* Ingo Molnar <mingo@xxxxxxxxxx> [2012-12-10 13:44:58]:
> Srikar Dronamraju reported that the following assert triggers on
> his box:
>
> kernel BUG at ../kernel/sched/fair.c:2371!
>
> Call Trace:
> [<ffffffff8113cd0e>] __do_numa_page+0xde/0x160
> [<ffffffff8113de9e>] handle_pte_fault+0x32e/0xcd0
> [<ffffffffa01c22c0>] ? drop_large_spte+0x30/0x30 [kvm]
> [<ffffffffa01bf215>] ? kvm_set_spte_hva+0x25/0x30 [kvm]
> [<ffffffff8113eab9>] handle_mm_fault+0x279/0x760
> [<ffffffff8115c024>] break_ksm+0x74/0xa0
> [<ffffffff8115c222>] break_cow+0xa2/0xb0
> [<ffffffff8115e38c>] ksm_scan_thread+0xb5c/0xd50
> [<ffffffff810771c0>] ? wake_up_bit+0x40/0x40
> [<ffffffff8115d830>] ? run_store+0x340/0x340
> [<ffffffff8107692e>] kthread+0xce/0xe0
>
> This means that task_numa_fault() was called for a kernel thread
> which has no fault tracking.
>
> This scenario is actually possible if a kernel thread does
> fault processing on behalf of a user-space task - ignore
> the page fault in that case.
>
> Also remove the (now never triggering) assert and robustify
> a nearby assert.
>
I do confirm that with this change, I dont see the assert anymore.
> Reported-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
> kernel/sched/fair.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9d11a8a..61c7a10 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2351,6 +2351,13 @@ void task_numa_fault(unsigned long addr, int node, int last_cpupid, int pages, b
> int priv;
> int idx;
>
> + /*
> + * Kernel threads might not have an mm but might still
> + * do fault processing (such as KSM):
> + */
> + if (!p->numa_faults)
> + return;
> +
> if (last_cpupid != cpu_pid_to_cpupid(-1, -1)) {
> /* Did we access it last time around? */
> if (last_pid == this_pid) {
> @@ -2367,8 +2374,8 @@ void task_numa_fault(unsigned long addr, int node, int last_cpupid, int pages, b
>
> idx = 2*node + priv;
>
> - WARN_ON_ONCE(last_cpu == -1 || node == -1);
> - BUG_ON(!p->numa_faults);
> + if (WARN_ON_ONCE(last_cpu == -1 || node == -1))
> + return;
>
> p->numa_faults_curr[idx] += pages;
> shared_fault_tick(p, node, last_cpu, pages);
>
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