Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161144AbVKRT7m (ORCPT ); Fri, 18 Nov 2005 14:59:42 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1161140AbVKRT7l (ORCPT ); Fri, 18 Nov 2005 14:59:41 -0500 Received: from gold.veritas.com ([143.127.12.110]:50546 "EHLO gold.veritas.com") by vger.kernel.org with ESMTP id S1161144AbVKRT7k (ORCPT ); Fri, 18 Nov 2005 14:59:40 -0500 Date: Fri, 18 Nov 2005 19:58:17 +0000 (GMT) From: Hugh Dickins X-X-Sender: hugh@goblin.wat.veritas.com To: Ingo Oeser cc: linux-kernel@vger.kernel.org, Andrew Morton , Nick Piggin Subject: Re: [PATCH 09/11] unpaged: ZERO_PAGE in VM_UNPAGED In-Reply-To: <200511172225.31973.ioe-lkml@rameria.de> Message-ID: References: <200511172225.31973.ioe-lkml@rameria.de> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-OriginalArrivalTime: 18 Nov 2005 19:59:35.0487 (UTC) FILETIME=[9977FCF0:01C5EC7A] Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1628 Lines: 41 On Thu, 17 Nov 2005, Ingo Oeser wrote: > > We do we refcount ZERO_PAGE at all? We never used to. They were, and for the moment still are, marked PageReserved. Prior to 2.6.15-rc we didn't refcount reserved pages, but now we're trying to move away from PageReserved (some differences of opinion how far to go), so refcounting them. We're currently refcounting the ZERO_PAGE(s) simply because the common case is refcounted: it would just be extra tests and code NOT to refcount the ZERO_PAGE(s). If they were a commoner case, then it would indeed be worth avoiding refcounting them, but it currently doesn't appear to be worth the effort. But it is still up in the air: there is or may be an issue with refcounts overflowing, and if it's clear that the ZERO_PAGE is the only one vulnerable on any architecture, then I'm sure we'd deal with it by not refcounting them. However, I believe the issue extends to mapped file pages too: though you need a huge amount of RAM to reach overflow, so it's not something we need to resolve this week. > Ok, there may be multiple, but they exist always and always at > the same physical addresses, right? Right. > So why do we care at all? > Memory hotplug? > Doesn't it suffice there, that they are reverse mappable? Actually, they're not reverse mappable: we tend to find there's not much gained by swapping out the ZERO_PAGE ;) Hugh - 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/