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

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Date:	Thu, 17 May 2012 18:59:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Catalin Marinas <catalin.marinas@....com>
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, 2012-05-17 at 18:44 +0200, Peter Zijlstra wrote:
> 
> So the RCU code can from ppc in commit
> 267239116987d64850ad2037d8e0f3071dc3b5ce, which has similar behaviour.
> Also I suspect the mm_users < 2 test will be incorrect for ARM since
> even the one user can be concurrent with your speculation engine.
> 
> 
Right, last mail, I promise, I've confused myself enough already! :-)

OK, so ppc/sparc are special (forgot all about s390) I think by the time
they are done with unmap_page_range() their hardware hash-tables are
empty and nobody but software page-table walkers will still access the
linux page tables.

So when we do free_pgtables() to clean up the actual page-tables.
Power/Sparc need to RCU free this to allow concurrent software
page-table walkers like gup_fast.

Thus I don't think they need to tlb flush again because their hardware
doesn't actually walk the link page-tables, it walks hash-tables, which
by this time are empty.

Now if x86/Xen were to use this, it would indeed also need to TLB flush
when freeing the page-tables, since its hardware walkers do indeed
traverse these pages and we need to sync against them.

So my first patch in the tlb-unify tree is actually buggy.

Humm,. what to do adding a tlb flush in there might slow down ppc/sparc
unnecessarily.. dave/ben? I guess we need more knobs :-(

Now its quite possible I've utterly confused myself and everybody
reading, apologies for that, I shall rest and purge all from memory and
start over before commenting more.. 
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