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SubjectRe: [PATCH] mm, numa: Fix bad pmd by atomically check for pmd_trans_huge when marking page tables prot_numa
On Mon 10-04-17 10:48:25, 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

s@helpful@helper@ I suspect

> (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@techsingularity.net>
> Cc: stable@vger.kernel.org

Acked-by: Michal Hocko <mhocko@suse.com>

you will probably win the_longest_function_name_contest but I do not
have much better suggestion.

> ---
> include/asm-generic/pgtable.h | 25 +++++++++++++++++++++++++
> mm/mprotect.c | 12 ++++++++++--
> 2 files changed, 35 insertions(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> index 1fad160f35de..597fa482cd4a 100644
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -819,6 +819,31 @@ static inline int pmd_none_or_trans_huge_or_clear_bad(pmd_t *pmd)
> }
>
> /*
> + * Used when setting automatic NUMA hinting protection where it is
> + * critical that a numa hinting PMD is not confused with a bad PMD.
> + */
> +static inline int pmd_none_or_clear_bad_unless_trans_huge(pmd_t *pmd)
> +{
> + pmd_t pmdval = pmd_read_atomic(pmd);
> +
> + /* See pmd_none_or_trans_huge_or_clear_bad for info on barrier */
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> + barrier();
> +#endif
> +
> + if (pmd_none(pmdval))
> + return 1;
> + if (pmd_trans_huge(pmdval))
> + return 0;
> + if (unlikely(pmd_bad(pmdval))) {
> + pmd_clear_bad(pmd);
> + return 1;
> + }
> + return 0;
> +}
> +
> +
> +/*
> * This is a noop if Transparent Hugepage Support is not built into
> * the kernel. Otherwise it is equivalent to
> * pmd_none_or_trans_huge_or_clear_bad(), and shall only be called in
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 8edd0d576254..821ff2904cdb 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -150,8 +150,16 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
> unsigned long this_pages;
>
> next = pmd_addr_end(addr, end);
> - if (!pmd_trans_huge(*pmd) && !pmd_devmap(*pmd)
> - && pmd_none_or_clear_bad(pmd))
> +
> + /*
> + * Automatic NUMA balancing walks the tables with mmap_sem
> + * held for read. It's possible a parallel update
> + * to occur between pmd_trans_huge and a pmd_none_or_clear_bad
> + * check leading to a false positive and clearing. Hence, it's
> + * necessary to atomically read the PMD value for all the
> + * checks.
> + */
> + if (!pmd_devmap(*pmd) && pmd_none_or_clear_bad_unless_trans_huge(pmd))
> continue;
>
> /* invoke the mmu notifier if the pmd is populated */

--
Michal Hocko
SUSE Labs

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