lkml.org 
[lkml]   [2019]   [May]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: mm/compaction: BUG: NULL pointer dereference
    From
    Date


    On 05/24/2019 02:50 PM, 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 ?
    >
    >
    > diff --git a/mm/compaction.c b/mm/compaction.c
    > index 9febc8c..9e1b9ac 100644
    > --- a/mm/compaction.c
    > +++ b/mm/compaction.c
    > @@ -1399,7 +1399,7 @@ fast_isolate_freepages(struct compact_control *cc)
    > page = pfn_to_page(highest);
    > cc->free_pfn = highest;
    > } else {
    > - if (cc->direct_compaction) {
    > + if (cc->direct_compaction && pfn_valid(min_pfn)) {
    > page = pfn_to_page(min_pfn);

    pfn_to_online_page() here would be better as it does not add pfn_valid() cost on
    architectures which does not subscribe to CONFIG_HOLES_IN_ZONE. But regardless if
    the compaction is trying to scan pfns in zone holes, then it should be avoided.

    \
     
     \ /
      Last update: 2019-05-24 12:56    [W:5.187 / U:0.048 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site