Re: [PATCH] mm/slub: fix panic in slab_alloc_node()

[Michal Hocko]

On Tue 27-10-20 15:39:46, Laurent Dufour wrote:
> Le 27/10/2020 à 15:24, Michal Hocko a écrit :
> > [Cc Vlastimil]
> > 
> > On Tue 27-10-20 15:09:26, Laurent Dufour wrote:
> > > While doing memory hot-unplug operation on a PowerPC VM running 1024 CPUs
> > > with 11TB of ram, I hit the following panic:
> > > 
> > > BUG: Kernel NULL pointer dereference on read at 0x00000007
> > > Faulting instruction address: 0xc000000000456048
> > > Oops: Kernel access of bad area, sig: 11 [#2]
> > > LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
> > > Modules linked in: rpadlpar_io rpaphp
> > > CPU: 160 PID: 1 Comm: systemd Tainted: G      D           5.9.0 #1
> > > NIP:  c000000000456048 LR: c000000000455fd4 CTR: c00000000047b350
> > > REGS: c00006028d1b77a0 TRAP: 0300   Tainted: G      D            (5.9.0)
> > > MSR:  8000000000009033 <SF,EE,ME,IR,DR,RI,LE>  CR: 24004228  XER: 00000000
> > > CFAR: c00000000000f1b0 DAR: 0000000000000007 DSISR: 40000000 IRQMASK: 0
> > > GPR00: c000000000455fd4 c00006028d1b7a30 c000000001bec800 0000000000000000
> > > GPR04: 0000000000000dc0 0000000000000000 00000000000374ef c00007c53df99320
> > > GPR08: 000007c53c980000 0000000000000000 000007c53c980000 0000000000000000
> > > GPR12: 0000000000004400 c00000001e8e4400 0000000000000000 0000000000000f6a
> > > GPR16: 0000000000000000 c000000001c25930 c000000001d62528 00000000000000c1
> > > GPR20: c000000001d62538 c00006be469e9000 0000000fffffffe0 c0000000003c0ff8
> > > GPR24: 0000000000000018 0000000000000000 0000000000000dc0 0000000000000000
> > > GPR28: c00007c513755700 c000000001c236a4 c00007bc4001f800 0000000000000001
> > > NIP [c000000000456048] __kmalloc_node+0x108/0x790
> > > LR [c000000000455fd4] __kmalloc_node+0x94/0x790
> > > Call Trace:
> > > [c00006028d1b7a30] [c00007c51af92000] 0xc00007c51af92000 (unreliable)
> > > [c00006028d1b7aa0] [c0000000003c0ff8] kvmalloc_node+0x58/0x110
> > > [c00006028d1b7ae0] [c00000000047b45c] mem_cgroup_css_online+0x10c/0x270
> > > [c00006028d1b7b30] [c000000000241fd8] online_css+0x48/0xd0
> > > [c00006028d1b7b60] [c00000000024af14] cgroup_apply_control_enable+0x2c4/0x470
> > > [c00006028d1b7c40] [c00000000024e838] cgroup_mkdir+0x408/0x5f0
> > > [c00006028d1b7cb0] [c0000000005a4ef0] kernfs_iop_mkdir+0x90/0x100
> > > [c00006028d1b7cf0] [c0000000004b8168] vfs_mkdir+0x138/0x250
> > > [c00006028d1b7d40] [c0000000004baf04] do_mkdirat+0x154/0x1c0
> > > [c00006028d1b7dc0] [c000000000032b38] system_call_exception+0xf8/0x200
> > > [c00006028d1b7e20] [c00000000000c740] system_call_common+0xf0/0x27c
> > > Instruction dump:
> > > e93e0000 e90d0030 39290008 7cc9402a e94d0030 e93e0000 7ce95214 7f89502a
> > > 2fbc0000 419e0018 41920230 e9270010 <89290007> 7f994800 419e0220 7ee6bb78
> > > 
> > > This pointing to the following code:
> > > 
> > > mm/slub.c:2851
> > >          if (unlikely(!object || !node_match(page, node))) {
> > > c000000000456038:       00 00 bc 2f     cmpdi   cr7,r28,0
> > > c00000000045603c:       18 00 9e 41     beq     cr7,c000000000456054 <__kmalloc_node+0x114>
> > > node_match():
> > > mm/slub.c:2491
> > >          if (node != NUMA_NO_NODE && page_to_nid(page) != node)
> > > c000000000456040:       30 02 92 41     beq     cr4,c000000000456270 <__kmalloc_node+0x330>
> > > page_to_nid():
> > > include/linux/mm.h:1294
> > > c000000000456044:       10 00 27 e9     ld      r9,16(r7)
> > > c000000000456048:       07 00 29 89     lbz     r9,7(r9)	<<<< r9 = NULL
> > > node_match():
> > > mm/slub.c:2491
> > > c00000000045604c:       00 48 99 7f     cmpw    cr7,r25,r9
> > > c000000000456050:       20 02 9e 41     beq     cr7,c000000000456270 <__kmalloc_node+0x330>
> > > 
> > > The panic occurred in slab_alloc_node() when checking for the page's node:
> > > 	object = c->freelist;
> > > 	page = c->page;
> > > 	if (unlikely(!object || !node_match(page, node))) {
> > > 		object = __slab_alloc(s, gfpflags, node, addr, c);
> > > 		stat(s, ALLOC_SLOWPATH);
> > > 
> > > The issue is that object is not NULL while page is NULL which is odd but
> > > may happen if the cache flush happened after loading object but before
> > > loading page. Thus checking for the page pointer is required too.
> > 
> > Could you be more specific? I am especially confused how the memory
> > hotplug is involved here. What kind of flush are we talking about?
> 
> This happens when flush_cpu_slab() is called when a memory block is about to
> be offlined, see slab_mem_going_offline_callback() called by the
> MEM_GOING_OFFLINE's callback triggered by offline_pages().

This would be a very valuable information for the changelog. I have to
admit that a more detailed description would help somebody not really
familiar with slub internals like me.

I still fail to see why do we get an inconsistent state though. I
thought that no object is associated with an offlined page so how come
we have an object without any page?

How does this allocation path synchronizes with the offline callback?

> > > In commit 6159d0f5c03e ("mm/slub.c: page is always non-NULL in
> > > node_match()") check on the page pointer has been removed assuming that
> > > page is always valid when it is called. It happens that this is not true in
> > > that particular case, so check for page before calling node_match() here.
> > > 
> > > Fixes: 6159d0f5c03e ("mm/slub.c: page is always non-NULL in node_match()")
> > > Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
> > > Cc: Christoph Lameter <cl@xxxxxxxxx>
> > > Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> > > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > > Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > ---
> > >   mm/slub.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/slub.c b/mm/slub.c
> > > index 8f66de8a5ab3..7dc5c6aaf4b7 100644
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -2852,7 +2852,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
> > >   	object = c->freelist;
> > >   	page = c->page;
> > > -	if (unlikely(!object || !node_match(page, node))) {
> > > +	if (unlikely(!object || !page || !node_match(page, node))) {
> > >   		object = __slab_alloc(s, gfpflags, node, addr, c);
> > >   	} else {
> > >   		void *next_object = get_freepointer_safe(s, object);
> > > -- 
> > > 2.29.1
> > > 
> > 
> 

-- 
Michal Hocko
SUSE Labs