Re: [PATCH v6 4/8] init: add support for zstd compressed kernel

[Norbert Lange]

Am Di., 7. Juli 2020 um 17:32 Uhr schrieb Nick Terrell <terrelln@xxxxxx>:
>
>
>
> > On Jul 7, 2020, at 3:19 AM, Norbert Lange <nolange79@xxxxxxxxx> wrote:
> >
> > Thanks for the respin.
> >
> > Am Di., 7. Juli 2020 um 05:51 Uhr schrieb Nick Terrell <nickrterrell@xxxxxxxxx>:
> >>
> >> From: Nick Terrell <terrelln@xxxxxx>
> >>
> >> * Adds the zstd cmd to scripts/Makefile.lib
> >> * Adds the HAVE_KERNEL_ZSTD and KERNEL_ZSTD options
> >>
> >> Architecture specific support is still needed for decompression.
> >>
> >> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> >> Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> >> Signed-off-by: Nick Terrell <terrelln@xxxxxx>
> >> ---
> >> init/Kconfig         | 15 ++++++++++++++-
> >> scripts/Makefile.lib | 15 +++++++++++++++
> >> 2 files changed, 29 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/init/Kconfig b/init/Kconfig
> >> index 0498af567f70..8d99f0c5e240 100644
> >> --- a/init/Kconfig
> >> +++ b/init/Kconfig
> >> @@ -191,13 +191,16 @@ config HAVE_KERNEL_LZO
> >> config HAVE_KERNEL_LZ4
> >>        bool
> >>
> >> +config HAVE_KERNEL_ZSTD
> >> +       bool
> >> +
> >> config HAVE_KERNEL_UNCOMPRESSED
> >>        bool
> >>
> >> choice
> >>        prompt "Kernel compression mode"
> >>        default KERNEL_GZIP
> >> -       depends on HAVE_KERNEL_GZIP || HAVE_KERNEL_BZIP2 || HAVE_KERNEL_LZMA || HAVE_KERNEL_XZ || HAVE_KERNEL_LZO || HAVE_KERNEL_LZ4 || HAVE_KERNEL_UNCOMPRESSED
> >> +       depends on HAVE_KERNEL_GZIP || HAVE_KERNEL_BZIP2 || HAVE_KERNEL_LZMA || HAVE_KERNEL_XZ || HAVE_KERNEL_LZO || HAVE_KERNEL_LZ4 || HAVE_KERNEL_ZSTD || HAVE_KERNEL_UNCOMPRESSED
> >>        help
> >>          The linux kernel is a kind of self-extracting executable.
> >>          Several compression algorithms are available, which differ
> >> @@ -276,6 +279,16 @@ config KERNEL_LZ4
> >>          is about 8% bigger than LZO. But the decompression speed is
> >>          faster than LZO.
> >>
> >> +config KERNEL_ZSTD
> >> +       bool "ZSTD"
> >> +       depends on HAVE_KERNEL_ZSTD
> >> +       help
> >> +         ZSTD is a compression algorithm targeting intermediate compression
> >> +         with fast decompression speed. It will compress better than GZIP and
> >> +         decompress around the same speed as LZO, but slower than LZ4. You
> >> +         will need at least 192 KB RAM or more for booting. The zstd command
> >> +         line tools is required for compression.
> >> +
> >> config KERNEL_UNCOMPRESSED
> >>        bool "None"
> >>        depends on HAVE_KERNEL_UNCOMPRESSED
> >> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> >> index 916b2f7f7098..d960f8815f87 100644
> >> --- a/scripts/Makefile.lib
> >> +++ b/scripts/Makefile.lib
> >> @@ -413,6 +413,21 @@ quiet_cmd_xzkern = XZKERN  $@
> >> quiet_cmd_xzmisc = XZMISC  $@
> >>       cmd_xzmisc = cat $(real-prereqs) | $(XZ) --check=crc32 --lzma2=dict=1MiB > $@
> >>
> >> +# ZSTD
> >> +# ---------------------------------------------------------------------------
> >> +# Appends the uncompressed size of the data using size_append. The .zst
> >> +# format has the size information available at the beginning of the file too,
> >> +# but it's in a more complex format and it's good to avoid changing the part
> >> +# of the boot code that reads the uncompressed size.
> >> +# Note that the bytes added by size_append will make the zstd tool think that
> >> +# the file is corrupt. This is expected.
> >> +
> >> +quiet_cmd_zstd = ZSTD    $@
> >> +cmd_zstd = (cat $(filter-out FORCE,$^) | \
> >> +       zstd -19 && \
> >> +        $(call size_append, $(filter-out FORCE,$^))) > $@ || \
> >> +       (rm -f $@ ; false)
> >
> > Is there any reason not to use '--ultra -22' ?
> > As far as I understand the other patches, the decompression should be
> > able to handle it,
> > and in terms of time required for a kernel build the difference is
> > insignificant.
>
> For kernel compression there isnât a strong reason not to use `--ultra -22`.
> It may slow down decompression a small amount, because the difference
> is that it has a 128 MB window size instead of a 8 MB window size.
>
> However, that probably isnât want you want for initramfs compression,
> which can optionally now use this same command. We could go like xz
> and have both cmd_zstdmisc and cmd_zstdkern, and only use `-22` for the
> kernel version.

Realistically, that's only relevant for an compressed internal rd with an
uncompressed kernel (means not really realistic, I haven't seen such a
thing in ages)
Means time is not the issue, but memory for a way to small
decompression window is.

>
> It also looks like there were a few minor changes made to the other
> compress cmds in this file while these patches have been out, so I will apply
> them to zstd as well.
>
> Iâll submit a new version with these changes today.
>
> > And would it be better to run zstd on a prepared file instead of
> > stream enconding?
> > windowsize would be adjusted to min(windowsize, filesize) for one.
>
> Yeah, that would be helpful for initramfs compression when the file is
> smaller than the window size, since it would limit the memory necessary
> For decompression. But, it wouldnât help kernel compression.

It would also allow you to unconditionally use -22, as the drawback
for the internal rd would be removed.

>
> For that weâd have to create a temporary file, because it looks like these
> commands potentially accept more than one input file. Do you know the
> standard practice for temporary files in the build system?

Nope, from peeking around (grep -r '\bmv\b' | grep Make),
I'd use something like this (completely untested, sorry)

cmd_zstd = ( trap "rm -f $(@D)/.tmp_$(@F)" 0 && \
       cat $(filter-out FORCE,$^) > $(@D)/.tmp_$(@F) && \
       zstd --ultra -22 -c $(@D)/.tmp_$(@F)  && \
        $(call size_append, $(filter-out FORCE,$^))) > $@ || \
       (rm -f $@ ; false)

On the other hand I would hate if this tinkering further delays upstreaming.

>
> Thanks for the review,
> Nick

Thanks for the patch ;)
Norbert