Messages in this thread | | | Date | Thu, 15 Nov 2012 22:23:35 +0800 | From | Wen Congyang <> | Subject | Re: [PATCH] mm: fix a regression with HIGHMEM introduced by changeset 7f1290f2f2a4d |
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At 2012/11/15 19:28, Bob Liu Wrote: > On Thu, Nov 15, 2012 at 5:22 PM, Wen Congyang<wency@cn.fujitsu.com> 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<jiang.liu@huawei.com> 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. >> > > I might miss something, but if memory hotplug is the only user depends on > fixup_zone_present_pages().
IIRC, water_mask depends on zone->present_pages. But I don't meet any problem even if zone->present_pages is wrong.
> Why not reverting the changeset 7f1290f2f2a4d2c3f1b7ce8e87256e052ca23125 > And add checking to offline_pages() like: > if (zone->present_pages>= offlined_page) > zone->present_pages -= offlined_pages; > else > zone->present_pages = 0; > > It's more simple and can minimize the effect to other parts of kernel.
Hmm, zone->present_pages may be 0 when there is memory in this zone which is onlined and in use. If zone->present_pages becomes to 0, we will free pcp list for this zone. It will cause some unexpected error.
> >> 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. >> >> Is it OK? >> >> If it is OK, I will resend the patch for step1(the patch is from laijs). >> >
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