From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx145.postini.com [74.125.245.145]) by kanga.kvack.org (Postfix) with SMTP id ADA2E6B005A for ; Thu, 15 Nov 2012 10:40:48 -0500 (EST) Received: by mail-pa0-f41.google.com with SMTP id fa10so1258183pad.14 for ; Thu, 15 Nov 2012 07:40:48 -0800 (PST) Message-ID: <50A50CF8.9040207@gmail.com> Date: Thu, 15 Nov 2012 23:40:40 +0800 From: Jiang Liu MIME-Version: 1.0 Subject: Re: [PATCH] mm: fix a regression with HIGHMEM introduced by changeset 7f1290f2f2a4d References: <1352165517-9732-1-git-send-email-jiang.liu@huawei.com> <20121106124315.79deb2bc.akpm@linux-foundation.org> <50A3B013.4030207@gmail.com> <50A4B45D.5000905@cn.fujitsu.com> In-Reply-To: <50A4B45D.5000905@cn.fujitsu.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Wen Congyang Cc: Andrew Morton , Jiang Liu , Maciej Rutecki , Jianguo Wu , Chris Clayton , "Rafael J. Wysocki" , Mel Gorman , Minchan Kim , KAMEZAWA Hiroyuki , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Daniel Vetter On 11/15/2012 05:22 PM, Wen Congyang wrote: > Hi, Liu Jiang > > At 11/14/2012 10:52 PM, Jiang Liu Wrote: >> On 11/07/2012 04:43 AM, Andrew Morton wrote: >>> On Tue, 6 Nov 2012 09:31:57 +0800 >>> Jiang Liu wrote: >>> >>>> Changeset 7f1290f2f2 tries to fix a issue when calculating >>>> zone->present_pages, but it causes a regression to 32bit systems with >>>> HIGHMEM. With that changeset, function reset_zone_present_pages() >>>> resets all zone->present_pages to zero, and fixup_zone_present_pages() >>>> is called to recalculate zone->present_pages when boot allocator frees >>>> core memory pages into buddy allocator. Because highmem pages are not >>>> freed by bootmem allocator, all highmem zones' present_pages becomes >>>> zero. >>>> >>>> Actually there's no need to recalculate present_pages for highmem zone >>>> because bootmem allocator never allocates pages from them. So fix the >>>> regression by skipping highmem in function reset_zone_present_pages() >>>> and fixup_zone_present_pages(). >>>> >>>> ... >>>> >>>> --- a/mm/page_alloc.c >>>> +++ b/mm/page_alloc.c >>>> @@ -6108,7 +6108,8 @@ void reset_zone_present_pages(void) >>>> for_each_node_state(nid, N_HIGH_MEMORY) { >>>> for (i = 0; i < MAX_NR_ZONES; i++) { >>>> z = NODE_DATA(nid)->node_zones + i; >>>> - z->present_pages = 0; >>>> + if (!is_highmem(z)) >>>> + z->present_pages = 0; >>>> } >>>> } >>>> } >>>> @@ -6123,10 +6124,11 @@ void fixup_zone_present_pages(int nid, unsigned long start_pfn, >>>> >>>> for (i = 0; i < MAX_NR_ZONES; i++) { >>>> z = NODE_DATA(nid)->node_zones + i; >>>> + if (is_highmem(z)) >>>> + continue; >>>> + >>>> zone_start_pfn = z->zone_start_pfn; >>>> zone_end_pfn = zone_start_pfn + z->spanned_pages; >>>> - >>>> - /* if the two regions intersect */ >>>> if (!(zone_start_pfn >= end_pfn || zone_end_pfn <= start_pfn)) >>>> z->present_pages += min(end_pfn, zone_end_pfn) - >>>> max(start_pfn, zone_start_pfn); >>> >>> This ... isn't very nice. It is embeds within >>> reset_zone_present_pages() and fixup_zone_present_pages() knowledge >>> about their caller's state. Or, more specifically, it is emebedding >>> knowledge about the overall state of the system when these functions >>> are called. >>> >>> I mean, a function called "reset_zone_present_pages" should reset >>> ->present_pages! >>> >>> The fact that fixup_zone_present_page() has multiple call sites makes >>> this all even more risky. And what are the interactions between this >>> and memory hotplug? >>> >>> Can we find a cleaner fix? >>> >>> Please tell us more about what's happening here. Is it the case that >>> reset_zone_present_pages() is being called *after* highmem has been >>> populated? If so, then fixup_zone_present_pages() should work >>> correctly for highmem? Or is it the case that highmem hasn't yet been >>> setup? IOW, what is the sequence of operations here? >>> >>> Is the problem that we're *missing* a call to >>> fixup_zone_present_pages(), perhaps? If we call >>> fixup_zone_present_pages() after highmem has been populated, >>> fixup_zone_present_pages() should correctly fill in the highmem zone's >>> ->present_pages? >> Hi Andrew, >> Sorry for the late response:( >> I have done more investigations according to your suggestions. Currently >> we have only called fixup_zone_present_pages() for memory freed by bootmem >> allocator and missed HIGHMEM pages. We could also call fixup_zone_present_pages() >> for HIGHMEM pages, but that will need to change arch specific code for x86, powerpc, >> sparc, microblaze, arm, mips, um and tile etc. Seems a little overhead. >> And sadly enough, I found the quick fix is still incomplete. The original >> patch still have another issue that, reset_zone_present_pages() is only called >> for IA64, so it will cause trouble for other arches which make use of "bootmem.c". >> Then I feel a little guilty and tried to find a cleaner solution without >> touching arch specific code. But things are more complex than my expectation and >> I'm still working on that. >> So how about totally reverting the changeset 7f1290f2f2a4d2c3f1b7ce8e87256e052ca23125 >> and I will post another version once I found a cleaner way? > > I think fixup_zone_present_pages() are very useful for memory hotplug. > > We calculate zone->present_pages in free_area_init_core(), but its value is wrong. > So it is why we fix it in fixup_zone_present_pages(). > > What about this: > 1. init zone->present_pages to the present pages in this zone(include bootmem) > 2. don't reset zone->present_pages for HIGHMEM pages > > We don't allocate bootmem from HIGHMEM. So its present pages is inited in step1 > and there is no need to fix it in step2. Hi Congyang, I feel that zone->present_pages has been abused. I guess it means "physical pages present in this zone" originally, but now sometimes zone->present_pages is used as "pages in this zone managed by the buddy system". So I'm trying to add a new field "managed_pages" into zone, which accounts for pages managed by buddy system. That's why I thought the clean solution is a little complex:( Why do we need "managed_pages"? With HIGHMEM enabled, there may be bigger difference between "present_pages" and "managed_pages" on ZONE_NORMAL because it also hosts page array for ZONE_HIGHMEM. That may cause disturbance to page allocator or scanner. What's your thoughts? Thanks Gerry -- 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