Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756059AbaJXIdW (ORCPT ); Fri, 24 Oct 2014 04:33:22 -0400 Received: from mail-yh0-f54.google.com ([209.85.213.54]:53186 "EHLO mail-yh0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752865AbaJXIdS (ORCPT ); Fri, 24 Oct 2014 04:33:18 -0400 MIME-Version: 1.0 In-Reply-To: <1414107635.364.91.camel@pasglop> References: <1413520687-31729-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <20141022160224.9c2268795e55d5a2eff5b94d@linux-foundation.org> <20141023.184035.388557314666522484.davem@davemloft.net> <1414107635.364.91.camel@pasglop> Date: Fri, 24 Oct 2014 09:33:17 +0100 Message-ID: Subject: Re: [PATCH V2 1/2] mm: Update generic gup implementation to handle hugepage directory From: Steve Capper To: Benjamin Herrenschmidt Cc: David Miller , "akpm@linux-foundation.org" , "Aneesh Kumar K.V" , Andrea Arcangeli , mpe@ellerman.id.au, "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , linuxppc-dev@lists.ozlabs.org, "linux-arch@vger.kernel.org" , Johannes Weiner Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 24 October 2014 00:40, Benjamin Herrenschmidt wrote: > On Thu, 2014-10-23 at 18:40 -0400, David Miller wrote: >> Hey guys, was looking over the generic GUP while working on a sparc64 >> issue and I noticed that you guys do speculative page gets, and after >> talking with Johannes Weiner (CC:'d) about this we don't see how it >> could be necessary. >> >> If interrupts are disabled during the page table scan (which they >> are), no IPI tlb flushes can arrive. Therefore any removal from the >> page tables is guarded by interrupts being re-enabled. And as a >> result, page counts of pages we see in the page tables must always >> have a count > 0. >> >> x86 does direct atomic_add() on &page->_count because of this >> invariant and I would rather see the generic version do this too. > > This is of course only true of archs who use IPIs for TLB flushes, so if > we are going down the path of not being speculative, powerpc would have > to go back to doing its own since our broadcast TLB flush means we > aren't protected (we are only protected vs. the page tables themselves > being freed since we do that via sched RCU). > > AFAIK, ARM also broadcasts TLB flushes... Indeed, for most ARM cores we have hardware TLB broadcasts, thus we need the speculative path. > > Another option would be to make the generic code use something defined > by the arch to decide whether to use speculative get or > not. I like the idea of keeping the bulk of that code generic... It would be nice to have the code generalised further. In addition to the speculative/atomic helpers the implementation would need to be renamed from GENERIC_RCU_GUP to GENERIC_GUP. The other noteworthy assumption made in the RCU GUP is that pte's can be read atomically. For x86 this isn't true when running with 64-bit pte's, thus a helper would be needed. Cheers, -- Steve -- 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/