linux-kernel - Re: [PATCH v2 2/2] mm: zbud: remove zbud

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Message-ID: <63bf6cenq2sg55jorlvwtstyt6xrsi3yayidh3uvdtyib3flbh@ni2ezwfvpggl>
Date: Wed, 29 Jan 2025 11:26:11 -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>, Seth Jennings <sjenning@...hat.com>, 
	Dan Streetman <ddstreet@...e.org>, 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>, Heiko Carstens <hca@...ux.ibm.com>, 
	Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev <agordeev@...ux.ibm.com>, 
	Christian Borntraeger <borntraeger@...ux.ibm.com>, Sven Schnelle <svens@...ux.ibm.com>, 
	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	loongarch@...ts.linux.dev, linux-s390@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm: zbud: remove zbud

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@...ux.dev>

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