On Sun, Feb 16, 2020 at 02:18:00PM -0500, Rafael Aquini wrote: > From: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > A user reported a bug against a distribution kernel while running > a proprietary workload described as "memory intensive that is not > swapping" that is expected to apply to mainline kernels. The workload > is read/write/modifying ranges of memory and checking the contents. They > reported that within a few hours that a bad PMD would be reported followed > by a memory corruption where expected data was all zeros. A partial report > of the bad PMD looked like > > [ 5195.338482] ../mm/pgtable-generic.c:33: bad pmd ffff8888157ba008(000002e0396009e2) > [ 5195.341184] ------------[ cut here ]------------ > [ 5195.356880] kernel BUG at ../mm/pgtable-generic.c:35! > .... > [ 5195.410033] Call Trace: > [ 5195.410471] [<ffffffff811bc75d>] change_protection_range+0x7dd/0x930 > [ 5195.410716] [<ffffffff811d4be8>] change_prot_numa+0x18/0x30 > [ 5195.410918] [<ffffffff810adefe>] task_numa_work+0x1fe/0x310 > [ 5195.411200] [<ffffffff81098322>] task_work_run+0x72/0x90 > [ 5195.411246] [<ffffffff81077139>] exit_to_usermode_loop+0x91/0xc2 > [ 5195.411494] [<ffffffff81003a51>] prepare_exit_to_usermode+0x31/0x40 > [ 5195.411739] [<ffffffff815e56af>] retint_user+0x8/0x10 > > Decoding revealed that the PMD was a valid prot_numa PMD and the bad PMD > was a false detection. The bug does not trigger if automatic NUMA balancing > or transparent huge pages is disabled. > > The bug is due a race in change_pmd_range between a pmd_trans_huge and > pmd_nond_or_clear_bad check without any locks held. During the pmd_trans_huge > check, a parallel protection update under lock can have cleared the PMD > and filled it with a prot_numa entry between the transhuge check and the > pmd_none_or_clear_bad check. > > While this could be fixed with heavy locking, it's only necessary to > make a copy of the PMD on the stack during change_pmd_range and avoid > races. A new helper is created for this as the check if quite subtle and the > existing similar helpful is not suitable. This passed 154 hours of testing > (usually triggers between 20 minutes and 24 hours) without detecting bad > PMDs or corruption. A basic test of an autonuma-intensive workload showed > no significant change in behaviour. > > Although Mel withdrew the patch on the face of LKML comment https://lkml.org/lkml/2017/4/10/922 > the race window aforementioned is still open, and we have reports of Linpack test reporting bad > residuals after the bad PMD warning is observed. In addition to that, bad rss-counter and > non-zero pgtables assertions are triggered on mm teardown for the task hitting the bad PMD. > > host kernel: mm/pgtable-generic.c:40: bad pmd 00000000b3152f68(8000000d2d2008e7) > .... > host kernel: BUG: Bad rss-counter state mm:00000000b583043d idx:1 val:512 > host kernel: BUG: non-zero pgtables_bytes on freeing mm: 4096 > > The issue is observed on a v4.18-based distribution kernel, but the race window is > expected to be applicable to mainline kernels, as well. > > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > Cc: stable@xxxxxxxxxxxxxxx > Signed-off-by: Rafael Aquini <aquini@xxxxxxxxxx> It's curious that it took so long for this to be caught again. Unfortunately I cannot find exactly what it's racing against but maybe it's not worth chasing down and the patch is simply the safer option :( -- Mel Gorman SUSE Labs
