Re: [PATCH v4 2/8] mm/memory_hotplug: Don't access uninitialized memmaps in shrink_zone_span()

[David Hildenbrand]

On 26.09.19 11:12, Aneesh Kumar K.V wrote:
> David Hildenbrand <david@xxxxxxxxxx> writes:
> 
>> Let's limit shrinking to !ZONE_DEVICE so we can fix the current code. We
>> should never try to touch the memmap of offline sections where we could
>> have uninitialized memmaps and could trigger BUGs when calling
>> page_to_nid() on poisoned pages.
>>
>> Stopping to shrink the ZONE_DEVICE is fine as set_zone_contiguous() cannot
>> deal with ZONE_DEVICE either way. The zones will always be !contiguous
>> already and zone shrinking is therefore of limited use.
> 
> Can you add more details w.r.t ZONE_DEVICE?
I can convert that to

"There is no reliable way to distinguish an uninitialized memmap from an
initialized memmap that belongs to ZONE_DEVICE, as we don't have
anything like SECTION_IS_ONLINE we can use similar to
pfn_to_online_section() for !ZONE_DEVICE memory. E.g.,
set_zone_contiguous() similarly relies on pfn_to_online_section() and
will therefore never set a ZONE_DEVICE zone consecutive. Stopping to
shrink the ZONE_DEVICE therefore results in no observable changes,
besides /proc/zoneinfo indicating different boundaries - something we
can totally live with."


> 
>>
>> Before commit d0dc12e86b31 ("mm/memory_hotplug: optimize memory
>> hotplug"), the memmap was initialized with 0 and the node with the
>> right value. So the zone might be wrong but not garbage. After that
>> commit, both the zone and the node will be garbage when touching
>> uninitialized memmaps.
>>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Oscar Salvador <osalvador@xxxxxxx>
>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>> Fixes: d0dc12e86b31 ("mm/memory_hotplug: optimize memory hotplug")
>> Reported-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
>> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>> ---
>>  mm/memory_hotplug.c | 14 +++++++++++---
>>  1 file changed, 11 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index ddba8d786e4a..e0d1f6a9dfeb 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -331,7 +331,7 @@ static unsigned long find_smallest_section_pfn(int nid, struct zone *zone,
>>  				     unsigned long end_pfn)
>>  {
>>  	for (; start_pfn < end_pfn; start_pfn += PAGES_PER_SUBSECTION) {
>> -		if (unlikely(!pfn_valid(start_pfn)))
>> +		if (unlikely(!pfn_to_online_page(start_pfn)))
>>  			continue;
>>  
>>  		if (unlikely(pfn_to_nid(start_pfn) != nid))
>> @@ -356,7 +356,7 @@ static unsigned long find_biggest_section_pfn(int nid, struct zone *zone,
>>  	/* pfn is the end pfn of a memory section. */
>>  	pfn = end_pfn - 1;
>>  	for (; pfn >= start_pfn; pfn -= PAGES_PER_SUBSECTION) {
>> -		if (unlikely(!pfn_valid(pfn)))
>> +		if (unlikely(!pfn_to_online_page(pfn)))
>>  			continue;
>>  
>>  		if (unlikely(pfn_to_nid(pfn) != nid))
>> @@ -415,7 +415,7 @@ static void shrink_zone_span(struct zone *zone, unsigned long start_pfn,
>>  	 */
>>  	pfn = zone_start_pfn;
>>  	for (; pfn < zone_end_pfn; pfn += PAGES_PER_SUBSECTION) {
>> -		if (unlikely(!pfn_valid(pfn)))
>> +		if (unlikely(!pfn_to_online_page(pfn)))
>>  			continue;
>>  
>>  		if (page_zone(pfn_to_page(pfn)) != zone)
>> @@ -463,6 +463,14 @@ static void __remove_zone(struct zone *zone, unsigned long start_pfn,
>>  	struct pglist_data *pgdat = zone->zone_pgdat;
>>  	unsigned long flags;
>>  
>> +	/*
>> +	 * Zone shrinking code cannot properly deal with ZONE_DEVICE. So
>> +	 * we will not try to shrink the zones - which is okay as
>> +	 * set_zone_contiguous() cannot deal with ZONE_DEVICE either way.
>> +	 */
>> +	if (zone_idx(zone) == ZONE_DEVICE)
>> +		return;
> 
> Can you describe here what is special about ZONE_DEVICE?

I think adding more details to the description is sufficient, especially
as this also applies to set_zone_contiguous().

Thanks!

-- 

Thanks,

David / dhildenb