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With CONFIG_SLUB=y, following commit 6edf2576 ("mm/slub: enable
debugging memory wasting of kmalloc") mt7621 failed to boot very early,
without showing any console messages.
This exposed the pre-existing bug of mt7621.c using kzalloc before normal
memory management was available.
Prior to this slub change, there existed the unintended protection against
"kmem_cache *s" being NULL as slab_pre_alloc_hook() happened to
return NULL and bailed out of slab_alloc_node().
This allowed mt7621 prom_soc_init to fail in the soc_dev_init kzalloc,
but continue booting without the SOC_BUS driver device registered.

Console output from a DEBUG_ZBOOT vmlinuz kernel loading,
with mm/slub modified to warn on kmem_cache zero or null:

zimage at:     80B842A0 810B4BC0
Uncompressing Linux at load address 80001000
Copy device tree to address  80B80EE0
Now, booting the kernel...

[    0.000000] Linux version 6.1.0-rc3+ (john@john)
(mipsel-buildroot-linux-gnu-gcc.br_real (Buildroot
2021.11-4428-g6b6741b) 12.2.0, GNU ld (GNU Binutils) 2.39) #73 SMP Wed
     Nov  2 05:10:01 AEST 2022
[    0.000000] ------------[ cut here ]------------
[    0.000000] WARNING: CPU: 0 PID: 0 at mm/slub.c:3416
kmem_cache_alloc+0x5a4/0x5e8
[    0.000000] Modules linked in:
[    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 6.1.0-rc3+ #73
[    0.000000] Stack : 810fff78 80084d98 00000000 00000004 00000000
00000000 80889d04 80c90000
[    0.000000]         80920000 807bd328 8089d368 80923bd3 00000000
00000001 80889cb0 00000000
[    0.000000]         00000000 00000000 807bd328 8084bcb1 00000002
00000002 00000001 6d6f4320
[    0.000000]         00000000 80c97d3d 80c97d68 fffffffc 807bd328
00000000 00000000 00000000
[    0.000000]         00000000 a0000000 80910000 8110a0b4 00000000
00000020 80010000 80010000
[    0.000000]         ...
[    0.000000] Call Trace:
[    0.000000] [<80008260>] show_stack+0x28/0xf0
[    0.000000] [<8070c958>] dump_stack_lvl+0x60/0x80
[    0.000000] [<8002e184>] __warn+0xc4/0xf8
[    0.000000] [<8002e210>] warn_slowpath_fmt+0x58/0xa4
[    0.000000] [<801c0fac>] kmem_cache_alloc+0x5a4/0x5e8
[    0.000000] [<8092856c>] prom_soc_init+0x1fc/0x2b4
[    0.000000] [<80928060>] prom_init+0x44/0xf0
[    0.000000] [<80929214>] setup_arch+0x4c/0x6a8
[    0.000000] [<809257e0>] start_kernel+0x88/0x7c0
[    0.000000]
[    0.000000] ---[ end trace 0000000000000000 ]---
[    0.000000] SoC Type: MediaTek MT7621 ver:1 eco:3
[    0.000000] printk: bootconsole [early0] enabled

Allowing soc_device_register to work exposed oops in the mt7621 phy pci,
and pci controller drivers from soc_device_match_attr, due to missing
sentinels in the quirks tables. These were fixed with:
commit 819b885c ("phy: ralink: mt7621-pci: add sentinel to quirks
table")
not yet applied ("PCI: mt7621: add sentinel to quirks table")

Link: https://lore.kernel.org/linux-mm/becf2ac3-2a90-4f3a-96d9-a70f67c66e4a@app.fastmail.com/
Fixes: 71b9b5e0

 ("MIPS: ralink: mt7621: introduce 'soc_device' initialization")
Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>