Re: mm/compaction: BUG: NULL pointer dereference

[Suzuki K Poulose <suzuki.poulose@arm.com>]

Hi Mel,

Thanks for your quick response.

On 24/05/2019 11:39, Mel Gorman wrote:
> On Fri, May 24, 2019 at 10:20:19AM +0100, Suzuki K Poulose wrote:
>> Hi,
>>
>> We are hitting NULL pointer dereferences while running stress tests with KVM.
>> See splat [0]. The test is to spawn 100 VMs all doing standard debian
>> installation (Thanks to Marc's automated scripts, available here [1] ).
>> The problem has been reproduced with a better rate of success from 5.1-rc6
>> onwards.
>>
>> The issue is only reproducible with swapping enabled and the entire
>> memory is used up, when swapping heavily. Also this issue is only reproducible
>> on only one server with 128GB, which has the following memory layout:
>>
>> [32GB@4GB, hole , 96GB@544GB]
>>
>> Here is my non-expert analysis of the issue so far.
>>
>> Under extreme memory pressure, the kswapd could trigger reset_isolation_suitable()
>> to figure out the cached values for migrate/free pfn for a zone, by scanning through
>> the entire zone. On our server it does so in the range of [ 0x10_0000, 0xa00_0000 ],
>> with the following area of holes : [ 0x20_0000, 0x880_0000 ].
>> In the failing case, we end up setting the cached migrate pfn as : 0x508_0000, which
>> is right in the center of the zone pfn range. i.e ( 0x10_0000 + 0xa00_0000 ) / 2,
>> with reset_migrate = 0x88_4e00, reset_free = 0x10_0000.
>>
>> Now these cached values are used by the fast_isolate_freepages() to find a pfn. However,
>> since we cant find anything during the search we fall back to using the page belonging
>> to the min_pfn (which is the migrate_pfn), without proper checks to see if that is valid
>> PFN or not. This is then passed on to fast_isolate_around() which tries to do :
>> set_pageblock_skip(page) on the page which blows up due to an NULL mem_section pointer.
>>
>> The following patch seems to fix the issue for me, but I am not quite convinced that
>> it is the right fix. Thoughts ?
>>
> 
> I think the patch is valid and the alternatives would be unnecessarily
> complicated. During a normal scan for free pages to isolate, there
> is a check for pageblock_pfn_to_page() which uses a pfn_valid check
> for non-contiguous zones in __pageblock_pfn_to_page. Now, while the

I had the initial version with the pageblock_pfn_to_page(), but as you said,
it is a complicated way of perform the same check as pfn_valid().

> non-contiguous check could be made in the area you highlight, it would be a
> relatively small optimisation that would be unmeasurable overall. However,
> it is definitely the case that if the PFN you highlight is invalid that
> badness happens. If you want to express this as a signed-off patch with
> an adjusted changelog then I'd be happy to add

Sure, will send it right away.

> 
> Reviewed-by: Mel Gorman <mgorman@techsingularity.net>
> 

Thanks.

Suzuki
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