[PATCH] powerpc/mobility: Fix node detach/rename problem

Michael Bringmann mwb at linux.vnet.ibm.com
Tue Jul 31 07:02:54 AEST 2018

See below.

On 07/30/2018 01:31 AM, Michael Ellerman wrote:
> Michael Bringmann <mwb at linux.vnet.ibm.com> writes:
>> During LPAR migration, the content of the device tree/sysfs may
>> be updated including deletion and replacement of nodes in the
>> tree.  When nodes are added to the internal node structures, they
>> are appended in FIFO order to a list of nodes maintained by the
>> OF code APIs.
> That hasn't been true for several years. The data structure is an n-ary
> tree. What kernel version are you working on?

Sorry for an error in my description.  I oversimplified based on the
name of a search iterator.  Let me try to provide a better explanation
of the problem, here.

This is the problem.  The PPC mobility code receives RTAS requests to
delete nodes with platform-/hardware-specific attributes when restarting
the kernel after a migration.  My example is for migration between a
P8 Alpine and a P8 Brazos.   Nodes to be deleted may include 'ibm,random-v1',
'ibm,compression-v1', 'ibm,platform-facilities', 'ibm,sym-encryption-v1',
or others.

The mobility.c code calls 'of_detach_node' for the nodes and their children.
This makes calls to detach the properties and to try to remove the associated
sysfs/kernfs files.

Then new copies of the same nodes are next provided by the PHYP, local
copies are built, and a pointer to the 'struct device_node' is passed to
of_attach_node.  Before the call to of_attach_node, the phandle is initialized
to 0 when the data structure is alloced.  During the call to of_attach_node,
it calls __of_attach_node which pulls the actual name and phandle from just
created sub-properties named something like 'name' and 'ibm,phandle'.

This is all fine for the first migration.  The problem occurs with the
second and subsequent migrations when the PHYP on the new system wants to
replace the same set of nodes again, referenced with the same names and
phandle values.

>> When nodes are removed from the device tree, they
>> are marked OF_DETACHED, but not actually deleted from the system
>> to allow for pointers cached elsewhere in the kernel.  The order
>> and content of the entries in the list of nodes is not altered,
>> though.
> Something is going wrong if this is actually happening.
> When the node is detached it should be *detached* from the tree of all
> nodes, so it should not be discoverable other than by having an existing
> pointer to it.
On the second and subsequent migrations, the PHYP tells the system
to again delete the nodes 'ibm,platform-facilities', 'ibm,random-v1',
'ibm,compression-v1', 'ibm,sym-encryption-v1'.  It specifies these
nodes by its known set of phandle values -- the same handles used
by the PHYP on the source system are known on the target system.
The mobility.c code calls of_find_node_by_phandle() with these values
and ends up locating the first instance of each node that was added
during the original boot, instead of the second instance of each node
created after the first migration.  The detach during the second
migration fails with errors like,

[ 4565.030704] WARNING: CPU: 3 PID: 4787 at drivers/of/dynamic.c:252 __of_detach_node+0x8/0xa0
[ 4565.030708] Modules linked in: nfsv3 nfs_acl nfs tcp_diag udp_diag inet_diag unix_diag af_packet_diag netlink_diag lockd grace fscache sunrpc xts vmx_crypto sg pseries_rng binfmt_misc ip_tables xfs libcrc32c sd_mod ibmveth ibmvscsi scsi_transport_srp dm_mirror dm_region_hash dm_log dm_mod
[ 4565.030733] CPU: 3 PID: 4787 Comm: drmgr Tainted: G        W         4.18.0-rc1-wi107836-v05-120+ #201
[ 4565.030737] NIP:  c0000000007c1ea8 LR: c0000000007c1fb4 CTR: 0000000000655170
[ 4565.030741] REGS: c0000003f302b690 TRAP: 0700   Tainted: G        W          (4.18.0-rc1-wi107836-v05-120+)
[ 4565.030745] MSR:  800000010282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE,TM[E]>  CR: 22288822  XER: 0000000a
[ 4565.030757] CFAR: c0000000007c1fb0 IRQMASK: 1
[ 4565.030757] GPR00: c0000000007c1fa4 c0000003f302b910 c00000000114bf00 c0000003ffff8e68
[ 4565.030757] GPR04: 0000000000000001 ffffffffffffffff 800000c008e0b4b8 ffffffffffffffff
[ 4565.030757] GPR08: 0000000000000000 0000000000000001 0000000080000003 0000000000002843
[ 4565.030757] GPR12: 0000000000008800 c00000001ec9ae00 0000000040000000 0000000000000000
[ 4565.030757] GPR16: 0000000000000000 0000000000000008 0000000000000000 00000000f6ffffff
[ 4565.030757] GPR20: 0000000000000007 0000000000000000 c0000003e9f1f034 0000000000000001
[ 4565.030757] GPR24: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[ 4565.030757] GPR28: c000000001549d28 c000000001134828 c0000003ffff8e68 c0000003f302b930
[ 4565.030804] NIP [c0000000007c1ea8] __of_detach_node+0x8/0xa0
[ 4565.030808] LR [c0000000007c1fb4] of_detach_node+0x74/0xd0
[ 4565.030811] Call Trace:
[ 4565.030815] [c0000003f302b910] [c0000000007c1fa4] of_detach_node+0x64/0xd0 (unreliable)
[ 4565.030821] [c0000003f302b980] [c0000000000c33c4] dlpar_detach_node+0xb4/0x150
[ 4565.030826] [c0000003f302ba10] [c0000000000c3ffc] delete_dt_node+0x3c/0x80
[ 4565.030831] [c0000003f302ba40] [c0000000000c4380] pseries_devicetree_update+0x150/0x4f0
[ 4565.030836] [c0000003f302bb70] [c0000000000c479c] post_mobility_fixup+0x7c/0xf0
[ 4565.030841] [c0000003f302bbe0] [c0000000000c4908] migration_store+0xf8/0x130
[ 4565.030847] [c0000003f302bc70] [c000000000998160] kobj_attr_store+0x30/0x60
[ 4565.030852] [c0000003f302bc90] [c000000000412f14] sysfs_kf_write+0x64/0xa0
[ 4565.030857] [c0000003f302bcb0] [c000000000411cac] kernfs_fop_write+0x16c/0x240
[ 4565.030862] [c0000003f302bd00] [c000000000355f20] __vfs_write+0x40/0x220
[ 4565.030867] [c0000003f302bd90] [c000000000356358] vfs_write+0xc8/0x240
[ 4565.030872] [c0000003f302bde0] [c0000000003566cc] ksys_write+0x5c/0x100
[ 4565.030880] [c0000003f302be30] [c00000000000b288] system_call+0x5c/0x70
[ 4565.030884] Instruction dump:
[ 4565.030887] 38210070 38600000 e8010010 eb61ffd8 eb81ffe0 eba1ffe8 ebc1fff0 ebe1fff8
[ 4565.030895] 7c0803a6 4e800020 e9230098 7929f7e2 <0b090000> 2f890000 4cde0020 e9030040
[ 4565.030903] ---[ end trace 5bd54cb1df9d2976 ]---

The mobility.c code continues on during the second migration, accepts the
definitions of the new nodes from the PHYP and ends up renaming the new
properties e.g.

[ 4565.827296] Duplicate name in base, renamed to "ibm,platform-facilities#1"

I don't see any check like 'of_node_check_flag(np, OF_DETACHED)' within
of_find_node_by_phandle to skip nodes that are detached, but still present
due to caching or use count considerations.  Another possibility to consider
is that of_find_node_by_phandle also uses something called 'phandle_cache'
which may have outdated data as of_detach_node() does not have access to
that cache for the 'OF_DETACHED' nodes.

> That's what __of_detach_node() does:
> 	parent = np->parent;
> 	if (WARN_ON(!parent))
> 		return;
> 	if (parent->child == np)
> 		parent->child = np->sibling;
> 	else {
> 		struct device_node *prevsib;
> 		for (prevsib = np->parent->child;
> 		     prevsib->sibling != np;
> 		     prevsib = prevsib->sibling)
> 			;
> 		prevsib->sibling = np->sibling;
> 	}
> ie. the node must already have a NULL parent, and then it is spliced out
> of its parent's child list.
> Please give us more info so we can work out what's actually happening.

I duplicated the problem in the 4.18 raw kernel on the second and subsequent
legs of migration of an LPAR back and forth between an Alpine to a Brazos.
Again, entries like  'ibm,random-v1', 'ibm,compression-v1',
'ibm,platform-facilities', 'ibm,sym-encryption-v1', yield errors as described

My patch was attempting to resolve the issue wholly within the powerpc code,
at this time.

> cheers


Michael W. Bringmann
Linux Technology Center
IBM Corporation
Tie-Line  363-5196
External: (512) 286-5196
Cell:       (512) 466-0650
mwb at linux.vnet.ibm.com

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