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

[Michal Hocko]

[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?

> 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