linux-kernel - Re: [RFC PATCH 2/2] mm/mmu_gather: Avoid multiple page walk cache flush

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Message-ID: <20191217085854.GW2844@hirez.programming.kicks-ass.net>
Date:   Tue, 17 Dec 2019 09:58:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     akpm@...ux-foundation.org, npiggin@...il.com, mpe@...erman.id.au,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH 2/2] mm/mmu_gather: Avoid multiple page walk cache
 flush

On Tue, Dec 17, 2019 at 12:47:13PM +0530, Aneesh Kumar K.V wrote:
> On tlb_finish_mmu() kernel does a tlb flush before  mmu gather table invalidate.
> The mmu gather table invalidate depending on kernel config also does another
> TLBI. Avoid the later on tlb_finish_mmu().

That is already avoided, if you look at tlb_flush_mmu_tlbonly() it does
__tlb_range_reset(), which results in ->end = 0, which then triggers the
early exit on the next invocation:

	if (!tlb->end)
		return;