Re: [PATCH] memory_hotplug: fix the panic when memory end is not on the section boundary

[Zaslonko Mikhail <zaslonko@linux.vnet.ibm.com>]

On 10.09.2018 15:17, Michal Hocko wrote:
> [Cc Pavel]
>
> On Mon 10-09-18 14:35:27, Mikhail Zaslonko wrote:
>> If memory end is not aligned with the linux memory section boundary, such
>> a section is only partly initialized. This may lead to VM_BUG_ON due to
>> uninitialized struct pages access from is_mem_section_removable() or
>> test_pages_in_a_zone() function.
>>
>> Here is one of the panic examples:
>>   CONFIG_DEBUG_VM_PGFLAGS=y
>>   kernel parameter mem=3075M
> OK, so the last memory section is not full and we have a partial memory
> block right?

Right. In my example above, I define 3075M (3Gig + 3Meg) of base memory 
in the
kernel parameters. As a result we end up with the last memory block 
having only
3 megabytes initialized. The initialization takes place within
memmap_init_zone(unsigned long size, ...) function called from
free_area_init_core() with the size = zone->spanned_pages. Thus, only three
megabytes of the last memory block are initialized (till the end of the zone
Normal). And with the page poisoning introduced by Pavel we fail on such a
memory block processing in memory_hotplug code (no actual memory hotplug
is involved here).

>
>>   page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
> OK, this means that the struct page is not fully initialized. Do you
> have a specific place which has triggered this assert?

This assert is triggered in page_to_nid() function when it is called for
uninitialized page. I found two places where that can happen:
1) is_pageblock_removable_nolock() - direct call
2) test_pages_in_a_zone() - via page_zone() call

>
>>   ------------[ cut here ]------------
>>   Call Trace:
>>   ([<000000000039b8a4>] is_mem_section_removable+0xcc/0x1c0)
>>    [<00000000009558ba>] show_mem_removable+0xda/0xe0
>>    [<00000000009325fc>] dev_attr_show+0x3c/0x80
>>    [<000000000047e7ea>] sysfs_kf_seq_show+0xda/0x160
>>    [<00000000003fc4e0>] seq_read+0x208/0x4c8
>>    [<00000000003cb80e>] __vfs_read+0x46/0x180
>>    [<00000000003cb9ce>] vfs_read+0x86/0x148
>>    [<00000000003cc06a>] ksys_read+0x62/0xc0
>>    [<0000000000c001c0>] system_call+0xdc/0x2d8
>>
>> This fix checks if the page lies within the zone boundaries before
>> accessing the struct page data. The check is added to both functions.
>> Actually similar check has already been present in
>> is_pageblock_removable_nolock() function but only after the struct page
>> is accessed.
>>
> Well, I am afraid this is not the proper solution. We are relying on the
> full pageblock worth of initialized struct pages at many other place. We
> used to do that in the past because we have initialized the full
> section but this has been changed recently. Pavel, do you have any ideas
> how to deal with this partial mem sections now?

I think this is not related to the recent changes of memory 
initialization. If
you mean deferred init case, the problem exists even without
CONFIG_DEFERRED_STRUCT_PAGE_INIT kernel option.

>> Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
>> Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
>> Cc: <stable@vger.kernel.org>
>> ---
>>   mm/memory_hotplug.c | 20 +++++++++++---------
>>   1 file changed, 11 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 9eea6e809a4e..8e20e8fcc3b0 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1229,9 +1229,8 @@ static struct page *next_active_pageblock(struct page *page)
>>   	return page + pageblock_nr_pages;
>>   }
>>   
>> -static bool is_pageblock_removable_nolock(struct page *page)
>> +static bool is_pageblock_removable_nolock(struct page *page, struct zone **zone)
>>   {
>> -	struct zone *zone;
>>   	unsigned long pfn;
>>   
>>   	/*
>> @@ -1241,15 +1240,14 @@ static bool is_pageblock_removable_nolock(struct page *page)
>>   	 * We have to take care about the node as well. If the node is offline
>>   	 * its NODE_DATA will be NULL - see page_zone.
>>   	 */
>> -	if (!node_online(page_to_nid(page)))
>> -		return false;
>> -
>> -	zone = page_zone(page);
>>   	pfn = page_to_pfn(page);
>> -	if (!zone_spans_pfn(zone, pfn))
>> +	if (*zone && !zone_spans_pfn(*zone, pfn))
>>   		return false;
>> +	if (!node_online(page_to_nid(page)))
>> +		return false;
>> +	*zone = page_zone(page);
>>   
>> -	return !has_unmovable_pages(zone, page, 0, MIGRATE_MOVABLE, true);
>> +	return !has_unmovable_pages(*zone, page, 0, MIGRATE_MOVABLE, true);
>>   }
>>   
>>   /* Checks if this range of memory is likely to be hot-removable. */
>> @@ -1257,10 +1255,11 @@ bool is_mem_section_removable(unsigned long start_pfn, unsigned long nr_pages)
>>   {
>>   	struct page *page = pfn_to_page(start_pfn);
>>   	struct page *end_page = page + nr_pages;
>> +	struct zone *zone = NULL;
>>   
>>   	/* Check the starting page of each pageblock within the range */
>>   	for (; page < end_page; page = next_active_pageblock(page)) {
>> -		if (!is_pageblock_removable_nolock(page))
>> +		if (!is_pageblock_removable_nolock(page, &zone))
>>   			return false;
>>   		cond_resched();
>>   	}
>> @@ -1296,6 +1295,9 @@ int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn,
>>   				i++;
>>   			if (i == MAX_ORDER_NR_PAGES || pfn + i >= end_pfn)
>>   				continue;
>> +			/* Check if we got outside of the zone */
>> +			if (zone && !zone_spans_pfn(zone, pfn))
>> +				return 0;
>>   			page = pfn_to_page(pfn + i);
>>   			if (zone && page_zone(page) != zone)
>>   				return 0;
>> -- 
>> 2.16.4