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, 4 insertions, 6 deletions

diff --git a/Documentation/admin-guide/mm/zswap.rst b/Documentation/admin-guide/mm/zswap.rst
index 3598dcd7dbe7..fd3370aa43fe 100644
--- a/Documentation/admin-guide/mm/zswap.rst
+++ b/Documentation/admin-guide/mm/zswap.rst
@@ -60,15 +60,13 @@ accessed.  The compressed memory pool grows on demand and shrinks as compressed
 pages are freed.  The pool is not preallocated.  By default, a zpool
 of type selected in ``CONFIG_ZSWAP_ZPOOL_DEFAULT`` Kconfig option is created,
 but it can be overridden at boot time by setting the ``zpool`` attribute,
-e.g. ``zswap.zpool=zbud``. It can also be changed at runtime using the sysfs
+e.g. ``zswap.zpool=zsmalloc``. It can also be changed at runtime using the sysfs
 ``zpool`` attribute, e.g.::
 
-	echo zbud > /sys/module/zswap/parameters/zpool
+	echo zsmalloc > /sys/module/zswap/parameters/zpool
 
-The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which
-means the compression ratio will always be 2:1 or worse (because of half-full
-zbud pages).  The zsmalloc type zpool has a more complex compressed page
-storage method, and it can achieve greater storage densities.
+The zsmalloc type zpool has a complex compressed page storage method, and it
+can achieve great storage densities.
 
 When a swap page is passed from swapout to zswap, zswap maintains a mapping
 of the swap entry, a combination of the swap type and swap offset, to the zpool