Hello,
On Tue, Nov 09, 2021 at 06:15:33PM +0100, Michal Hocko wrote:
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:
- not present.(nid == NUMA_NO_NODE)
- present, but offline (nid > NUMA_NO_NODE, node_online(nid) == 0, NODE_DATA(nid) == NULL)
- 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.
Sorry, I briefly saw this thread last week but was on jury duty and got sequestered when the fix fell into percpu-vm.c.
I'm also not involved with any hotplug work, so my forgive my limited understanding.
I'm understanding this as a cpu/mem hotplug problem that we're papering over with this fix. Given that, I should be looking to take this out when the proper fix to the hotplug subsystem is added. Is that right?
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.
Percpu has always allocated for_each_possible_cpu(). This means even before a cpu online and corresponding numa node online, we're not allocating on the right node anyway. But to me this just seems like a straight up bug we're papering over as I said above for memoryless node cpu hotplug.
To me, I don't see the importance of hotplug in situations where performance is utmost. But it is not exactly ideal contract wise with percpu. However, the trade off is really halting the system for a period of time for any hotplug even to correctly add/free existing percpu allocations and that doesn't seem great. I need to understand the importance of hotplug and then we can figure out how we can fit that in with the percpu allocator.
Thanks, Dennis