Messages in this thread | | | Subject | Re: mm/compaction: BUG: NULL pointer dereference | From | Suzuki K Poulose <> | Date | Fri, 24 May 2019 11:42:54 +0100 |
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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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