linux-kernel - Re: [RFC][PATCH 4/6] arm, mm: Convert arm to generic tlb

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Date:	Thu, 17 May 2012 17:00:12 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Russell King <rmk@....linux.org.uk>,
	Paul Mundt <lethal@...ux-sh.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Chris Metcalf <cmetcalf@...era.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [RFC][PATCH 4/6] arm, mm: Convert arm to generic tlb

On Thu, May 17, 2012 at 12:28:06PM +0100, Peter Zijlstra wrote:
> On Thu, 2012-05-17 at 10:51 +0100, Russell King wrote:
> > On Thu, May 17, 2012 at 10:30:23AM +0100, Catalin Marinas wrote:
> > > Another minor thing is that on newer ARM processors (Cortex-A15) we
> > > need the TLB shootdown even on UP systems, so tlb_fast_mode should
> > > always return 0. Something like below (untested):
> > 
> > No Catalin, we need this for virtually all ARMv7 CPUs whether they're UP
> > or SMP, not just for A15, because of the speculative prefetch which can
> > re-load TLB entries from the page tables at _any_ time.
> 
> Hmm,. so this is mostly because of the confusion/coupling between
> tlb_remove_page() and tlb_remove_table() I guess. Since I don't see the
> freeing of the actual pages being a problem with speculative TLB
> reloads, just the page-tables.
> 
> Should we introduce a tlb_remove_table() regardless of
> HAVE_RCU_TABLE_FREE which always queues the tables regardless of
> tlb_fast_mode()? 

BTW, looking at your tlb-unify branch, does tlb_remove_table() call
tlb_flush/tlb_flush_mmu before freeing the tables?  I can only see
tlb_remove_page() doing this. On ARM, even UP, we need the TLB flushing
after clearing the pmd and before freeing the pte page table (and
ideally doing it less often than at every pte_free_tlb() call).

-- 
Catalin
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