Re: [FIX PATCH v0] powerpc: Fix memory unplug failure on radix guest

Bharata B Rao [off-list ref] writes:

For a PowerKVM guest, it is possible to specify a DIMM device in
addition to the system RAM at boot time. When such a cold plugged DIMM
device is removed from a radix guest, we hit the following warning in the
guest kernel resulting in the eventual failure of memory unplug:

remove_pud_table: unaligned range
WARNING: CPU: 3 PID: 164 at arch/powerpc/mm/pgtable-radix.c:597 remove_pagetable+0x468/0xca0
Call Trace:
remove_pagetable+0x464/0xca0 (unreliable)
radix__remove_section_mapping+0x24/0x40
remove_section_mapping+0x28/0x60
arch_remove_memory+0xcc/0x120
remove_memory+0x1ac/0x270
dlpar_remove_lmb+0x1ac/0x210
dlpar_memory+0xbc4/0xeb0
pseries_hp_work_fn+0x1a4/0x230
process_one_work+0x1cc/0x660
worker_thread+0xac/0x6d0
kthread+0x16c/0x1b0
ret_from_kernel_thread+0x5c/0x74

The DIMM memory that is cold plugged gets merged to the same memblock
region as RAM and hence gets mapped at 1G alignment. However since the
removal is done for one LMB (lmb size 256MB) at a time, the address
of the LMB (which is 256MB aligned) would get flagged as unaligned
in remove_pud_table() resulting in the above failure.

This problem is not seen for hot plugged memory because for the
hot plugged memory, the mappings are created separately for each
LMB and hence they all get aligned at 256MB.

To fix this problem for the cold plugged memory, let us mark the
cold plugged memblock region explicitly as HOTPLUGGED so that the
region doesn't get merged with RAM. All the memory that is discovered
via ibm,dynamic-memory-configuration is marked so(1). Next identify
such regions in radix_init_pgtable() and create separate mappings
within that region for each LMB so that they get don't get aligned
like RAM region at 1G (2).

(1) For PowerKVM guests, all boot time memory is represented via
memory@XXXX nodes and hot plugged/pluggable memory is represented via
ibm,dynamic-memory-reconfiguration property. We are marking all
hotplugged memory that is in ASSIGNED state during boot as HOTPLUGGED.
With this only cold plugged memory gets marked for PowerKVM but
need to check how this will affect PowerVM guests.

Can you verify this on PowerVM too ? ie we should in most case not find
anything under ibm,dynamic-memory-reconfiguration ?


-aneesh