linux-kernel - Re: [PATCH 1/2] mm: zbud: deprecate CONFIG

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Message-ID: <b2txiy4sggqlmvdunv4xacciu2klcgfdzq3rp4y3vzbo27ugsq@few2vub23zmj>
Date: Tue, 28 Jan 2025 08:57:39 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Vitaly Wool <vitaly.wool@...sulko.com>, Miaohe Lin <linmiaohe@...wei.com>, 
	Johannes Weiner <hannes@...xchg.org>, Nhat Pham <nphamcs@...il.com>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, Huacai Chen <chenhuacai@...nel.org>, 
	WANG Xuerui <kernel@...0n.name>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	loongarch@...ts.linux.dev
Subject: Re: [PATCH 1/2] mm: zbud: deprecate CONFIG_ZBUD

On Mon, Jan 27, 2025 at 11:58:21PM +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.
> 
> 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@...ux.dev>

Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>