Re: [PATCH] mm, numa: Fix bad pmd by atomically check for pmd_trans_huge when marking page tables prot_numa

[Zi Yan]

On 10 Apr 2017, at 4:48, Mel Gorman wrote:

> 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.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Does this patch fix the same problem fixed by Kirill's patch here?
https://lkml.org/lkml/2017/3/2/347

--
Best Regards
Yan Zi

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