From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with SMTP id 0F40C6B004F for ; Tue, 21 Jul 2009 10:11:05 -0400 (EDT) From: "Rafael J. Wysocki" Subject: Re: [PATCH] hibernate / memory hotplug: always use for_each_populated_zone() Date: Tue, 21 Jul 2009 16:11:08 +0200 References: <1248103551.23961.0.camel@localhost.localdomain> <20090721071508.GB12734@osiris.boeblingen.de.ibm.com> <20090721163846.2a8001c1.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <20090721163846.2a8001c1.kamezawa.hiroyu@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200907211611.09525.rjw@sisk.pl> Sender: owner-linux-mm@kvack.org To: KAMEZAWA Hiroyuki Cc: Heiko Carstens , Nigel Cunningham , Gerald Schaefer , Andrew Morton , linux-kernel@vger.kernel.org, Martin Schwidefsky , KOSAKI Motohiro , Yasunori Goto , Nick Piggin , linux-mm@kvack.org List-ID: On Tuesday 21 July 2009, KAMEZAWA Hiroyuki wrote: > On Tue, 21 Jul 2009 09:15:08 +0200 > Heiko Carstens wrote: > > > On Tue, Jul 21, 2009 at 07:29:58AM +1000, Nigel Cunningham wrote: > > > Hi. > > > > > > Gerald Schaefer wrote: > > > > From: Gerald Schaefer > > > > > > > > Use for_each_populated_zone() instead of for_each_zone() in hibernation > > > > code. This fixes a bug on s390, where we allow both config options > > > > HIBERNATION and MEMORY_HOTPLUG, so that we also have a ZONE_MOVABLE > > > > here. We only allow hibernation if no memory hotplug operation was > > > > performed, so in fact both features can only be used exclusively, but > > > > this way we don't need 2 differently configured (distribution) kernels. > > > > > > > > If we have an unpopulated ZONE_MOVABLE, we allow hibernation but run > > > > into a BUG_ON() in memory_bm_test/set/clear_bit() because hibernation > > > > code iterates through all zones, not only the populated zones, in > > > > several places. For example, swsusp_free() does for_each_zone() and > > > > then checks for pfn_valid(), which is true even if the zone is not > > > > populated, resulting in a BUG_ON() later because the pfn cannot be > > > > found in the memory bitmap. > > > > > > I agree with your logic and patch, but doesn't this also imply that the > > > s390 implementation pfn_valid should be changed to return false for > > > those pages? > > > > For CONFIG_SPARSEMEM, which s390 uses, there is no architecture specific > > pfn_valid() implementation. > > Also it looks like the semantics of pfn_valid() aren't clear. > > At least for sparsemem it means nothing but "the memmap for the section > > this page belongs to exists". So it just means the struct page for the > > pfn exists. > > Historically, pfn_valid() just means "there is a memmap." no other meanings > in any configs/archs. Is this documented anywhere actually? > > We still have pfn_present() for CONFIG_SPARSEMEM. But that just means > > "some pages in the section this pfn belongs to are present." > > It just exists for sparsemem internal purpose IIUC. > > > > So it looks like checking for pfn_valid() and afterwards checking > > for PG_Reserved (?) might give what one would expect. > I think so, too. If memory is offline, PG_reserved is always set. > > In general, it's expected that "page is contiguous in MAX_ORDER range" > and no memory holes in MAX_ORDER. In most case, PG_reserved is checked > for skipping not-existing memory. PG_reserved is also set for kernel text, at least on some architectures, and for some other areas that we want to save. > > Looks all a bit confusing to me. > > Or maybe it's just me who is confused? :) > > > IIRC, there are no generic interface to know whether there is a physical page. We need to know that for hibernation, though. Well, there is a mechanism for marking making address ranges that are never to be saved, but they need to be known during initialisation already. > pfn_valid() is only for memmap and people have used > if (pfn_valid(pfn) && !PageReserved(page)) > check. > But, hmm, If hibernation have to save PG_reserved memory, general solution is > use copy_user_page() and handle fault. That's not exactly straightforward IMHO. > Alternative is making use of walk_memory_resource() as memory hotplug does. > It checks resource information registered. I'd be fine with any _simple_ mechanism allowing us to check whether there's a physical page frame for given page (or given PFN). Best, Rafael -- 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