Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754489Ab2F1LGa (ORCPT ); Thu, 28 Jun 2012 07:06:30 -0400 Received: from merlin.infradead.org ([205.233.59.134]:51120 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752460Ab2F1LG1 convert rfc822-to-8bit (ORCPT ); Thu, 28 Jun 2012 07:06:27 -0400 Message-ID: <1340881511.28750.19.camel@twins> Subject: Re: [PATCH 02/20] mm: Add optional TLB flush to generic RCU page-table freeing From: Peter Zijlstra To: Benjamin Herrenschmidt Cc: Linus Torvalds , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, Thomas Gleixner , Ingo Molnar , akpm@linux-foundation.org, Rik van Riel , Hugh Dickins , Mel Gorman , Nick Piggin , Alex Shi , "Nikunj A. Dadhania" , Konrad Rzeszutek Wilk , David Miller , Russell King , Catalin Marinas , Chris Metcalf , Martin Schwidefsky , Tony Luck , Paul Mundt , Jeff Dike , Richard Weinberger , Ralf Baechle , Kyle McMartin , James Bottomley , Chris Zankel Date: Thu, 28 Jun 2012 13:05:11 +0200 In-Reply-To: <1340867364.20977.65.camel@pasglop> References: <20120627211540.459910855@chello.nl> <20120627212830.693232452@chello.nl> <1340838106.10063.85.camel@twins> <1340867364.20977.65.camel@pasglop> Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-Mailer: Evolution 3.2.2- Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2250 Lines: 49 On Thu, 2012-06-28 at 17:09 +1000, Benjamin Herrenschmidt wrote: > On Thu, 2012-06-28 at 01:01 +0200, Peter Zijlstra wrote: > > On Wed, 2012-06-27 at 15:23 -0700, Linus Torvalds wrote: > > > > > Plus it really isn't about hardware page table walkers at all. It's > > > more about the possibility of speculative TLB fils, it has nothing to > > > do with *how* they are done. Sure, it's likely that a software > > > pagetable walker wouldn't be something that gets called speculatively, > > > but it's not out of the question. > > > > > Hmm, I would call gup_fast() as speculative as we can get in software. > > It does a lock-less walk of the page-tables. That's what the RCU free'd > > page-table stuff is for to begin with. > > Strictly speaking it's not :-) To *begin with* (as in the origin of that > code) it comes from powerpc hash table code which walks the linux page > tables locklessly :-) It then came in handy with gup_fast :-) Ah, ok my bad. > > > IOW, if Sparc/PPC really want to guarantee that they never fill TLB > > > entries speculatively, and that if we are in a kernel thread they will > > > *never* fill the TLB with anything else, then make them enable > > > CONFIG_STRICT_TLB_FILL or something in their architecture Kconfig > > > files. > > > > Since we've dealt with the speculative software side by using RCU-ish > > stuff, the only thing that's left is hardware, now neither sparc64 nor > > ppc actually know about the linux page-tables from what I understood, > > they only look at their hash-table thing. > > Some embedded ppc's know about the lowest level (SW loaded PMD) but > that's not an issue here. We flush these special TLB entries > specifically and synchronously in __pte_free_tlb(). OK, I missed that.. is that arch/powerpc/mm/tlb_nohash.c:tlb_flush_pgtable() ? > > So even if the hardware did do speculative tlb fills, it would do them > > from the hash-table, but that's already cleared out. > > Right, Phew at least I got the important thing right ;-) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/