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Date:   Sun, 16 Feb 2020 23:32:33 +0000
From:   Mel Gorman <>
To:     Rafael Aquini <>
Subject: Re: [PATCH] mm, numa: fix bad pmd by atomically check for
 pmd_trans_huge when marking page tables prot_numa

On Sun, Feb 16, 2020 at 02:18:00PM -0500, Rafael Aquini wrote:
> From: Mel Gorman <>
>   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
> 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 <>
> Cc:
> Signed-off-by: Rafael Aquini <>

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

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