On Wed, Jan 29, 2025 at 06:06:32PM +0000, Yosry Ahmed 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.

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/ (local)
[2]https://lore.kernel.org/lkml/Z5gdnSX5Lv-nfjQL@google.com/ (local)
[3]https://lore.kernel.org/lkml/20240904233343.933462-1-yosryahmed@google.com/ (local)

Signed-off-by: Yosry Ahmed <redacted>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>