linux-kernel - Re: [PATCH v2] mm: z3fold: deprecate CONFIG

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Message-ID: <20240905100557.GA2748692@cmpxchg.org>
Date: Thu, 5 Sep 2024 06:05:57 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Vitaly Wool <vitaly.wool@...sulko.com>,
	Miaohe Lin <linmiaohe@...wei.com>, Nhat Pham <nphamcs@...il.com>,
	Huacai Chen <chenhuacai@...nel.org>,
	WANG Xuerui <kernel@...0n.name>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	"Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Chris Down <chris@...isdown.name>, loongarch@...ts.linux.dev,
	linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: z3fold: deprecate CONFIG_Z3FOLD

On Wed, Sep 04, 2024 at 11:33:43PM +0000, Yosry Ahmed wrote:
> The z3fold compressed pages allocator is rarely used, most users use
> zsmalloc. The only disadvantage of zsmalloc in comparison is the
> dependency on MMU, and zbud is a more common option for !MMU as it was
> the default zswap allocator for a long time.
> 
> Historically, zsmalloc had 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]. That analysis showed that z3fold does not
> have any advantage over zsmalloc or zbud considering both performance
> and memory usage. In a kernel build test on tmpfs in a limited cgroup,
> z3fold took 3% more time and used 1.8% more memory. The latency of
> zswap_load() was 7% higher, and that of zswap_store() was 10% higher.
> Zsmalloc is better in all metrics.
> 
> Moreover, z3fold apparently has latent bugs, which was made noticeable
> by a recent soft lockup bug report with z3fold [2]. Switching to
> zsmalloc not only fixed the problem, but also reduced the swap usage
> from 6~8G to 1~2G. Other users have also reported being bitten by
> mistakenly enabling z3fold.
> 
> Other than hurting users, z3fold is repeatedly causing wasted
> engineering effort. Apart from investigating the above bug, it came up
> in multiple development discussions (e.g. [3]) as something we need to
> handle, when there aren't any legit users (at least not intentionally).
> 
> The natural course of action is to deprecate z3fold, and remove in a few
> cycles if no objections are raised from active users. Next on the list
> should be zbud, as it offers marginal latency gains at the cost of huge
> memory waste when compared to zsmalloc. That one will need to wait until
> zsmalloc does not depend on MMU.
> 
> Rename the user-visible config option from CONFIG_Z3FOLD to
> CONFIG_Z3FOLD_DEPRECATED so that users with CONFIG_Z3FOLD=y get a new
> prompt with explanation during make oldconfig. Also, remove
> CONFIG_Z3FOLD=y from defconfigs.
> 
> [1]https://lore.kernel.org/lkml/CAJD7tkbRF6od-2x_L8-A1QL3=2Ww13sCj4S3i4bNndqF+3+_Vg@mail.gmail.com/
> [2]https://lore.kernel.org/lkml/EF0ABD3E-A239-4111-A8AB-5C442E759CF3@gmail.com/
> [3]https://lore.kernel.org/lkml/CAJD7tkbnmeVugfunffSovJf9FAgy9rhBVt_tx=nxUveLUfqVsA@mail.gmail.com/
> 
> Acked-by: Chris Down <chris@...isdown.name>
> Acked-by: Nhat Pham <nphamcs@...il.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>

zsmalloc's CONFIG_MMU requirement was a concern in the past, but
z3fold appears so bitrotted at this point that it's simply not a
usable option anymore.

> I think it should actually be fine to remove z3fold without deprecating
> it first, but I am doing the due diligence.

Yeah, you never know for sure if users exist. Deprecating it for a few
cycles is the safer option.

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