Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751988AbdFVO7a (ORCPT ); Thu, 22 Jun 2017 10:59:30 -0400 Received: from mail.skyhub.de ([5.9.137.197]:59756 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750812AbdFVO73 (ORCPT ); Thu, 22 Jun 2017 10:59:29 -0400 Date: Thu, 22 Jun 2017 16:59:15 +0200 From: Borislav Petkov To: Andy Lutomirski Cc: X86 ML , "linux-kernel@vger.kernel.org" , Linus Torvalds , Andrew Morton , Mel Gorman , "linux-mm@kvack.org" , Nadav Amit , Rik van Riel , Dave Hansen , Arjan van de Ven , Peter Zijlstra Subject: Re: [PATCH v3 05/11] x86/mm: Track the TLB's tlb_gen and update the flushing algorithm Message-ID: <20170622145914.tzqdulshlssiywj4@pd.tnic> References: <91f24a6145b2077f992902891f8fa59abe5c8696.1498022414.git.luto@kernel.org> <20170621184424.eixb2jdyy66xq4hg@pd.tnic> <20170622072449.4rc4bnvucn7usuak@pd.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2495 Lines: 72 On Thu, Jun 22, 2017 at 07:48:21AM -0700, Andy Lutomirski wrote: > On Thu, Jun 22, 2017 at 12:24 AM, Borislav Petkov wrote: > > On Wed, Jun 21, 2017 at 07:46:05PM -0700, Andy Lutomirski wrote: > >> > I'm certainly still missing something here: > >> > > >> > We have f->new_tlb_gen and mm_tlb_gen to control the flushing, i.e., we > >> > do once > >> > > >> > bump_mm_tlb_gen(mm); > >> > > >> > and once > >> > > >> > info.new_tlb_gen = bump_mm_tlb_gen(mm); > >> > > >> > and in both cases, the bumping is done on mm->context.tlb_gen. > >> > > >> > So why isn't that enough to do the flushing and we have to consult > >> > info.new_tlb_gen too? > >> > >> The issue is a possible race. Suppose we start at tlb_gen == 1 and > >> then two concurrent flushes happen. The first flush is a full flush > >> and sets tlb_gen to 2. The second is a partial flush and sets tlb_gen > >> to 3. If the second flush gets propagated to a given CPU first and it > > > > Maybe I'm still missing something, which is likely... > > > > but if the second flush gets propagated to the CPU first, the CPU will > > have local tlb_gen 1 and thus enforce a full flush anyway because we > > will go 1 -> 3 on that particular CPU. Or? > > > > Yes, exactly. Which means I'm probably just misunderstanding your > original question. Can you re-ask it? Ah, simple: we control the flushing with info.new_tlb_gen and mm->context.tlb_gen. I.e., this check: if (f->end != TLB_FLUSH_ALL && f->new_tlb_gen == local_tlb_gen + 1 && f->new_tlb_gen == mm_tlb_gen) { why can't we write: if (f->end != TLB_FLUSH_ALL && mm_tlb_gen == local_tlb_gen + 1) ? If mm_tlb_gen is + 2, then we'll do a full flush, if it is + 1, then partial. If the second flush, as you say is a partial one and still gets propagated first, the check will force a full flush anyway. When the first flush propagates after the second, we'll ignore it because local_tlb_gen has advanced adready due to the second flush. As a matter of fact, we could simplify the logic: if local_tlb_gen is only mm_tlb_gen - 1, then do the requested flush type. If mm_tlb_gen has advanced more than 1 generation, just do a full flush unconditionally. ... and I think we do something like that already but I think the logic could be simplified, unless I'm missing something, that is. Thanks. -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.