Re: [PATCH] sched/numa, mm: do not promote folios to nodes not set N_MEMORY

[Phil Auld]

Hi,

On Wed, Feb 14, 2024 at 12:53:55PM +0900 Byungchul Park wrote:
> While running qemu with a configuration where some CPUs don't have their
> local memory and with a kernel numa balancing on, the following oops has
> been observed. It's because of null pointers of ->zone_pgdat of zones of
> those nodes that are not initialized at booting time. So should avoid
> nodes not set N_MEMORY from getting promoted.
> 
> > BUG: unable to handle page fault for address: 00000000000033f3
> > #PF: supervisor read access in kernel mode
> > #PF: error_code(0x0000) - not-present page
> > PGD 0 P4D 0
> > Oops: 0000 [#1] PREEMPT SMP NOPTI
> > CPU: 2 PID: 895 Comm: masim Not tainted 6.6.0-dirty #255
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> >    rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
> > RIP: 0010:wakeup_kswapd (./linux/mm/vmscan.c:7812)
> > Code: (omitted)
> > RSP: 0000:ffffc90004257d58 EFLAGS: 00010286
> > RAX: ffffffffffffffff RBX: ffff88883fff0480 RCX: 0000000000000003
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88883fff0480
> > RBP: ffffffffffffffff R08: ff0003ffffffffff R09: ffffffffffffffff
> > R10: ffff888106c95540 R11: 0000000055555554 R12: 0000000000000003
> > R13: 0000000000000000 R14: 0000000000000000 R15: ffff88883fff0940
> > FS:  00007fc4b8124740(0000) GS:ffff888827c00000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00000000000033f3 CR3: 000000026cc08004 CR4: 0000000000770ee0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > PKRU: 55555554
> > Call Trace:
> >  <TASK>
> > ? __die
> > ? page_fault_oops
> > ? __pte_offset_map_lock
> > ? exc_page_fault
> > ? asm_exc_page_fault
> > ? wakeup_kswapd
> > migrate_misplaced_page
> > __handle_mm_fault
> > handle_mm_fault
> > do_user_addr_fault
> > exc_page_fault
> > asm_exc_page_fault
> > RIP: 0033:0x55b897ba0808
> > Code: (omitted)
> > RSP: 002b:00007ffeefa821a0 EFLAGS: 00010287
> > RAX: 000055b89983acd0 RBX: 00007ffeefa823f8 RCX: 000055b89983acd0
> > RDX: 00007fc2f8122010 RSI: 0000000000020000 RDI: 000055b89983acd0
> > RBP: 00007ffeefa821a0 R08: 0000000000000037 R09: 0000000000000075
> > R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000000
> > R13: 00007ffeefa82410 R14: 000055b897ba5dd8 R15: 00007fc4b8340000
> >  </TASK>
> > Modules linked in:
> > CR2: 00000000000033f3
> > ---[ end trace 0000000000000000  ]---
> > RIP: 0010:wakeup_kswapd (./linux/mm/vmscan.c:7812)
> > Code: (omitted)
> > RSP: 0000:ffffc90004257d58 EFLAGS: 00010286
> > RAX: ffffffffffffffff RBX: ffff88883fff0480 RCX: 0000000000000003
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88883fff0480
> > RBP: ffffffffffffffff R08: ff0003ffffffffff R09: ffffffffffffffff
> > R10: ffff888106c95540 R11: 0000000055555554 R12: 0000000000000003
> > R13: 0000000000000000 R14: 0000000000000000 R15: ffff88883fff0940
> > FS:  00007fc4b8124740(0000) GS:ffff888827c00000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00000000000033f3 CR3: 000000026cc08004 CR4: 0000000000770ee0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > PKRU: 55555554
> > note: masim[895] exited with irqs disabled

I think you could trim the down a little bit.


> 
> Signed-off-by: Byungchul Park <byungchul@xxxxxx>
> Reported-by: hyeongtak.ji@xxxxxx
> ---
>  kernel/sched/fair.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d7a3c63a2171..6d215cc85f14 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1828,6 +1828,23 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
>  	int dst_nid = cpu_to_node(dst_cpu);
>  	int last_cpupid, this_cpupid;
>  
> +	/*
> +	 * A node of dst_nid might not have its local memory. Promoting
> +	 * a folio to the node is meaningless. What's even worse, oops
> +	 * can be observed by the null pointer of ->zone_pgdat in
> +	 * various points of the code during migration.
> +	 *

> +	 * For instance, oops has been observed at CPU2 while qemu'ing:
> +	 *
> +	 * {qemu} \
> +	 *    -numa node,nodeid=0,mem=1G,cpus=0-1 \
> +	 *    -numa node,nodeid=1,cpus=2-3 \
> +	 *    -numa node,nodeid=2,mem=8G \
> +	 *    ...

This part above should probably be in the commit message not in the code.
The first paragraph of comment is plenty.

Otherwise, I think the check probably makes sense.


Cheers,
Phil

> +	 */
> +	if (!node_state(dst_nid, N_MEMORY))
> +		return false;
> +
>  	/*
>  	 * The pages in slow memory node should be migrated according
>  	 * to hot/cold instead of private/shared.
> -- 
> 2.17.1
> 
> 

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