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... [2]https://lore.kernel.org/lkml/EF0ABD3E-A239-4111-A8AB-5C442E759CF3@gmail.com/ [3]https://lore.kernel.org/lkml/CAJD7tkbnmeVugfunffSovJf9FAgy9rhBVt_tx=nxUveLUf...
[arnd@arndb.de: deprecate ZSWAP_ZPOOL_DEFAULT_Z3FOLD as well] Link: https://lkml.kernel.org/r/20240909202625.1054880-1-arnd@kernel.org Link: https://lkml.kernel.org/r/20240904233343.933462-1-yosryahmed@google.com Signed-off-by: Yosry Ahmed yosryahmed@google.com Signed-off-by: Arnd Bergmann arnd@arndb.de Acked-by: Chris Down chris@chrisdown.name Acked-by: Nhat Pham nphamcs@gmail.com Acked-by: Johannes Weiner hannes@cmpxchg.org Acked-by: Vitaly Wool vitaly.wool@konsulko.com Acked-by: Christoph Hellwig hch@lst.de Cc: Aneesh Kumar K.V aneesh.kumar@kernel.org Cc: Christophe Leroy christophe.leroy@csgroup.eu Cc: Huacai Chen chenhuacai@kernel.org Cc: Miaohe Lin linmiaohe@huawei.com Cc: Michael Ellerman mpe@ellerman.id.au Cc: Naveen N. Rao naveen.n.rao@linux.ibm.com Cc: Nicholas Piggin npiggin@gmail.com Cc: Sergey Senozhatsky senozhatsky@chromium.org Cc: WANG Xuerui kernel@xen0n.name Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org (cherry picked from commit 7a2369b74abf76cd3e54c45b30f6addb497f831b) Signed-off-by: Yosry Ahmed yosryahmed@google.com --- arch/loongarch/configs/loongson3_defconfig | 1 - mm/Kconfig | 25 ++++++++++++++++------ 2 files changed, 19 insertions(+), 7 deletions(-)
diff --git a/arch/loongarch/configs/loongson3_defconfig b/arch/loongarch/configs/loongson3_defconfig index 3540e9c0a6310..15b7420c868ba 100644 --- a/arch/loongarch/configs/loongson3_defconfig +++ b/arch/loongarch/configs/loongson3_defconfig @@ -82,7 +82,6 @@ CONFIG_ZSWAP=y CONFIG_ZSWAP_COMPRESSOR_DEFAULT_ZSTD=y CONFIG_ZPOOL=y CONFIG_ZBUD=y -CONFIG_Z3FOLD=y CONFIG_ZSMALLOC=m CONFIG_NET=y CONFIG_PACKET=y diff --git a/mm/Kconfig b/mm/Kconfig index a65145fe89f2b..cf782e0474910 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -132,12 +132,15 @@ config ZSWAP_ZPOOL_DEFAULT_ZBUD help Use the zbud allocator as the default allocator.
-config ZSWAP_ZPOOL_DEFAULT_Z3FOLD - bool "z3fold" - select Z3FOLD +config ZSWAP_ZPOOL_DEFAULT_Z3FOLD_DEPRECATED + bool "z3foldi (DEPRECATED)" + select Z3FOLD_DEPRECATED help Use the z3fold allocator as the default allocator.
+ Deprecated and scheduled for removal in a few cycles, + see CONFIG_Z3FOLD_DEPRECATED. + config ZSWAP_ZPOOL_DEFAULT_ZSMALLOC bool "zsmalloc" select ZSMALLOC @@ -149,7 +152,7 @@ config ZSWAP_ZPOOL_DEFAULT string depends on ZSWAP default "zbud" if ZSWAP_ZPOOL_DEFAULT_ZBUD - default "z3fold" if ZSWAP_ZPOOL_DEFAULT_Z3FOLD + default "z3fold" if ZSWAP_ZPOOL_DEFAULT_Z3FOLD_DEPRECATED default "zsmalloc" if ZSWAP_ZPOOL_DEFAULT_ZSMALLOC default ""
@@ -163,15 +166,25 @@ 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@kvack.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