[RFC PATCH] mm: z3fold: rename CONFIG_Z3FOLD to CONFIG_Z3FOLD_DEPRECATED

[Yosry Ahmed <yosryahmed@google.com>]

The z3fold compressed pages allocator is not widely used, most users use
zsmalloc. The only disadvantage of zsmalloc in comparison is the
dependency on MMU, and zbud is a more common option for !MMU as it was
the default zswap allocator for a long time.

In hopes of having a single compressed pages allocator at some point,
and following in the footsteps of SLAB, deprecate z3fold. Rename the
user-visible option so that users with CONFIG_Z3FOLD=y get a new prompt
with explanation during make oldconfig. Remove CONFIG_Z3FOLD=y from
defconfigs.

Existing users, if any, should voice their objections. Otherwise, we can
remove z3fold in a few releases.

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
---
I have limited understanding of Kconfigs. I modelled this after commit
eb07c4f39c3e ("mm/slab: rename CONFIG_SLAB to CONFIG_SLAB_DEPRECATED"),
but one difference is that CONFIG_Z3FOLD is a tristate. I made
CONFIG_Z3FOLD_DEPRECATED a boolean config, and CONFIG_Z3FOLD default y
so that it is on by default if CONFIG_Z3FOLD_DEPRECATED is selected. I
am not sure if that's the correct way to do this.

---
 arch/loongarch/configs/loongson3_defconfig |  1 -
 arch/powerpc/configs/ppc64_defconfig       |  1 -
 mm/Kconfig                                 | 13 +++++++++++--
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/loongarch/configs/loongson3_defconfig b/arch/loongarch/configs/loongson3_defconfig
index 33795e4a5bd63..89b66b6c6a1d5 100644
--- a/arch/loongarch/configs/loongson3_defconfig
+++ b/arch/loongarch/configs/loongson3_defconfig
@@ -85,7 +85,6 @@ CONFIG_ZPOOL=y
 CONFIG_ZSWAP=y
 CONFIG_ZSWAP_COMPRESSOR_DEFAULT_ZSTD=y
 CONFIG_ZBUD=y
-CONFIG_Z3FOLD=y
 CONFIG_ZSMALLOC=m
 # CONFIG_COMPAT_BRK is not set
 CONFIG_MEMORY_HOTPLUG=y
diff --git a/arch/powerpc/configs/ppc64_defconfig b/arch/powerpc/configs/ppc64_defconfig
index 544a65fda77bc..d39284489aa26 100644
--- a/arch/powerpc/configs/ppc64_defconfig
+++ b/arch/powerpc/configs/ppc64_defconfig
@@ -81,7 +81,6 @@ CONFIG_MODULE_SIG_SHA512=y
 CONFIG_PARTITION_ADVANCED=y
 CONFIG_BINFMT_MISC=m
 CONFIG_ZSWAP=y
-CONFIG_Z3FOLD=y
 CONFIG_ZSMALLOC=y
 # CONFIG_SLAB_MERGE_DEFAULT is not set
 CONFIG_SLAB_FREELIST_RANDOM=y
diff --git a/mm/Kconfig b/mm/Kconfig
index 1902cfe4cc4f5..bc6cc97c08349 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -193,15 +193,24 @@ config ZBUD
 	  deterministic reclaim properties that make it preferable to a higher
 	  density approach when reclaim will be used.
 
-config Z3FOLD
-	tristate "3:1 compression allocator (z3fold)"
+config Z3FOLD_DEPRECATED
+	bool "3:1 compression allocator (z3fold) (DEPRECATED)"
 	depends on ZSWAP
 	help
+	  Deprecated and scheduled for removal in a few cycles. If you have
+	  a good reason for using Z3FOLD rather than ZSMALLOC or ZBUD, please
+	  contact linux-mm@kvack.org and the zswap maintainers.
+
 	  A special purpose allocator for storing compressed pages.
 	  It is designed to store up to three compressed pages per physical
 	  page. It is a ZBUD derivative so the simplicity and determinism are
 	  still there.
 
+config Z3FOLD
+	tristate
+	default y
+	depends on Z3FOLD_DEPRECATED
+
 config ZSMALLOC
 	tristate
 	prompt "N:1 compression allocator (zsmalloc)" if ZSWAP
-- 
2.43.0.275.g3460e3d667-goog