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SubjectRe: [PATCH] initramfs: Fix disabling of initramfs (and its compression)

On 08/28/2017 08:09 PM, Nicholas Piggin wrote:
> On Mon, 28 Aug 2017 13:03:31 -0700
> Florian Fainelli <> wrote:
>> On 05/21/2017 07:46 PM, Nicholas Piggin wrote:
>>> On Sat, 20 May 2017 20:33:35 -0700
>>> Florian Fainelli <> wrote:
>>>> Commit db2aa7fd15e8 ("initramfs: allow again choice of the embedded
>>>> initram compression algorithm") introduced the possibility to select the
>>>> initramfs compression algorithm from Kconfig and while this is a nice
>>>> feature it broke the use case described below.
>>>> Here is what my build system does:
>>>> - kernel is initially configured not to have an initramfs included
>>>> - build the user space root file system
>>>> - re-configure the kernel to have an initramfs included
>>>> (CONFIG_INITRAMFS_SOURCE="/path/to/romfs") and set relevant
>>>> CONFIG_INITRAMFS options, in my case, no compression option
>>>> - kernel is re-built with these options -> kernel+initramfs image is
>>>> copied
>>>> - kernel is re-built again without these options -> kernel image is
>>>> copied
>>>> Building a kernel without an initramfs means setting this option:
>>>> CONFIG_INITRAMFS_SOURCE="" (and this one only)
>>>> whereas building a kernel with an initramfs means setting these options:
>>>> CONFIG_INITRAMFS_SOURCE="/home/fainelli/work/uclinux-rootfs/romfs
>>>> /home/fainelli/work/uclinux-rootfs/misc/"
>>>> Commit db2aa7fd15e857891cefbada8348c8d938c7a2bc ("initramfs: allow again
>>>> choice of the embedded initram compression algorithm") is problematic
>>>> because CONFIG_INITRAMFS_COMPRESSION which is used to determine the
>>>> initramfs_data.cpio extension/compression is a string, and due to how
>>>> Kconfig works it will evaluate in order, how to assign it.
>>>> CONFIG_INITRAMFS_SOURCE="" cannot possibly work (because of the depends
>>>> still get CONFIG_INITRAMFS_COMPRESSION assigned to ".gz" because
>>>> CONFIG_RD_GZIP=y is set in my kernel, even when there is no initramfs
>>>> being built.
>>>> So we basically end-up generating two initramfs_data.cpio* files, one
>>>> without extension, and one with .gz. This causes usr/Makefile to track
>>>> usr/initramfs_data.cpio.gz, and not usr/initramfs_data.cpio anymore,
>>>> that is also largely problematic after
>>>> 9e3596b0c6539e28546ff7c72a06576627068353 ("kbuild: initramfs cleanup,
>>>> set target from Kconfig") because we used to track all possible
>>>> initramfs_data files in the $(targets) variable before that commit.
>>>> The end result is that the kernel with an initramfs clearly does not
>>>> contain what we expect it to, it has a stale initramfs_data.cpio file
>>>> built into it, and we keep re-generating an initramfs_data.cpio.gz file
>>>> which is not the one that we want to include in the kernel image proper.
>>>> The fix consists in hiding CONFIG_INITRAMFS_COMPRESSION when
>>>> CONFIG_INITRAMFS_SOURCE="". This puts us back in a state to the pre-4.10
>>>> behavior where we can properly disable and re-enable initramfs within
>>>> the same kernel .config file, and be in control of what
>>>> Fixes: db2aa7fd15e8 ("initramfs: allow again choice of the embedded initram compression algorithm")
>>>> Fixes: 9e3596b0c653 ("kbuild: initramfs cleanup, set target from Kconfig")
>>>> Signed-off-by: Florian Fainelli <>
>>> This is very thorough, thank you for tracking it down and fixing it.
>>> I can't say I've worked through the problem in the code, but your
>>> changelog and the proposed fix seem reasonable to me. So for what
>>> it's worth:
>>> Acked-by: Nicholas Piggin <>
>> Well, I am looking at this again, and it's still broken, the same test
>> case is involved, except this time, I am switching beween no-initramfs
>> and initramfs with gzip compression (the key thing is using a
>> compression of some sort). The end result is the following:
>> - change stuff in the rootfs
>> - build the kernel with initramfs, CONFIG_INITRAMFS_COMPRESSION_GZIP=y,
>> usr/initramfs_data.cpio.gz gets generated correctly the first time
>> - build the kernel without initramfs,
>> CONFIG_INITRAMFS_COMPRESSION_NONE=y, usr/initramfs_data.cpio gets generated
>> Now back to step 1 add some files, and we can see that
>> usr/initramfs_data.cpio.gz is now stale from before...
>> So while my earlier fix switched the initramfs w/o compression to no
>> initramfs rebuild, now this does not work because we still have two
>> files left to be tracked:
>> usr/initramfs_data.cpio (no compression, or when initramfs is disabled)
>> and usr/initramfs_data.cpio.$(suffix_y)
>> How would you go about solving this?
> I don't see the problem. When I change back to .gz, modify the source
> directory, and rebuild, it rebuilds a new initramfs_data.cpio.gz here.

Can you share the different steps involved here? Here is what the build
system I am using is doing.


The script that does the config option switching is here:

When you say change the source directory, you mean setting
CONFIG_INITRAMFS_SOURCE to one directory that contains your rootfs, and
then setting it to empty to disable that, right?

Here is what I have so far, only caveat (still triple checking) is that
we seem to be always re-generating usr/initramfs_cpio.$(suffix_y) which
is still an improvement over the current situation where we have a stale
file after the first creation.

diff --git a/usr/Makefile b/usr/Makefile
index 0b87e71c00fc..2d7d7f4b91f1 100644
--- a/usr/Makefile
+++ b/usr/Makefile
@@ -7,6 +7,7 @@ PHONY += klibcdirs

suffix_y = $(subst $\",,$(CONFIG_INITRAMFS_COMPRESSION))
datafile_y = initramfs_data.cpio$(suffix_y)
+datafile_d_y = .$(datafile_y).d
AFLAGS_initramfs_data.o += -DINITRAMFS_IMAGE="usr/$(datafile_y)"

@@ -29,12 +30,12 @@ ramfs-args := \

-# .initramfs_data.cpio.d is used to identify all files included
+# $(datafile_d_y) is used to identify all files included
# in initramfs and to detect if any files are added/removed.
# Removed files are identified by directory timestamp being updated
# The dependency list is generated by -l
-ifneq ($(wildcard $(obj)/.initramfs_data.cpio.d),)
- include $(obj)/.initramfs_data.cpio.d
+ifneq ($(wildcard $(obj)/$(datafile_d_y)),)
+ include $(obj)/$(datafile_d_y)

quiet_cmd_initfs = GEN $@
@@ -52,5 +53,5 @@ $(deps_initramfs): klibcdirs
# 3) If gen_init_cpio are newer than initramfs_data.cpio
# 4) arguments to changes
$(obj)/$(datafile_y): $(obj)/gen_init_cpio $(deps_initramfs) klibcdirs
- $(Q)$(initramfs) -l $(ramfs-input) > $(obj)/.initramfs_data.cpio.d
+ $(Q)$(initramfs) -l $(ramfs-input) > $(obj)/$(datafile_d_y)
$(call if_changed,initfs)
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