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

Dennis Zhou
2021-11-09T19:06:20Z

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:
> > > 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.
>

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