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Subject: Re: [PATCH v2 1/1] mm, memory_hotplug: Initialize struct pages for
 the full memory section
From:   David Hildenbrand <david@redhat.com>
To:     Mikhail Zaslonko <zaslonko@linux.ibm.com>,
        akpm@linux-foundation.org
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        mhocko@kernel.org, Pavel.Tatashin@microsoft.com,
        schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com,
        gerald.schaefer@de.ibm.com
References: <20181212172712.34019-1-zaslonko@linux.ibm.com>
 <20181212172712.34019-2-zaslonko@linux.ibm.com>
 <476a80cb-5524-16c1-6dd5-da5febbd6139@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <bcd0c49c-e417-ef8b-996f-99ecef540d9c@redhat.com>
Date:   Fri, 14 Dec 2018 16:49:14 +0100
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On 14.12.18 16:22, David Hildenbrand wrote:
> On 12.12.18 18:27, Mikhail Zaslonko wrote:
>> If memory end is not aligned with the sparse memory section boundary, the
>> mapping of such a section is only partly initialized. This may lead to
>> VM_BUG_ON due to uninitialized struct page access from
>> is_mem_section_removable() or test_pages_in_a_zone() function triggered by
>> memory_hotplug sysfs handlers:
>>
>> Here are the the panic examples:
>>  CONFIG_DEBUG_VM=y
>>  CONFIG_DEBUG_VM_PGFLAGS=y
>>
>>  kernel parameter mem=2050M
>>  --------------------------
>>  page:000003d082008000 is uninitialized and poisoned
>>  page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
>>  Call Trace:
>>  ([<0000000000385b26>] test_pages_in_a_zone+0xde/0x160)
>>   [<00000000008f15c4>] show_valid_zones+0x5c/0x190
>>   [<00000000008cf9c4>] dev_attr_show+0x34/0x70
>>   [<0000000000463ad0>] sysfs_kf_seq_show+0xc8/0x148
>>   [<00000000003e4194>] seq_read+0x204/0x480
>>   [<00000000003b53ea>] __vfs_read+0x32/0x178
>>   [<00000000003b55b2>] vfs_read+0x82/0x138
>>   [<00000000003b5be2>] ksys_read+0x5a/0xb0
>>   [<0000000000b86ba0>] system_call+0xdc/0x2d8
>>  Last Breaking-Event-Address:
>>   [<0000000000385b26>] test_pages_in_a_zone+0xde/0x160
>>  Kernel panic - not syncing: Fatal exception: panic_on_oops
>>
>>  kernel parameter mem=3075M
>>  --------------------------
>>  page:000003d08300c000 is uninitialized and poisoned
>>  page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
>>  Call Trace:
>>  ([<000000000038596c>] is_mem_section_removable+0xb4/0x190)
>>   [<00000000008f12fa>] show_mem_removable+0x9a/0xd8
>>   [<00000000008cf9c4>] dev_attr_show+0x34/0x70
>>   [<0000000000463ad0>] sysfs_kf_seq_show+0xc8/0x148
>>   [<00000000003e4194>] seq_read+0x204/0x480
>>   [<00000000003b53ea>] __vfs_read+0x32/0x178
>>   [<00000000003b55b2>] vfs_read+0x82/0x138
>>   [<00000000003b5be2>] ksys_read+0x5a/0xb0
>>   [<0000000000b86ba0>] system_call+0xdc/0x2d8
>>  Last Breaking-Event-Address:
>>   [<000000000038596c>] is_mem_section_removable+0xb4/0x190
>>  Kernel panic - not syncing: Fatal exception: panic_on_oops
>>
>> Fix the problem by initializing the last memory section of each zone
>> in memmap_init_zone() till the very end, even if it goes beyond the zone
>> end.
>>
>> Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
>> Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
>> Cc: <stable@vger.kernel.org>
>> ---
>>  mm/page_alloc.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 2ec9cc407216..e2afdb2dc2c5 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5542,6 +5542,18 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>>  			cond_resched();
>>  		}
>>  	}
>> +#ifdef CONFIG_SPARSEMEM
>> +	/*
>> +	 * If the zone does not span the rest of the section then
>> +	 * we should at least initialize those pages. Otherwise we
>> +	 * could blow up on a poisoned page in some paths which depend
>> +	 * on full sections being initialized (e.g. memory hotplug).
>> +	 */
>> +	while (end_pfn % PAGES_PER_SECTION) {
>> +		__init_single_page(pfn_to_page(end_pfn), end_pfn, zone, nid);
>> +		end_pfn++;
> 
> This page will not be marked as PG_reserved - although it is a physical
> memory gap. Do we care?
> 

Hm, or do we even have any idea what this is (e.g. could it also be
something not a gap)?

For physical memory gaps within a section, architectures usually exclude
that memory from getting passed to e.g. the page allocator by
memblock_reserve().

Before handing all free pages to the page allocator, all such reserved
memblocks will be marked reserved.

But this here seems to be different. We don't have a previous
memblock_reserve(), because otherwise these pages would have properly
been initialized already when marking them reserved.

-- 

Thanks,

David / dhildenb