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

[Chris Down <chris@chrisdown.name>]

Yosry Ahmed writes:
>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>

Acked-by: Chris Down <chris@chrisdown.name>

Thanks. We are definitely hurting users as well by keeping this around with its 
known issues when writeback is disabled, for example.

>---
>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
>
>