Re: [PATCH v2 2/2] mm: zbud: remove zbud

Johannes Weiner
2025-01-30T16:28:46Z

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/
> [2]https://lore.kernel.org/lkml/Z5gdnSX5Lv-nfjQL@google.com/
> [3]https://lore.kernel.org/lkml/20240904233343.933462-1-yosryahmed@google.com/
>
> Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>

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