From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qc0-f177.google.com (mail-qc0-f177.google.com [209.85.216.177]) by kanga.kvack.org (Postfix) with ESMTP id CD75D6B0073 for ; Thu, 23 Oct 2014 19:41:23 -0400 (EDT) Received: by mail-qc0-f177.google.com with SMTP id l6so1390705qcy.22 for ; Thu, 23 Oct 2014 16:41:23 -0700 (PDT) Received: from gate.crashing.org (gate.crashing.org. [63.228.1.57]) by mx.google.com with ESMTPS id 20si5307402qgn.61.2014.10.23.16.41.20 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Thu, 23 Oct 2014 16:41:20 -0700 (PDT) Message-ID: <1414107635.364.91.camel@pasglop> Subject: Re: [PATCH V2 1/2] mm: Update generic gup implementation to handle hugepage directory From: Benjamin Herrenschmidt Date: Fri, 24 Oct 2014 10:40:35 +1100 In-Reply-To: <20141023.184035.388557314666522484.davem@davemloft.net> References: <1413520687-31729-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <20141022160224.9c2268795e55d5a2eff5b94d@linux-foundation.org> <20141023.184035.388557314666522484.davem@davemloft.net> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: David Miller Cc: akpm@linux-foundation.org, aneesh.kumar@linux.vnet.ibm.com, steve.capper@linaro.org, aarcange@redhat.com, mpe@ellerman.id.au, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-arch@vger.kernel.org, hannes@cmpxchg.org 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... 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... Cheers, Ben. > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email@kvack.org -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org