resource: improve child resource handling in release_mem_region_adjustable()

resource: improve child resource handling in release_mem_region_adjustable()

When memory block is removed via try_remove_memory(), it eventually
reaches release_mem_region_adjustable().  The current implementation
assumes that when a busy memory resource is split into two, all child
resources remain in the lower address range.

This simplification causes problems when child resources actually belong
to the upper split.  For example:

* Initial memory layout:
lsmem
RANGE                                 SIZE   STATE REMOVABLE  BLOCK
0x0000000000000000-0x00000002ffffffff  12G  online       yes   0-95

* /proc/iomem
00000000-2dfefffff : System RAM
  158834000-1597b3fff : Kernel code
  1597b4000-159f50fff : Kernel data
  15a13c000-15a218fff : Kernel bss
2dff00000-2ffefffff : Crash kernel
2fff00000-2ffffffff : System RAM

* After offlining and removing range
  0x150000000-0x157ffffff
lsmem -o RANGE,SIZE,STATE,BLOCK,CONFIGURED
(output according to upcoming lsmem changes with the configured column:
s390)
RANGE                                  SIZE   STATE  BLOCK  CONFIGURED
0x0000000000000000-0x000000014fffffff  5.3G  online   0-41  yes
0x0000000150000000-0x0000000157ffffff  128M offline     42  no
0x0000000158000000-0x00000002ffffffff  6.6G  online  43-95  yes

The iomem resource gets split into two entries, but kernel code, kernel
data, and kernel bss remain attached to the lower resource [0–5376M]
instead of the correct upper resource [5504M–12288M].

As a result, WARN_ON() triggers in release_mem_region_adjustable()
("Usecase: split into two entries - we need a new resource")
------------[ cut here ]------------
WARNING: CPU: 5 PID: 858 at kernel/resource.c:1486
release_mem_region_adjustable+0x210/0x280
Modules linked in:
CPU: 5 UID: 0 PID: 858 Comm: chmem Not tainted 6.17.0-rc2-11707-g2c36aaf3ba4e
Hardware name: IBM 3906 M04 704 (z/VM 7.3.0)
Krnl PSW : 0704d00180000000 0000024ec0dae0e4
           (release_mem_region_adjustable+0x214/0x280)
           R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:1 PM:0 RI:0 EA:3
Krnl GPRS: 0000000000000000 00000002ffffafc0 fffffffffffffff0 0000000000000000
           000000014fffffff 0000024ec2257608 0000000000000000 0000024ec2301758
           0000024ec22680d0 00000000902c9140 0000000150000000 00000002ffffafc0
           000003ffa61d8d18 0000024ec21fb478 0000024ec0dae014 000001cec194fbb0
Krnl Code: 0000024ec0dae0d8: af000000            mc      0,0
           0000024ec0dae0dc: a7f4ffc1            brc     15,0000024ec0dae05e
          #0000024ec0dae0e0: af000000            mc      0,0
          >0000024ec0dae0e4: a5defffd            llilh   %r13,65533
           0000024ec0dae0e8: c04000c6064c        larl    %r4,0000024ec266ed80
           0000024ec0dae0ee: eb1d400000f8        laa     %r1,%r13,0(%r4)
           0000024ec0dae0f4: 07e0                bcr     14,%r0
           0000024ec0dae0f6: a7f4ffc0            brc     15,0000024ec0dae076

 [<0000024ec0dae0e4>] release_mem_region_adjustable+0x214/0x280
([<0000024ec0dadf3c>] release_mem_region_adjustable+0x6c/0x280)
 [<0000024ec10a2130>] try_remove_memory+0x100/0x140
 [<0000024ec10a4052>] __remove_memory+0x22/0x40
 [<0000024ec18890f6>] config_mblock_store+0x326/0x3e0
 [<0000024ec11f7056>] kernfs_fop_write_iter+0x136/0x210
 [<0000024ec1121e86>] vfs_write+0x236/0x3c0
 [<0000024ec11221b8>] ksys_write+0x78/0x110
 [<0000024ec1b6bfbe>] __do_syscall+0x12e/0x350
 [<0000024ec1b782ce>] system_call+0x6e/0x90
Last Breaking-Event-Address:
 [<0000024ec0dae014>] release_mem_region_adjustable+0x144/0x280
---[ end trace 0000000000000000 ]---

Also, resource adjustment doesn't happen and stale resources still cover
[0-12288M].  Later, memory re-add fails in register_memory_resource() with
-EBUSY.

i.e: /proc/iomem is still:
00000000-2dfefffff : System RAM
  158834000-1597b3fff : Kernel code
  1597b4000-159f50fff : Kernel data
  15a13c000-15a218fff : Kernel bss
2dff00000-2ffefffff : Crash kernel
2fff00000-2ffffffff : System RAM

Enhance release_mem_region_adjustable() to reassign child resources to the
correct parent after a split.  Children are now assigned based on their
actual range: If they fall within the lower split, keep them in the lower
parent.  If they fall within the upper split, move them to the upper
parent.

Kernel code/data/bss regions are not offlined, so they will always reside
entirely within one parent and never span across both.

Output after the enhancement:
* Initial state /proc/iomem (before removal of memory block):
00000000-2dfefffff : System RAM
  1f94f8000-1fa477fff : Kernel code
  1fa478000-1fac14fff : Kernel data
  1fae00000-1faedcfff : Kernel bss
2dff00000-2ffefffff : Crash kernel
2fff00000-2ffffffff : System RAM

* Offline and remove 0x1e8000000-0x1efffffff memory range
* /proc/iomem
00000000-1e7ffffff : System RAM
1f0000000-2dfefffff : System RAM
  1f94f8000-1fa477fff : Kernel code
  1fa478000-1fac14fff : Kernel data
  1fae00000-1faedcfff : Kernel bss
2dff00000-2ffefffff : Crash kernel
2fff00000-2ffffffff : System RAM

Link: https://lkml.kernel.org/r/20250912123021.3219980-1-sumanthk@linux.ibm.com
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Andriy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>