Re: Linux 4.14.36

["Theodore Y. Ts'o" <tytso@mit.edu>]

On Thu, Apr 26, 2018 at 01:36:57PM +0200, Fran�ois Valenduc wrote:
> 
> xzcat /boot/initramfs-genkernel-x86_64-4.14.37-rc1 | cpio --list | grep
> crc32
> lib/modules/4.14.37-rc1/kernel/lib/libcrc32c.ko
> 69823 blocs
> 
> So the libcrc32 module is included, but crc32c_generic�� is also needed.

So both libcrc32c and ext4 modules both have softdeps in crc32c, which
will drag in either crc32c_generic or crc32c_intel.  What initramfs
utilities are you use to generate the initramfs, and what distribution
are you using?

I have seen one other user have a problem where their initramfs had
crc32c_intel, but not crc32c_generic, but their scripts weren't
regenerating the modueles.dep.bin file, so when the modprobe tries to
load crc32c, even though crc32c_intel is present (and preferred), the
modprobe bombs out because it was trying to load crc32c_generic first.

You seem to have a different failure mode, but it looks like there are
many ways in which in the initramfs tools will fail to include the
right crc32c module(s).  I was able to find a Debian bug going back to
2011(!) which described problems with this, and Debian ultimately
dealt with this by just unconditionally force-including libcrc32c,
crc32c_generic, and crc32c_intel into the initramfs.  :-(

	https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=608538

I'll note that it's actually really silly for distros to be building
these as modules.  crc32c_generic is 900 bytes, and crc32c_intel is
11k --- but the modules consume 16k (each) of memory when loaded, and
each module burns a TLB cache entry when the code is called.  So it's
actually a much smarter idea to just build the crc32c code into the
kernel, instead of keeping them as modules.  It's what I do just
because it's a Really Good Idea, and apparently Debian has hacked
around the problem years ago so it's not an issue I would have seen
either if I did build them as modules.

Still, if we have a lot of buggy distros out there, and 4.16.4 is
going to trigger it, we're going to have to figure out some kind of
workaround for everyone....

					- Ted