Re: [PATCH] memory_hotplug: fix kernel_panic on offline page processing

[Pasha Tatashin]

On 8/28/18 7:25 AM, Michal Hocko wrote:
> On Tue 28-08-18 11:05:39, Mikhail Zaslonko wrote:
>> Within show_valid_zones() the function test_pages_in_a_zone() should be
>> called for online memory blocks only. Otherwise it might lead to the
>> VM_BUG_ON due to uninitialized struct pages (when CONFIG_DEBUG_VM_PGFLAGS
>> kernel option is set):
>>
>>  page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
>>  ------------[ cut here ]------------
>>  Call Trace:
>>  ([<000000000038f91e>] test_pages_in_a_zone+0xe6/0x168)
>>   [<0000000000923472>] show_valid_zones+0x5a/0x1a8
>>   [<0000000000900284>] dev_attr_show+0x3c/0x78
>>   [<000000000046f6f0>] sysfs_kf_seq_show+0xd0/0x150
>>   [<00000000003ef662>] seq_read+0x212/0x4b8
>>   [<00000000003bf202>] __vfs_read+0x3a/0x178
>>   [<00000000003bf3ca>] vfs_read+0x8a/0x148
>>   [<00000000003bfa3a>] ksys_read+0x62/0xb8
>>   [<0000000000bc2220>] system_call+0xdc/0x2d8
>>
>> That VM_BUG_ON was triggered by the page poisoning introduced in
>> mm/sparse.c with the git commit d0dc12e86b31 ("mm/memory_hotplug: optimize
>> memory hotplug")
>> With the same commit the new 'nid' field has been added to the struct
>> memory_block in order to store and later on derive the node id for offline
>> pages (instead of accessing struct page which might be uninitialized). But
>> one reference to nid in show_valid_zones() function has been overlooked.
>> Fixed with current commit.
>> Also, nr_pages will not be used any more after test_pages_in_a_zone() call,
>> do not update it.
>>
>> Fixes: d0dc12e86b31 ("mm/memory_hotplug: optimize memory hotplug")
>> Cc: <stable@xxxxxxxxxxxxxxx> # v4.17+
>> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
>> Signed-off-by: Mikhail Zaslonko <zaslonko@xxxxxxxxxxxxx>
> 
> Btw. this land mines which are basically impossible to find during the
> review are the reason why I was not all that happy about d0dc12e86b31.
> It added a margninal improvement but opened a can of warms. On the other

Hi Michal,

I agree, the hotplug code is very fragile. But, it only means that it
requires improvements.

I specifically added PagePoisoned() check with hotplug optimizations
changes in order to catch these kind of bugs, and I am glad we are
catching them.

Reviewed-by: Pavel Tatashin <pavel.tatashin@xxxxxxxxxxxxx>

Thank you,
Pavel

> hand maybe we just had to open that can one day...
> 
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> 
> Thanks!
> 
>> ---
>>  drivers/base/memory.c | 20 +++++++++-----------
>>  1 file changed, 9 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
>> index f5e560188a18..622ab8edc035 100644
>> --- a/drivers/base/memory.c
>> +++ b/drivers/base/memory.c
>> @@ -416,26 +416,24 @@ static ssize_t show_valid_zones(struct device *dev,
>>  	struct zone *default_zone;
>>  	int nid;
>>  
>> -	/*
>> -	 * The block contains more than one zone can not be offlined.
>> -	 * This can happen e.g. for ZONE_DMA and ZONE_DMA32
>> -	 */
>> -	if (!test_pages_in_a_zone(start_pfn, start_pfn + nr_pages, &valid_start_pfn, &valid_end_pfn))
>> -		return sprintf(buf, "none\n");
>> -
>> -	start_pfn = valid_start_pfn;
>> -	nr_pages = valid_end_pfn - start_pfn;
>> -
>>  	/*
>>  	 * Check the existing zone. Make sure that we do that only on the
>>  	 * online nodes otherwise the page_zone is not reliable
>>  	 */
>>  	if (mem->state == MEM_ONLINE) {
>> +		/*
>> +		 * The block contains more than one zone can not be offlined.
>> +		 * This can happen e.g. for ZONE_DMA and ZONE_DMA32
>> +		 */
>> +		if (!test_pages_in_a_zone(start_pfn, start_pfn + nr_pages,
>> +					  &valid_start_pfn, &valid_end_pfn))
>> +			return sprintf(buf, "none\n");
>> +		start_pfn = valid_start_pfn;
>>  		strcat(buf, page_zone(pfn_to_page(start_pfn))->name);
>>  		goto out;
>>  	}
>>  
>> -	nid = pfn_to_nid(start_pfn);
>> +	nid = mem->nid;
>>  	default_zone = zone_for_pfn_range(MMOP_ONLINE_KEEP, nid, start_pfn, nr_pages);
>>  	strcat(buf, default_zone->name);
>>  
>> -- 
>> 2.16.4
>