Re: [PATCH] init: add support for zstd compressed modules

[Nick Terrell]

> On Mar 31, 2021, at 10:48 AM, Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx> wrote:
> 
> Hello.
> 
> On Wed, Mar 31, 2021 at 05:39:25PM +0000, Nick Terrell wrote:
>> 
>> 
>>> On Mar 30, 2021, at 4:50 AM, Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx> wrote:
>>> 
>>> On Tue, Mar 30, 2021 at 01:32:35PM +0200, Piotr Gorski wrote:
>>>> kmod 28 supports modules compressed in zstd format so let's add this possibility to kernel.
>>>> 
>>>> Signed-off-by: Piotr Gorski <lucjan.lucjanov@xxxxxxxxx>
>>>> ---
>>>> Makefile     | 7 +++++--
>>>> init/Kconfig | 9 ++++++---
>>>> 2 files changed, 11 insertions(+), 5 deletions(-)
>>>> 
>>>> diff --git a/Makefile b/Makefile
>>>> index 5160ff8903c1..82f4f4cc2955 100644
>>>> --- a/Makefile
>>>> +++ b/Makefile
>>>> @@ -1156,8 +1156,8 @@ endif # INSTALL_MOD_STRIP
>>>> export mod_strip_cmd
>>>> 
>>>> # CONFIG_MODULE_COMPRESS, if defined, will cause module to be compressed
>>>> -# after they are installed in agreement with CONFIG_MODULE_COMPRESS_GZIP
>>>> -# or CONFIG_MODULE_COMPRESS_XZ.
>>>> +# after they are installed in agreement with CONFIG_MODULE_COMPRESS_GZIP,
>>>> +# CONFIG_MODULE_COMPRESS_XZ, or CONFIG_MODULE_COMPRESS_ZSTD.
>>>> 
>>>> mod_compress_cmd = true
>>>> ifdef CONFIG_MODULE_COMPRESS
>>>> @@ -1167,6 +1167,9 @@ ifdef CONFIG_MODULE_COMPRESS
>>>>  ifdef CONFIG_MODULE_COMPRESS_XZ
>>>>    mod_compress_cmd = $(XZ) --lzma2=dict=2MiB -f
>>>>  endif # CONFIG_MODULE_COMPRESS_XZ
>>>> +  ifdef CONFIG_MODULE_COMPRESS_ZSTD
>>>> +    mod_compress_cmd = $(ZSTD) -T0 --rm -f -q
>> 
>> This will use the default zstd level, level 3. I think it would make more sense to use a high
>> compression level. Level 19 would probably be a good choice. That will choose a window
>> size of up to 8MB, meaning the decompressor needs to allocate that much memory. If that
>> is unacceptable, you could use `zstd -T0 --rm -f -q -19 --zstd=wlog=21`, which will use a
>> window size of up to 2MB, to match the XZ command. Note that if the file is smaller than
>> the window size, it will be shrunk to the smallest power of two at least as large as the file.
> 
> Please no. We've already done that with initramfs in Arch, and it
> increased the time to generate it enormously.
> 
> I understand that building a kernel is a more rare operation than
> regenerating initramfs, but still I'd go against hard-coding the level.
> And if it should be specified anyway, I'd opt in for an explicit
> configuration option. Remember, not all the kernel are built on
> build farms...
> 
> FWIW, Piotr originally used level 9 which worked okay, but I insisted
> on sending the patch initially without specifying level at all like it is
> done for other compressors. If this is a wrong approach, then oh meh,
> mea culpa ;).
> 
> Whatever default non-standard compression level you choose, I'm fine
> as long as I can change it without editing Makefile.

That makes sense to me. I have a deep seated need to compress files as
efficiently as possible for widely distributed packages. But, I understand that
slow compression significantly impacts build times for quick iteration. I’d be
happy with a compression level parameter that defaults to a happy middle.

I’m also fine with taking this patch as-is if it is easier, and I can put up another
patch that adds a compression level parameter, since I don’t want to block
merging this.

Best,
Nick Terrell

> Thanks!
> 
>> 
>> Best,
>> Nick Terrell
>> 
>>>> +  endif # CONFIG_MODULE_COMPRESS_ZSTD
>>>> endif # CONFIG_MODULE_COMPRESS
>>>> export mod_compress_cmd
>>>> 
>>>> diff --git a/init/Kconfig b/init/Kconfig
>>>> index 8c2cfd88f6ef..86a452bc2747 100644
>>>> --- a/init/Kconfig
>>>> +++ b/init/Kconfig
>>>> @@ -2250,8 +2250,8 @@ config MODULE_COMPRESS
>>>> 	bool "Compress modules on installation"
>>>> 	help
>>>> 
>>>> -	  Compresses kernel modules when 'make modules_install' is run; gzip or
>>>> -	  xz depending on "Compression algorithm" below.
>>>> +	  Compresses kernel modules when 'make modules_install' is run; gzip,
>>>> +	  xz, or zstd depending on "Compression algorithm" below.
>>>> 
>>>> 	  module-init-tools MAY support gzip, and kmod MAY support gzip and xz.
>>>> 
>>>> @@ -2273,7 +2273,7 @@ choice
>>>> 	  This determines which sort of compression will be used during
>>>> 	  'make modules_install'.
>>>> 
>>>> -	  GZIP (default) and XZ are supported.
>>>> +	  GZIP (default), XZ, and ZSTD are supported.
>>>> 
>>>> config MODULE_COMPRESS_GZIP
>>>> 	bool "GZIP"
>>>> @@ -2281,6 +2281,9 @@ config MODULE_COMPRESS_GZIP
>>>> config MODULE_COMPRESS_XZ
>>>> 	bool "XZ"
>>>> 
>>>> +config MODULE_COMPRESS_ZSTD
>>>> +	bool "ZSTD"
>>>> +
>>>> endchoice
>>>> 
>>>> config MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS
>>>> -- 
>>>> 2.31.0.97.g1424303384
>>>> 
>>> 
>>> Great!
>>> 
>>> Reviewed-by: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx>
>>> 
>>> This works perfectly fine in Arch Linux if accompanied by the
>>> following mkinitcpio amendment: [1].
>>> 
>>> I'm also Cc'ing other people from get_maintainers output just
>>> to make this submission more visible.
>>> 
>>> Thanks.
>>> 
>>> [1] https://github.com/archlinux/mkinitcpio/pull/43
>>> 
>>> -- 
>>> Oleksandr Natalenko (post-factum)
>> 
> 
> -- 
>  Oleksandr Natalenko (post-factum)