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Message-ID: <20240904233343.933462-1-yosryahmed@google.com> Date: Wed, 4 Sep 2024 23:33:43 +0000 From: Yosry Ahmed <yosryahmed@...gle.com> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Vitaly Wool <vitaly.wool@...sulko.com>, Miaohe Lin <linmiaohe@...wei.com>, Johannes Weiner <hannes@...xchg.org>, 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, Yosry Ahmed <yosryahmed@...gle.com> Subject: [PATCH v2] mm: z3fold: deprecate CONFIG_Z3FOLD 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> --- I think it should actually be fine to remove z3fold without deprecating it first, but I am doing the due diligence. v1: https://lore.kernel.org/linux-mm/20240112193103.3798287-1-yosryahmed@google.com/ v1 -> v2: - Make CONFIG_Z3FOLD_DEPRECATED a tristate option to match CONFIG_Z3FOLD. - Update commit subject and log. --- arch/loongarch/configs/loongson3_defconfig | 1 - arch/powerpc/configs/ppc64_defconfig | 1 - mm/Kconfig | 14 ++++++++++++-- 3 files changed, 12 insertions(+), 4 deletions(-) diff --git a/arch/loongarch/configs/loongson3_defconfig b/arch/loongarch/configs/loongson3_defconfig index b4252c357c8e2..75b366407a60a 100644 --- a/arch/loongarch/configs/loongson3_defconfig +++ b/arch/loongarch/configs/loongson3_defconfig @@ -96,7 +96,6 @@ CONFIG_ZPOOL=y CONFIG_ZSWAP=y CONFIG_ZSWAP_COMPRESSOR_DEFAULT_ZSTD=y CONFIG_ZBUD=y -CONFIG_Z3FOLD=y CONFIG_ZSMALLOC=m # CONFIG_COMPAT_BRK is not set CONFIG_MEMORY_HOTPLUG=y diff --git a/arch/powerpc/configs/ppc64_defconfig b/arch/powerpc/configs/ppc64_defconfig index 544a65fda77bc..d39284489aa26 100644 --- a/arch/powerpc/configs/ppc64_defconfig +++ b/arch/powerpc/configs/ppc64_defconfig @@ -81,7 +81,6 @@ CONFIG_MODULE_SIG_SHA512=y CONFIG_PARTITION_ADVANCED=y CONFIG_BINFMT_MISC=m CONFIG_ZSWAP=y -CONFIG_Z3FOLD=y CONFIG_ZSMALLOC=y # CONFIG_SLAB_MERGE_DEFAULT is not set CONFIG_SLAB_FREELIST_RANDOM=y diff --git a/mm/Kconfig b/mm/Kconfig index b23913d4e47e2..536679d726417 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -177,15 +177,25 @@ config ZBUD deterministic reclaim properties that make it preferable to a higher density approach when reclaim will be used. -config Z3FOLD - tristate "3:1 compression allocator (z3fold)" +config Z3FOLD_DEPRECATED + tristate "3:1 compression allocator (z3fold) (DEPRECATED)" depends on ZSWAP help + Deprecated and scheduled for removal in a few cycles. If you have + a good reason for using Z3FOLD over ZSMALLOC, please contact + linux-mm@...ck.org and the zswap maintainers. + A special purpose allocator for storing compressed pages. It is designed to store up to three compressed pages per physical page. It is a ZBUD derivative so the simplicity and determinism are still there. +config Z3FOLD + tristate + default y if Z3FOLD_DEPRECATED=y + default m if Z3FOLD_DEPRECATED=m + depends on Z3FOLD_DEPRECATED + config ZSMALLOC tristate prompt "N:1 compression allocator (zsmalloc)" if ZSWAP -- 2.46.0.469.g59c65b2a67-goog
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