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Subject: Re: [PATCH] zswap: allow setting default status, compressor and
 allocator in Kconfig
To:     Dan Streetman <ddstreet@ieee.org>
Cc:     Seth Jennings <sjenning@redhat.com>,
        Vitaly Wool <vitalywool@gmail.com>,
        Linux-MM <linux-mm@kvack.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
References: <20191011234038.198995-1-mail@maciej.szmigiero.name>
 <CALZtONCx4L5K7+h3TuS1Ms_q_Mt_5JcZ+XTgACqdDyVsaCS76g@mail.gmail.com>
From:   "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
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Message-ID: <d7e1c0ca-80e9-233a-c4ea-d7fccaf56722@maciej.szmigiero.name>
Date:   Sat, 19 Oct 2019 00:16:43 +0200
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On 18.10.2019 13:19, Dan Streetman wrote:
> On Fri, Oct 11, 2019 at 7:40 PM Maciej S. Szmigiero
> <mail@maciej.szmigiero.name> wrote:
>>
>> The compressed cache for swap pages (zswap) currently needs from 1 to 3
>> extra kernel command line parameters in order to make it work: it has to be
>> enabled by adding a "zswap.enabled=1" command line parameter and if one
>> wants a different compressor or pool allocator than the default lzo / zbud
>> combination then these choices also need to be specified on the kernel
>> command line in additional parameters.
>>
>> Using a different compressor and allocator for zswap is actually pretty
>> common as guides often recommend using the lz4 / z3fold pair instead of
>> the default one.
>> In such case it is also necessary to remember to enable the appropriate
>> compression algorithm and pool allocator in the kernel config manually.
>>
>> Let's avoid the need for adding these kernel command line parameters and
>> automatically pull in the dependencies for the selected compressor
>> algorithm and pool allocator by adding an appropriate default switches to
>> Kconfig.
> 
> Who is the target for using these kernel build-time defaults?  I don't
> think any distribution would be defaulting zswap to enabled, and if
> the config defaults are intended for personal kernel builds, it is
> really so much harder to just configure it on the boot cmdline?

Appropriate kernel config defaults are normally provided for kernel
parameters that are reasonably expected to be configured for normal
operation (and sometimes for debugging parameters, too), so one does not
need to boot kernel with a lot of command line parameters to get the
desired behavior.

A quick, limited grep of Documentation/admin-guide/kernel-parameters.txt
gives me the following config options that control the default values of
command line parameters:
CONFIG_HPET_MMAP_DEFAULT
CONFIG_BOOTPARAM_HUNG_TASK_PANIC
CONFIG_INIT_ON_ALLOC_DEFAULT_ON
CONFIG_INIT_ON_FREE_DEFAULT_ON
CONFIG_IOMMU_DEFAULT_PASSTHROUGH
CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF
CONFIG_LSM
CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE
CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT
CONFIG_RANDOM_TRUST_CPU
CONFIG_SLUB_MEMCG_SYSFS_ON
CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC
CONFIG_SYSFS_DEPRECATED_V2
CONFIG_COMPAT_VDSO
CONFIG_WQ_POWER_EFFICIENT_DEFAULT
CONFIG_XEN_SCRUB_PAGES_DEFAULT

There are countless other ones that set the default values for module
parameters, for example in the "sound" directory alone there are four:
CONFIG_SND_PS3_DEFAULT_START_DELAY
CONFIG_SND_HDA_POWER_SAVE_DEFAULT
CONFIG_SND_AC97_POWER_SAVE_DEFAULT
CONFIG_SND_SEQ_HRTIMER_DEFAULT

Providing such selectable default also means that the dependencies for
the selected compressor algorithm and pool allocator are pulled in
automatically - the old way pulled in the LZO algorithm (and only the
LZO algorithm) into the kernel unconditionally, even if the user was
ultimately going to use another algorithm.

>>
>> The default values for these options match what the code was using
>> previously as its defaults.
>>
>> Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
>> ---
>>  mm/Kconfig | 103 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
>>  mm/zswap.c |  26 ++++++++------
>>  2 files changed, 117 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index a5dae9a7eb51..4309bcaaa29d 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -525,7 +525,6 @@ config MEM_SOFT_DIRTY
>>  config ZSWAP
>>         bool "Compressed cache for swap pages (EXPERIMENTAL)"
>>         depends on FRONTSWAP && CRYPTO=y
>> -       select CRYPTO_LZO
>>         select ZPOOL
>>         help
>>           A lightweight compressed cache for swap pages.  It takes
>> @@ -541,6 +540,108 @@ config ZSWAP
>>           they have not be fully explored on the large set of potential
>>           configurations and workloads that exist.
>>
>> +choice
> 
> Using choice becomes a bit of a maintenence issue...if we add this,
> wouldn't it be better to use string input so new compression algs can
> be added without having to update this Kconfig?

If this is changed to a string prompt we'll lose automatic pulling in of
an appropriate CONFIG_CRYPTO_* dependency.

Other similar options are presented as a choice, too, see for example
CONFIG_KERNEL_{GZIP,BZIP2,LZMA,XZ,LZO,LZ4,UNCOMPRESSED} and
CONFIG_INITRAMFS_COMPRESSION_{NONE,GZIP,BZIP2,LZMA,XZ,LZO,LZ4} (that
maps to an extension string table called CONFIG_INITRAMFS_COMPRESSION).

BTW: New compression algorithm being added to the kernel is a rare
event, I also envision that not every new algorithm will need to be
presented as a choice for zswap default.

>> +       prompt "Compressed cache for swap pages default compressor"
>> +       depends on ZSWAP
>> +       default ZSWAP_DEFAULT_COMP_LZO
>> +       help
>> +         Selects the default compression algorithm for the compressed cache
>> +         for swap pages.
>> +         If in doubt, select 'LZO'.
>> +
>> +         The selection made here can be overridden by using the kernel
>> +         command line 'zswap.compressor=' option.
>> +
>> +config ZSWAP_DEFAULT_COMP_DEFLATE
>> +       bool "Deflate"
>> +       select CRYPTO_DEFLATE
>> +       help
>> +         Use the Deflate algorithm as the default compression algorithm.
>> +
>> +config ZSWAP_DEFAULT_COMP_LZO
>> +       bool "LZO"
>> +       select CRYPTO_LZO
>> +       help
>> +         Use the LZO algorithm as the default compression algorithm.
>> +
>> +config ZSWAP_DEFAULT_COMP_842
>> +       bool "842"
>> +       select CRYPTO_842
>> +       help
>> +         Use the 842 algorithm as the default compression algorithm.
>> +
>> +config ZSWAP_DEFAULT_COMP_LZ4
>> +       bool "LZ4"
>> +       select CRYPTO_LZ4
>> +       help
>> +         Use the LZ4 algorithm as the default compression algorithm.
>> +
>> +config ZSWAP_DEFAULT_COMP_LZ4HC
>> +       bool "LZ4HC"
>> +       select CRYPTO_LZ4HC
>> +       help
>> +         Use the LZ4HC algorithm as the default compression algorithm.
>> +
>> +config ZSWAP_DEFAULT_COMP_ZSTD
>> +       bool "zstd"
>> +       select CRYPTO_ZSTD
>> +       help
>> +         Use the zstd algorithm as the default compression algorithm.
>> +endchoice
>> +
>> +config ZSWAP_DEFAULT_COMP_NAME
>> +       string
>> +       default "deflate" if ZSWAP_DEFAULT_COMP_DEFLATE
>> +       default "lzo" if ZSWAP_DEFAULT_COMP_LZO
>> +       default "842" if ZSWAP_DEFAULT_COMP_842
>> +       default "lz4" if ZSWAP_DEFAULT_COMP_LZ4
>> +       default "lz4hc" if ZSWAP_DEFAULT_COMP_LZ4HC
>> +       default "zstd" if ZSWAP_DEFAULT_COMP_ZSTD
>> +       default ""
>> +
>> +choice
>> +       prompt "Compressed cache for swap pages default allocator"
>> +       depends on ZSWAP
>> +       default ZSWAP_DEFAULT_ZPOOL_ZBUD
>> +       help
>> +         Selects the default allocator for the compressed cache for
>> +         swap pages.
>> +         The default is 'zbud' for compatibility, however please do
>> +         read the description of each of the allocators below before
>> +         making a right choice.
>> +
>> +         The selection made here can be overridden by using the kernel
>> +         command line 'zswap.zpool=' option.
>> +
>> +config ZSWAP_DEFAULT_ZPOOL_ZBUD
>> +       bool "zbud"
>> +       select ZBUD
>> +       help
>> +         Use the zbud allocator as the default allocator.
>> +
>> +config ZSWAP_DEFAULT_ZPOOL_Z3FOLD
>> +       bool "z3fold"
>> +       select Z3FOLD
>> +       help
>> +         Use the z3fold allocator as the default allocator.
>> +endchoice
>> +
>> +config ZSWAP_DEFAULT_ZPOOL_NAME
>> +       string
>> +       default "zbud" if ZSWAP_DEFAULT_ZPOOL_ZBUD
>> +       default "z3fold" if ZSWAP_DEFAULT_ZPOOL_Z3FOLD
>> +       default ""
>> +
>> +config ZSWAP_DEFAULT_ON
>> +       bool "Enable the compressed cache for swap pages by default"
>> +       depends on ZSWAP
>> +       help
>> +         If selected, the compressed cache for swap pages will be enabled
>> +         at boot, otherwise it will be disabled.
>> +
>> +         The selection made here can be overridden by using the kernel
>> +         command line 'zswap.enabled=' option.
>> +
>>  config ZPOOL
>>         tristate "Common API for compressed memory storage"
>>         help
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 46a322316e52..59231f6fb2ca 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -71,8 +71,12 @@ static u64 zswap_duplicate_entry;
>>
>>  #define ZSWAP_PARAM_UNSET ""
>>
>> -/* Enable/disable zswap (disabled by default) */
>> +/* Enable/disable zswap */
>> +#ifdef CONFIG_ZSWAP_DEFAULT_ON
>> +static bool zswap_enabled = true;
>> +#else
>>  static bool zswap_enabled;
>> +#endif
>>  static int zswap_enabled_param_set(const char *,
>>                                    const struct kernel_param *);
>>  static struct kernel_param_ops zswap_enabled_param_ops = {
>> @@ -82,8 +86,7 @@ static struct kernel_param_ops zswap_enabled_param_ops = {
>>  module_param_cb(enabled, &zswap_enabled_param_ops, &zswap_enabled, 0644);
>>
>>  /* Crypto compressor to use */
>> -#define ZSWAP_COMPRESSOR_DEFAULT "lzo"
>> -static char *zswap_compressor = ZSWAP_COMPRESSOR_DEFAULT;
>> +static char *zswap_compressor = CONFIG_ZSWAP_DEFAULT_COMP_NAME;
>>  static int zswap_compressor_param_set(const char *,
>>                                       const struct kernel_param *);
>>  static struct kernel_param_ops zswap_compressor_param_ops = {
>> @@ -95,8 +98,7 @@ module_param_cb(compressor, &zswap_compressor_param_ops,
>>                 &zswap_compressor, 0644);
>>
>>  /* Compressed storage zpool to use */
>> -#define ZSWAP_ZPOOL_DEFAULT "zbud"
>> -static char *zswap_zpool_type = ZSWAP_ZPOOL_DEFAULT;
>> +static char *zswap_zpool_type = CONFIG_ZSWAP_DEFAULT_ZPOOL_NAME;
>>  static int zswap_zpool_param_set(const char *, const struct kernel_param *);
>>  static struct kernel_param_ops zswap_zpool_param_ops = {
>>         .set =          zswap_zpool_param_set,
>> @@ -569,11 +571,12 @@ static __init struct zswap_pool *__zswap_pool_create_fallback(void)
>>         bool has_comp, has_zpool;
>>
>>         has_comp = crypto_has_comp(zswap_compressor, 0, 0);
>> -       if (!has_comp && strcmp(zswap_compressor, ZSWAP_COMPRESSOR_DEFAULT)) {
>> +       if (!has_comp && strcmp(zswap_compressor,
>> +                               CONFIG_ZSWAP_DEFAULT_COMP_NAME)) {
> 
> bit of bikeshedding, wouldn't CONFIG_ZSWAP_COMPRESSOR_DEFAULT be
> clearer than CONFIG_ZSWAP_DEFAULT_COMP_NAME?

I'm fine either way (used "COMP" instead of "COMPRESSOR" to shorten these
identifiers a bit), also CONFIG_ZSWAP_DEFAULT_ZPOOL_NAME then probably
would need renaming to CONFIG_ZSWAP_ZPOOL_DEFAULT for consistency.

Thanks,
Maciej