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Date: Fri, 24 Aug 2018 10:47:17 +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: Will Deacon <will.deacon@arm.com>
Cc: 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 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.