Hi Yosry,

On Tue, Jan 28, 2025 at 12:58 AM Yosry Ahmed <yosry.ahmed@linux.dev> wrote:

> The zbud compressed pages allocator is rarely used, most users use
> zsmalloc. zbud consumes much more memory (only stores 1 or 2 compressed
> pages per physical page). The only advantage of zbud is a marginal
> performance improvement that by no means justify the memory overhead.
>
> Historically, zsmalloc had significantly worse latency than zbud and
> z3fold but offered better memory savings.  This is no longer the case as
> shown by a simple recent analysis [1].  In a kernel build test on tmpfs
> in a limited cgroup, zbud 2-3% less time than zsmalloc, but at the cost
> of using ~32% more memory (1.5G vs 1.13G). The tradeoff does not make
> sense for zbud in any practical scenario.
>
> The only alleged advantage of zbud is not having the dependency on
> CONFIG_MMU, but CONFIG_SWAP already depends on CONFIG_MMU anyway, and
> zbud is only used by zswap.
>
> Following in the footsteps of [2], which deprecated z3fold, deprecated
> zbud as planned and remove it in a few cycles if no objections are
> raised from active users.
>
> Rename the user-visible config options so that users with CONFIG_ZBUD=y
> get a new prompt with explanation during make oldconfig. Also, remove
> CONFIG_ZBUD from defconfig.
>
> [1]
> https://lore.kernel.org/lkml/CAJD7tkbRF6od-2x_L8-A1QL3=2Ww13sCj4S3i4bNndqF+3+_Vg@mail.gmail.com/
> [2]
> https://lore.kernel.org/lkml/20240904233343.933462-1-yosryahmed@google.com/
>
> Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
>

Let me disagree here. The footprint for zbud and the ability to work in
MMU-less setups make it self sufficient and I don't support its deprecation.
I can give a formal NAK if that's required.

Thanks,
Vitaly


> ---
>  arch/loongarch/configs/loongson3_defconfig |  3 +--
>  mm/Kconfig                                 | 26 ++++++++++++++++------
>  2 files changed, 20 insertions(+), 9 deletions(-)
>
> diff --git a/arch/loongarch/configs/loongson3_defconfig
> b/arch/loongarch/configs/loongson3_defconfig
> index 73c77500ac467..7ce5beb3cbf34 100644
> --- a/arch/loongarch/configs/loongson3_defconfig
> +++ b/arch/loongarch/configs/loongson3_defconfig
> @@ -109,8 +109,7 @@ CONFIG_BINFMT_MISC=m
>  CONFIG_ZPOOL=y
>  CONFIG_ZSWAP=y
>  CONFIG_ZSWAP_COMPRESSOR_DEFAULT_ZSTD=y
> -CONFIG_ZBUD=y
> -CONFIG_ZSMALLOC=m
> +CONFIG_ZSMALLOC=y
>  # CONFIG_COMPAT_BRK is not set
>  CONFIG_MEMORY_HOTPLUG=y
>  # CONFIG_MHP_DEFAULT_ONLINE_TYPE_OFFLINE is not set
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 1b501db064172..e3e15ab0cba49 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -129,7 +129,6 @@ choice
>         prompt "Default allocator"
>         depends on ZSWAP
>         default ZSWAP_ZPOOL_DEFAULT_ZSMALLOC if MMU
> -       default ZSWAP_ZPOOL_DEFAULT_ZBUD
>         help
>           Selects the default allocator for the compressed cache for
>           swap pages.
> @@ -140,12 +139,15 @@ choice
>           The selection made here can be overridden by using the kernel
>           command line 'zswap.zpool=' option.
>
> -config ZSWAP_ZPOOL_DEFAULT_ZBUD
> -       bool "zbud"
> -       select ZBUD
> +config ZSWAP_ZPOOL_DEFAULT_ZBUD_DEPRECATED
> +       bool "zbud (DEPRECATED)"
> +       select ZBUD_DEPRECATED
>         help
>           Use the zbud allocator as the default allocator.
>
> +         Deprecated and scheduled for removal in a few cycles,
> +         see CONFIG_ZBUD_DEPRECATED.
> +
>  config ZSWAP_ZPOOL_DEFAULT_Z3FOLD_DEPRECATED
>         bool "z3foldi (DEPRECATED)"
>         select Z3FOLD_DEPRECATED
> @@ -165,21 +167,31 @@ endchoice
>  config ZSWAP_ZPOOL_DEFAULT
>         string
>         depends on ZSWAP
> -       default "zbud" if ZSWAP_ZPOOL_DEFAULT_ZBUD
> +       default "zbud" if ZSWAP_ZPOOL_DEFAULT_ZBUD_DEPRECATED
>         default "z3fold" if ZSWAP_ZPOOL_DEFAULT_Z3FOLD_DEPRECATED
>         default "zsmalloc" if ZSWAP_ZPOOL_DEFAULT_ZSMALLOC
>         default ""
>
> -config ZBUD
> -       tristate "2:1 compression allocator (zbud)"
> +config ZBUD_DEPRECATED
> +       tristate "2:1 compression allocator (zbud) (DEPRECATED)"
>         depends on ZSWAP
>         help
> +         Deprecated and scheduled for removal in a few cycles. If you have
> +         a good reason for using ZBUD over ZSMALLOC, 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 two compressed pages per physical
>           page.  While this design limits storage density, it has simple
> and
>           deterministic reclaim properties that make it preferable to a
> higher
>           density approach when reclaim will be used.
>
> +config ZBUD
> +       tristate
> +       default y if ZBUD_DEPRECATED=y
> +       default m if ZBUD_DEPRECATED=m
> +       depends on ZBUD_DEPRECATED
> +
>  config Z3FOLD_DEPRECATED
>         tristate "3:1 compression allocator (z3fold) (DEPRECATED)"
>         depends on ZSWAP
> --
> 2.48.1.262.g85cc9f2d1e-goog
>
>