Re: [PATCH 1/1] mm, memory_hotplug: Initialize struct pages for the full memory section

[Zaslonko Mikhail <zaslonko@linux.bm.com>]

Hello,

On 10.12.2018 14:24, Michal Hocko wrote:
> On Mon 10-12-18 14:07:12, 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.
> 
> It would be great to mention how you can end up in the situation like
> this(a user provided memmap or a strange HW).

Yes, I should probably keep full failure samples from my previous patch 
version.
 
> 
>> 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:
>>
>>  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
>>
>> Fix the problem by initializing the last memory section of the highest zone
>> in memmap_init_zone() till the very end, even if it goes beyond the zone
>> end.
> 
> Why do we need to restrict this to the highest zone? In other words, why
> cannot we do what I was suggesting earlier [1]. What does prevent other
> zones to have an incomplete section boundary?

Well, as you were also suggesting earlier: 'If we do not have a zone which
spans the rest of the section'. I'm not sure how else we should verify that.
Moreover, I was able to recreate the problem only with the highest zone
(memory end is not on the section boundary).

> 
> [1] http://lkml.kernel.org/r/20181105183533.GQ4361@dhcp22.suse.cz
> 
>> 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 | 15 +++++++++++++++
>>  1 file changed, 15 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 2ec9cc407216..41ef5508e5f1 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5542,6 +5542,21 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>>  			cond_resched();
>>  		}
>>  	}
>> +#ifdef CONFIG_SPARSEMEM
>> +	/*
>> +	 * If there is no zone spanning 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).
>> +	 */
>> +	if (end_pfn == max_pfn) {
>> +		while (end_pfn % PAGES_PER_SECTION) {
>> +			__init_single_page(pfn_to_page(end_pfn), end_pfn, zone,
>> +					   nid);
>> +			end_pfn++;
>> +		}
>> +	}
>> +#endif
>>  }
>>  
>>  #ifdef CONFIG_ZONE_DEVICE
>> -- 
>> 2.16.4
> 

Thanks,
Mikhail Zaslonko