mm: zbud: remove zbud

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.

Remove zbud after z3fold's removal, leaving zsmalloc as the one and only
zpool allocator.  Leave the removal of the zpool API (and its associated
config options) to a followup cleanup after no more allocators show up.

Deprecating zbud for a few cycles before removing it was initially
proposed [2], like z3fold was marked as deprecated for 2 cycles [3]. 
However, Johannes rightfully pointed out that the 2 cycles is too short
for most downstream consumers, and z3fold was deprecated first only as a
courtesy anyway.

[1]https://lore.kernel.org/lkml/CAJD7tkbRF6od-2x_L8-A1QL3=2Ww13sCj4S3i4bNndqF+3+_Vg@mail.gmail.com/
[2]https://lore.kernel.org/lkml/Z5gdnSX5Lv-nfjQL@google.com/
[3]https://lore.kernel.org/lkml/20240904233343.933462-1-yosryahmed@google.com/

Link: https://lkml.kernel.org/r/20250129180633.3501650-3-yosry.ahmed@linux.dev
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: WANG Xuerui <kernel@xen0n.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 0 insertions, 18 deletions

diff --git a/mm/Kconfig b/mm/Kconfig
index 6fa19022c09b..fba9757e5814 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 +139,6 @@ 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
-	help
-	  Use the zbud allocator as the default allocator.
-
 config ZSWAP_ZPOOL_DEFAULT_ZSMALLOC
 	bool "zsmalloc"
 	select ZSMALLOC
@@ -156,20 +149,9 @@ endchoice
 config ZSWAP_ZPOOL_DEFAULT
        string
        depends on ZSWAP
-       default "zbud" if ZSWAP_ZPOOL_DEFAULT_ZBUD
        default "zsmalloc" if ZSWAP_ZPOOL_DEFAULT_ZSMALLOC
        default ""
 
-config ZBUD
-	tristate "2:1 compression allocator (zbud)"
-	depends on ZSWAP
-	help
-	  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 ZSMALLOC
 	tristate
 	prompt "N:1 compression allocator (zsmalloc)" if (ZSWAP || ZRAM)