linux-kernel - Re: [PATCH] sched: Fix task_numa

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Date:	Thu, 13 Dec 2012 19:27:26 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mel Gorman <mgorman@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>, Hillf Danton <dhillf@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Alex Shi <lkml.alex@...il.com>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Fix task_numa_fault() + KSM crash

* Ingo Molnar <mingo@...nel.org> [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@...ux.vnet.ibm.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
>  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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