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 16:10, Wei Yang wrote:
> On Mon, Dec 10, 2018 at 02:07:12PM +0100, 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:
>>
>> 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.
>>
>> 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
> 
> If my understanding is correct, end_pfn is not a valid range.
> 
> memmap_init_zone() initialize the range [start_pfn, start_pfn + size). I
> am afraid this will break the syntax. 
> 
> And max_pfn is also not a valid one. For example, on x86,
I used pfn_max here to check for the highest zone. What would be a better way? 

> update_end_of_memory_vars() will update max_pfn, which is calculated by:
> 
>     end_pfn = PFN_UP(start + size);
> 
> BTW, as you mentioned this apply to hotplug case. And then why this couldn't
> happen during boot up? What differ these two cases?

Well, the pages left uninitialized during bootup (initial problem), but the panic itself takes 
place when we try to process memory_hotplug sysfs attributes (thus triggering sysfs handlers). 
You can find more details in the original thread:
https://marc.info/?t=153658306400001&r=1&w=2

> 
>> }
>>
>> #ifdef CONFIG_ZONE_DEVICE
>> -- 
>> 2.16.4
> 

Thanks,
Mikhail Zaslonko