On Thu, Sep 5, 2024 at 1:33 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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@xxxxxxxxxxxxxx/ > [2]https://lore.kernel.org/lkml/EF0ABD3E-A239-4111-A8AB-5C442E759CF3@xxxxxxxxx/ > [3]https://lore.kernel.org/lkml/CAJD7tkbnmeVugfunffSovJf9FAgy9rhBVt_tx=nxUveLUfqVsA@xxxxxxxxxxxxxx/ > > Acked-by: Chris Down <chris@xxxxxxxxxxxxxx> > Acked-by: Nhat Pham <nphamcs@xxxxxxxxx> > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> Acked-by: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx> > --- > > 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@xxxxxxxxxx/ > 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@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 > -- > 2.46.0.469.g59c65b2a67-goog >
