linux-kernel - Re: [PATCH] sched/numa, mm: do not promote folios to nodes not set N

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Date: Fri, 16 Feb 2024 16:45:51 +0900
From: Byungchul Park <byungchul@...com>
To: Phil Auld <pauld@...hat.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	bristot@...hat.com, vschneid@...hat.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kernel_team@...ynix.com, akpm@...ux-foundation.org
Subject: Re: [PATCH] sched/numa, mm: do not promote folios to nodes not set
 N_MEMORY

On Wed, Feb 14, 2024 at 03:03:18PM -0500, Phil Auld wrote:
> On Wed, Feb 14, 2024 at 07:31:37AM -0500 Phil Auld wrote:
> > 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@...com>
> > > Reported-by: hyeongtak.ji@...com
> > > ---
> > >  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.
> >
> 
> Actually, after looking at the memory.c code I wonder if this check should
> not be made farther up in the numa migrate machinery.

First of all, we cannot avoid hinting fault. It's because no one knows
which node a task eventually runs on until a hinting fault occurs. So
should let it go get hinting fault *and then* we can make the decision
if we can migrate the folio or not. Assuming that, IMHO,
should_numa_migrate_memory() is a good place to make it.

Thoughts? Am I missing something?

	Byungchul

> Cheers,
> Phil
> 
> > 
> > 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
> > > 
> > > 
> > 
> > -- 
> > 
> > 
> 
> --