linux-kernel - Re: [PATCH v10 4/8] usr: add support for zstd compressed initramfs

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Date:   Tue, 4 Aug 2020 11:19:49 +0200
From:   Adam Borowski <kilobyte@...band.pl>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Nick Terrell <nickrterrell@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Chris Mason <clm@...com>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Petr Malat <oss@...at.biz>, Kees Cook <keescook@...omium.org>,
        Kernel Team <Kernel-team@...com>,
        Patrick Williams <patrickw3@...com>, rmikey@...com,
        Ingo Molnar <mingo@...nel.org>,
        Patrick Williams <patrick@...cx.xyz>,
        Norbert Lange <nolange79@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alex Xu <alex_y_xu@...oo.ca>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Terrell <terrelln@...com>
Subject: Re: [PATCH v10 4/8] usr: add support for zstd compressed initramfs

On Tue, Aug 04, 2020 at 09:25:23AM +0200, Sedat Dilek wrote:
> On Tue, Aug 4, 2020 at 8:52 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > On Thu, Jul 30, 2020 at 9:13 PM Nick Terrell <nickrterrell@...il.com> wrote:
> > > From: Nick Terrell <terrelln@...com>
> > > * Add support for a zstd compressed initramfs.
> > > * Add compression for compressing built-in initramfs with zstd.

> > > --- a/usr/Kconfig
> > > +++ b/usr/Kconfig
> > > @@ -100,6 +100,15 @@ config RD_LZ4
> > >           Support loading of a LZ4 encoded initial ramdisk or cpio buffer
> > >           If unsure, say N.
> > >
> > > +config RD_ZSTD
> > > +       bool "Support initial ramdisk/ramfs compressed using ZSTD"
> > > +       default y
> > > +       depends on BLK_DEV_INITRD
> > > +       select DECOMPRESS_ZSTD
> > > +       help
> > > +         Support loading of a ZSTD encoded initial ramdisk or cpio buffer.
> > > +         If unsure, say N.
> >
> > I'm aware you copied this from the other entries, but IMHO "default y",
> > and "If unsure, say N" are not a good combination.

> you are right - for new stuff it should be "default n".

It got already applied to Linus' tree with "y", and I think it'd be nice
to have it as a default.  Let's disable other compressors instead.

On the other hand, having an unsupported rd compressor results in a boot
failure that's not immediately obvious, so that's a reason for keeping
the setting as "y".

On the third hand, distributions default to either gz or xz, thus I'd say:
* let's have gz xz zstd default to y, all others to n
* drop bzip2 lzma1 completely
* distros can't switch the mkinitramfs default yet, but if RD_ZSTD=y now,
  they'll be able to once they drop support for old kernels in a few years

> What I am missing - still - is a note - that your user-space should
> have the correct bits to support zstd-initramfs.
> Unsure where to place such an information.

Looks like INITRAMFS_COMPRESSION_* have lengthy prose but are not shown in
menuconfig, while RD_*, with no such prose, are shown.

The prose itself is grossly obsolete, too.  I have some updates in:
    https://github.com/kilobyte/linux/commits/nobz2-v3
but that patchset needs rebasing and refreshing.


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