mm, slob: rename CONFIG_SLOB to CONFIG_SLOB_DEPRECATED

As explained in [1], we would like to remove SLOB if possible.

- There are no known users that need its somewhat lower memory footprint
so much that they cannot handle SLUB (after some modifications by the
previous patches) instead.

- It is an extra maintenance burden, and a number of features are
incompatible with it.

- It blocks the API improvement of allowing kfree() on objects allocated
via kmem_cache_alloc().

As the first step, rename the CONFIG_SLOB option in the slab allocator
configuration choice to CONFIG_SLOB_DEPRECATED. Add CONFIG_SLOB
depending on CONFIG_SLOB_DEPRECATED as an internal option to avoid code
churn. This will cause existing .config files and defconfigs with
CONFIG_SLOB=y to silently switch to the default (and recommended
replacement) SLUB, while still allowing SLOB to be configured by anyone
that notices and needs it. But those should contact the slab maintainers
and [e-mail] as explained in the updated help. With no valid
objections, the plan is to update the existing defconfigs to SLUB and
remove SLOB in a few cycles.

To make SLUB more suitable replacement for SLOB, a CONFIG_SLUB_TINY
option was introduced to limit SLUB's memory overhead.
There is a number of defconfigs specifying CONFIG_SLOB=y. As part of
this patch, update them to select CONFIG_SLUB and CONFIG_SLUB_TINY.

[1] https://lore.kernel.org/all/[e-mail]/

Cc: Russell King <[e-mail]>
Cc: Aaro Koskinen <[e-mail]>
Cc: Janusz Krzysztofik <[e-mail]>
Cc: Tony Lindgren <[e-mail]>
Cc: Jonas Bonn <[e-mail]>
Cc: Stefan Kristiansson <[e-mail]>
Cc: Stafford Horne <[e-mail]>
Cc: Yoshinori Sato <[e-mail]>
Cc: Rich Felker <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Josh Triplett <[e-mail]>
Cc: Conor Dooley <[e-mail]>
Cc: Damien Le Moal <[e-mail]>
Cc: Christophe Leroy <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Vlastimil Babka <[e-mail]>
Acked-by: Aaro Koskinen <[e-mail]> # OMAP1
Reviewed-by: Damien Le Moal <[e-mail]> # riscv k210
Acked-by: Arnd Bergmann <[e-mail]> # arm
Acked-by: Roman Gushchin <[e-mail]>
Acked-by: Mike Rapoport <[e-mail]>
Reviewed-by: Christoph Lameter <[e-mail]>

arch/arm/configs/clps711x_defconfig

···
 CONFIG_ARCH_P720T=y
 CONFIG_AEABI=y
 # CONFIG_COREDUMP is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 CONFIG_NET=y
 CONFIG_PACKET=y
 CONFIG_UNIX=y

arch/arm/configs/collie_defconfig

···
 CONFIG_FPE_NWFPE=y
 CONFIG_PM=y
 # CONFIG_SWAP is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 CONFIG_NET=y
 CONFIG_PACKET=y
 CONFIG_UNIX=y

arch/arm/configs/multi_v4t_defconfig

···
 CONFIG_JUMP_LABEL=y
 CONFIG_PARTITION_ADVANCED=y
 # CONFIG_COREDUMP is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 CONFIG_MTD=y
 CONFIG_MTD_CMDLINE_PARTS=y
 CONFIG_MTD_BLOCK=y

arch/arm/configs/omap1_defconfig

···
 CONFIG_PARTITION_ADVANCED=y
 CONFIG_BINFMT_MISC=y
 # CONFIG_SWAP is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 # CONFIG_VM_EVENT_COUNTERS is not set
 CONFIG_NET=y
 CONFIG_PACKET=y

arch/arm/configs/pxa_defconfig

···
 CONFIG_LDM_PARTITION=y
 CONFIG_CMDLINE_PARTITION=y
 CONFIG_BINFMT_MISC=y
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 # CONFIG_COMPACTION is not set
 CONFIG_NET=y
 CONFIG_PACKET=y

arch/arm/configs/tct_hammer_defconfig

···
 CONFIG_MODULES=y
 CONFIG_MODULE_UNLOAD=y
 # CONFIG_SWAP is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 CONFIG_NET=y
 CONFIG_PACKET=y
 CONFIG_UNIX=y

arch/arm/configs/xcep_defconfig

···
 CONFIG_MODVERSIONS=y
 CONFIG_MODULE_SRCVERSION_ALL=y
 # CONFIG_BLOCK is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 # CONFIG_COMPAT_BRK is not set
 # CONFIG_VM_EVENT_COUNTERS is not set
 CONFIG_NET=y

arch/openrisc/configs/or1ksim_defconfig

···
 # CONFIG_AIO is not set
 # CONFIG_VM_EVENT_COUNTERS is not set
 # CONFIG_COMPAT_BRK is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 CONFIG_MODULES=y
 # CONFIG_BLOCK is not set
 CONFIG_OPENRISC_BUILTIN_DTB="or1ksim"

arch/openrisc/configs/simple_smp_defconfig

···
 # CONFIG_AIO is not set
 # CONFIG_VM_EVENT_COUNTERS is not set
 # CONFIG_COMPAT_BRK is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 CONFIG_MODULES=y
 # CONFIG_BLOCK is not set
 CONFIG_OPENRISC_BUILTIN_DTB="simple_smp"

arch/riscv/configs/nommu_k210_defconfig

···
 CONFIG_EMBEDDED=y
 # CONFIG_VM_EVENT_COUNTERS is not set
 # CONFIG_COMPAT_BRK is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 # CONFIG_MMU is not set
 CONFIG_SOC_CANAAN=y
 CONFIG_NONPORTABLE=y

arch/riscv/configs/nommu_k210_sdcard_defconfig

···
 CONFIG_EMBEDDED=y
 # CONFIG_VM_EVENT_COUNTERS is not set
 # CONFIG_COMPAT_BRK is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 # CONFIG_MMU is not set
 CONFIG_SOC_CANAAN=y
 CONFIG_NONPORTABLE=y

arch/riscv/configs/nommu_virt_defconfig

···
 # CONFIG_KALLSYMS is not set
 # CONFIG_VM_EVENT_COUNTERS is not set
 # CONFIG_COMPAT_BRK is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 # CONFIG_MMU is not set
 CONFIG_SOC_VIRT=y
 CONFIG_NONPORTABLE=y

arch/sh/configs/rsk7201_defconfig

···
 CONFIG_PID_NS=y
 CONFIG_BLK_DEV_INITRD=y
 # CONFIG_AIO is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 CONFIG_PROFILING=y
 CONFIG_MODULES=y
 # CONFIG_BLK_DEV_BSG is not set

arch/sh/configs/rsk7203_defconfig

···
 CONFIG_PID_NS=y
 CONFIG_BLK_DEV_INITRD=y
 CONFIG_KALLSYMS_ALL=y
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 CONFIG_PROFILING=y
 CONFIG_MODULES=y
 # CONFIG_BLK_DEV_BSG is not set

arch/sh/configs/se7206_defconfig

···
 CONFIG_KALLSYMS_ALL=y
 # CONFIG_ELF_CORE is not set
 # CONFIG_COMPAT_BRK is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 CONFIG_PROFILING=y
 CONFIG_MODULES=y
 CONFIG_MODULE_UNLOAD=y

arch/sh/configs/shmin_defconfig

···
 # CONFIG_FUTEX is not set
 # CONFIG_EPOLL is not set
 # CONFIG_SHMEM is not set
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 # CONFIG_BLK_DEV_BSG is not set
 CONFIG_CPU_SUBTYPE_SH7706=y
 CONFIG_MEMORY_START=0x0c000000

arch/sh/configs/shx3_defconfig

···
 CONFIG_PID_NS=y
 # CONFIG_CC_OPTIMIZE_FOR_SIZE is not set
 CONFIG_KALLSYMS_ALL=y
-CONFIG_SLOB=y
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y
 CONFIG_PROFILING=y
 CONFIG_KPROBES=y
 CONFIG_MODULES=y

kernel/configs/tiny.config

···
 # CONFIG_KERNEL_LZO is not set
 # CONFIG_KERNEL_LZ4 is not set
 # CONFIG_SLAB is not set
-# CONFIG_SLUB is not set
-CONFIG_SLOB=y
+# CONFIG_SLOB_DEPRECATED is not set
+CONFIG_SLUB=y
+CONFIG_SLUB_TINY=y

mm/Kconfig

···
 	   and has enhanced diagnostics. SLUB is the default choice for
 	   a slab allocator.
 
-config SLOB
+config SLOB_DEPRECATED
 	depends on EXPERT
-	bool "SLOB (Simple Allocator)"
+	bool "SLOB (Simple Allocator - DEPRECATED)"
 	depends on !PREEMPT_RT
 	help
+	   Deprecated and scheduled for removal in a few cycles. SLUB
+	   recommended as replacement. CONFIG_SLUB_TINY can be considered
+	   on systems with 16MB or less RAM.
+
+	   If you need SLOB to stay, please contact [e-mail] and
+	   people listed in the SLAB ALLOCATOR section of MAINTAINERS file,
+	   with your use case.
+
 	   SLOB replaces the stock allocator with a drastically simpler
 	   allocator. SLOB is generally more space efficient but
 	   does not perform as well on large systems.
 
 endchoice
+
+config SLOB
+	bool
+	default y
+	depends on SLOB_DEPRECATED
 
 config SLUB_TINY
 	bool "Configure SLUB for minimal memory footprint"