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Date: Fri, 24 Aug 2018 20:04:38 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for
 RCU_TABLE_FREE
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To: Nadav Amit <nadav.amit@gmail.com>
Cc: Will Deacon <will.deacon@arm.com>, Linus Torvalds <torvalds@linux-foundation.org>, Benjamin Herrenschmidt <benh@au1.ibm.com>, Nick Piggin <npiggin@gmail.com>, Andrew Lutomirski <luto@kernel.org>, the arch/x86 maintainers <x86@kernel.org>, Borislav Petkov <bp@alien8.de>, 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 Fri, Aug 24, 2018 at 10:26:50AM -0700, Nadav Amit wrote:
> at 1:47 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > On Thu, Aug 23, 2018 at 02:39:59PM +0100, Will Deacon wrote:
> >> The only problem with this approach is that we've lost track of the granule
> >> size by the point we get to the tlb_flush(), so we can't adjust the stride of
> >> the TLB invalidations for huge mappings, which actually works nicely in the
> >> synchronous case (e.g. we perform a single invalidation for a 2MB mapping,
> >> rather than iterating over it at a 4k granule).
> >> 
> >> One thing we could do is switch to synchronous mode if we detect a change in
> >> granule (i.e. treat it like a batch failure).
> > 
> > We could use tlb_start_vma() to track that, I think. Shouldn't be too
> > hard.
> 
> Somewhat unrelated, but I use this opportunity that TLB got your attention
> for something that bothers me for some time. clear_fixmap(), which is used
> in various places (e.g., text_poke()), ends up in doing only a local TLB
> flush (in __set_pte_vaddr()).
> 
> Is that sufficient?

Urgh.. weren't the fixmaps per cpu? Bah, I remember looking at this
during PTI, but I seem to have forgotten everything again.