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

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Message-ID: <Z5j5K6DqU5JnltyX@google.com>
Date: Tue, 28 Jan 2025 15:35:07 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Vitaly Wool <vitaly.wool@...sulko.com>
Cc: Andrew Morton <akpm@...ux-foundation.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>,
	Seth Jennings <sjenning@...hat.com>,
	Dan Streetman <ddstreet@...e.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 Tue, Jan 28, 2025 at 11:29:07AM +0100, Vitaly Wool wrote:
> Hi Yosry,
> 
> On Tue, Jan 28, 2025 at 12:58 AM Yosry Ahmed <yosry.ahmed@...ux.dev> 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>
> >
> 
> Let me disagree here. The footprint for zbud and the ability to work in
> MMU-less setups make it self sufficient and I don't support its deprecation.
> I can give a formal NAK if that's required.

What's the advtange of the ability to work on MMU-less setups when
CONFIG_SWAP itself depends on MMU? Please elaborate.

I agree that zbud has a smaller footprint, but I don't think that's
enough reason to keep it around if it's not being used, especially that
its memory utilization is really low.