Re: [PATCH v3] mm: fix panic in __alloc_pages

[Michal Hocko <mhocko@suse.com>]

On Mon 08-11-21 18:08:52, Eric Dumazet wrote:
> 
> 
> On 11/8/21 12:23 PM, Alexey Makhalov wrote:
> > There is a kernel panic caused by pcpu_alloc_pages() passing
> > offlined and uninitialized node to alloc_pages_node() leading
> > to panic by NULL dereferencing uninitialized NODE_DATA(nid).
> > 
> >  CPU2 has been hot-added
> >  BUG: unable to handle page fault for address: 0000000000001608
> >  #PF: supervisor read access in kernel mode
> >  #PF: error_code(0x0000) - not-present page
> >  PGD 0 P4D 0
> >  Oops: 0000 [#1] SMP PTI
> >  CPU: 0 PID: 1 Comm: systemd Tainted: G            E     5.15.0-rc7+ #11
> >  Hardware name: VMware, Inc. VMware7,1/440BX Desktop Reference Platform, BIOS VMW
> > 
> >  RIP: 0010:__alloc_pages+0x127/0x290
> >  Code: 4c 89 f0 5b 41 5c 41 5d 41 5e 41 5f 5d c3 44 89 e0 48 8b 55 b8 c1 e8 0c 83 e0 01 88 45 d0 4c 89 c8 48 85 d2 0f 85 1a 01 00 00 <45> 3b 41 08 0f 82 10 01 00 00 48 89 45 c0 48 8b 00 44 89 e2 81 e2
> >  RSP: 0018:ffffc900006f3bc8 EFLAGS: 00010246
> >  RAX: 0000000000001600 RBX: 0000000000000000 RCX: 0000000000000000
> >  RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000cc2
> >  RBP: ffffc900006f3c18 R08: 0000000000000001 R09: 0000000000001600
> >  R10: ffffc900006f3a40 R11: ffff88813c9fffe8 R12: 0000000000000cc2
> >  R13: 0000000000000000 R14: 0000000000000001 R15: 0000000000000cc2
> >  FS:  00007f27ead70500(0000) GS:ffff88807ce00000(0000) knlGS:0000000000000000
> >  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >  CR2: 0000000000001608 CR3: 000000000582c003 CR4: 00000000001706b0
> >  Call Trace:
> >   pcpu_alloc_pages.constprop.0+0xe4/0x1c0
> >   pcpu_populate_chunk+0x33/0xb0
> >   pcpu_alloc+0x4d3/0x6f0
> >   __alloc_percpu_gfp+0xd/0x10
> >   alloc_mem_cgroup_per_node_info+0x54/0xb0
> >   mem_cgroup_alloc+0xed/0x2f0
> >   mem_cgroup_css_alloc+0x33/0x2f0
> >   css_create+0x3a/0x1f0
> >   cgroup_apply_control_enable+0x12b/0x150
> >   cgroup_mkdir+0xdd/0x110
> >   kernfs_iop_mkdir+0x4f/0x80
> >   vfs_mkdir+0x178/0x230
> >   do_mkdirat+0xfd/0x120
> >   __x64_sys_mkdir+0x47/0x70
> >   ? syscall_exit_to_user_mode+0x21/0x50
> >   do_syscall_64+0x43/0x90
> >   entry_SYSCALL_64_after_hwframe+0x44/0xae
> > 
> > Panic can be easily reproduced by disabling udev rule for
> > automatic onlining hot added CPU followed by CPU with
> > memoryless node (NUMA node with CPU only) hot add.
> > 
> > Hot adding CPU and memoryless node does not bring the node
> > to online state. Memoryless node will be onlined only during
> > the onlining its CPU.
> > 
> > Node can be in one of the following states:
> > 1. not present.(nid == NUMA_NO_NODE)
> > 2. present, but offline (nid > NUMA_NO_NODE, node_online(nid) == 0,
> > 				NODE_DATA(nid) == NULL)
> > 3. present and online (nid > NUMA_NO_NODE, node_online(nid) > 0,
> > 				NODE_DATA(nid) != NULL)
> > 
> > Percpu code is doing allocations for all possible CPUs. The
> > issue happens when it serves hot added but not yet onlined
> > CPU when its node is in 2nd state. This node is not ready
> > to use, fallback to numa_mem_id().
> > 
> > Signed-off-by: Alexey Makhalov <amakhalov@vmware.com>
> > Reviewed-by: David Hildenbrand <david@redhat.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Oscar Salvador <osalvador@suse.de>
> > Cc: Dennis Zhou <dennis@kernel.org>
> > Cc: Tejun Heo <tj@kernel.org>
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: linux-mm@kvack.org
> > Cc: linux-kernel@vger.kernel.org
> > Cc: stable@vger.kernel.org
> > ---
> >  mm/percpu-vm.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/percpu-vm.c b/mm/percpu-vm.c
> > index 2054c9213..f58d73c92 100644
> > --- a/mm/percpu-vm.c
> > +++ b/mm/percpu-vm.c
> > @@ -84,15 +84,19 @@ static int pcpu_alloc_pages(struct pcpu_chunk *chunk,
> >  			    gfp_t gfp)
> >  {
> >  	unsigned int cpu, tcpu;
> > -	int i;
> > +	int i, nid;
> >  
> >  	gfp |= __GFP_HIGHMEM;
> >  
> >  	for_each_possible_cpu(cpu) {
> > +		nid = cpu_to_node(cpu);
> > +		if (nid == NUMA_NO_NODE || !node_online(nid))
> > +			nid = numa_mem_id();
> 
> Maybe we should fail this fallback if (gfp & __GFP_THISNODE) ?
> 
> Or maybe there is no support for this constraint in per-cpu allocator anyway.

I would be really curious about the usecase. Not to mention that pcp
allocation would be effectively unusable on any setups with memory less
nodes.

> I am a bit worried that we do not really know if pages are
> allocated on the right node or not.

There hasn't been any guarantee like that. Page allocator would fallback
to other nodes (in the node distance order) unless __GFP_THISNODE is
specified. This patch just papers over the fact that currently we can
end up having an invalid numa node associated with a cpu. This is a bug
in the initialization code. Even if that is fixed the node fallback is
still a real thing that might happen.

-- 
Michal Hocko
SUSE Labs