent you in this thread that said:

  Nick, Will is already looking at using this to remove the synchronous
  invalidation from __p*_free_tlb() for ARM, could you have a look to see
  if PowerPC-radix could benefit from that too?

  Basically, using a patch like the below, would give your tlb_flush()
  information on if tables were removed or not.

then, in that model, you do *not* need to override these
pte/pmd/pud/p4d_free_tlb() macros at all (well, you *can* if you want
to, for doing games with the range modification, but let's sayt that
you don't need that right now).

So instead, when you get to the actual "tlb_flush(tlb)", you do
exactly that - flush the tlb. And the mmu_gather structure shows you
how much you need to flush. If you see that "freed_tables" is set,
then you know that you need to also do the special instruction to
flush the inner level caches. The range continues to show the page
range.

           Linus