The patch titled
Subject: mm: z3fold: deprecate CONFIG_Z3FOLD
has been added to the -mm mm-unstable branch. Its filename is
mm-z3fold-deprecate-config_z3fold.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-z3fold-deprecate-config_z3fold.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Subject: mm: z3fold: deprecate CONFIG_Z3FOLD
Date: Wed, 4 Sep 2024 23:33:43 +0000
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@xxxxxxxxxxxxxx/
[2]https://lore.kernel.org/lkml/EF0ABD3E-A239-4111-A8AB-5C442E759CF3@xxxxxxxxx/
[3]https://lore.kernel.org/lkml/CAJD7tkbnmeVugfunffSovJf9FAgy9rhBVt_tx=nxUveLUfqVsA@xxxxxxxxxxxxxx/
Link: https://lkml.kernel.org/r/20240904233343.933462-1-yosryahmed@xxxxxxxxxx
Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Acked-by: Chris Down <chris@xxxxxxxxxxxxxx>
Acked-by: Nhat Pham <nphamcs@xxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx>
Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxx>
Cc: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Huacai Chen <chenhuacai@xxxxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Cc: WANG Xuerui <kernel@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/loongarch/configs/loongson3_defconfig | 1 -
arch/powerpc/configs/ppc64_defconfig | 1 -
mm/Kconfig | 14 ++++++++++++--
3 files changed, 12 insertions(+), 4 deletions(-)
--- a/arch/loongarch/configs/loongson3_defconfig~mm-z3fold-deprecate-config_z3fold
+++ a/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
--- a/arch/powerpc/configs/ppc64_defconfig~mm-z3fold-deprecate-config_z3fold
+++ a/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
--- a/mm/Kconfig~mm-z3fold-deprecate-config_z3fold
+++ a/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@xxxxxxxxx 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 || ZRAM)
_
Patches currently in -mm which might be from yosryahmed@xxxxxxxxxx are
mm-page_alloc-fix-missed-updates-of-pgfree-in-free_unref_page-folios.patch
mm-z3fold-deprecate-config_z3fold.patch
