Can't compile 2.3.4 with kernel 6.16.4 using llvm-21

[StormBytePP]

When trying to compile zfs-2.3.4 using (gentoo)kernel-6.16.4 I found the following during configure stage:

checking whether blkdev_get_by_path() exists and takes 3 args... no
checking whether blkdev_get_by_path() exists and takes 4 args... no
checking whether bdev_open_by_path() exists... no
checking whether bdev_file_open_by_path() exists... no
configure: error: 
	*** None of the expected "blkdev_get_by_path()" interfaces were detected.
	*** This may be because your kernel version is newer than what is
	*** supported, or you are using a patched custom kernel with
	*** incompatible modifications.
	***
	*** ZFS Version: zfs-2.3.4-r0-gentoo
	*** Compatible Kernels: 4.18 - 6.16

Maybe an internal change in API within minor versions despite version is listed as supported?

[StormBytePP]

Quickly finding in include/linux/blkdev.h line 1773:

struct file *bdev_file_open_by_path(const char *path, blk_mode_t mode,
                void *holder, const struct blk_holder_ops *hops);

But nothing about blkdev_get_by_path in kernel's code

[robn]

Builds and runs fine here with release 2.3.4 and upstream 6.16.4:

quiz# uname -a
Linux quiz 6.16.4 #1 SMP PREEMPT_DYNAMIC Fri Aug 29 19:23:33 AEST 2025 x86_64 GNU/Linux

quiz# zfs version
zfs-2.3.4-1
zfs-kmod-2.3.4-1

Relevant part of config.log is different to your output:

configure:105393: checking whether blkdev_get_by_path() exists and takes 3 args
configure:105439: result: no
configure:105441: checking whether blkdev_get_by_path() exists and takes 4 args
configure:105490: result: no
configure:105492: checking whether bdev_open_by_path() exists
configure:105541: result: no
configure:105543: checking whether bdev_file_open_by_path() exists
configure:105587: result: yes

So this is either something very different in the Linux or OpenZFS source you're using, or something is confused in your build environment. If you can post config.log and build/build.log.kabi I might be able to guess better what it is, but either way, for now, I'm not seeing an issue in any stock releases.

[comrade-meowski]

Ah ha, finally... I've been running into this bug (?) on/off for 6+ months.

I bet @StormBytePP is using LLVM to build with rather than GCC.

I maintain two kernel variants and build every new stable release - one generic x86_64 target with more conservative tuning for servers etc (built with GCC) and a more aggressively optimised ryzen targetted variant for workstations (built with LLVM). All these systems run the latest ZFS version which I also build (with GCC and LLVM). All systems are Ubuntu and I use DKMS for the ZFS kernel modules.

The GCC builds are super well behaved and never cause problems at all. However for the last 6 months or so when using the more recent releases of LLVM (20, 21 or 22) I run into the exact error above. To be clear, the kernels all build, install and run just fine except for one thing - compiling the ZFS kernel module. All other DKMS modules - including the Nvidia driver - compile just fine using LLVM.

I also know how to work around it - switch to LLVM-19 and DKMS will suddenly build the ZFS module on a LLVM kernel immediately with no None of the expected "blkdev_get_by_path()" interfaces were detected error.

So this interaction is specifically between ZFS and the latest versions of LLVM. This kind of feels more like a LLVM issue than a ZFS issue to be fair... but then ZFS is the only thing that triggers it, all other DKMS modules work fine.

This was starting to get to the point where I was ready to open an issue either here or more likely on the LLVM project about 3 months ago which was pretty late in the LLVM-21 development phase... and then the issue just disappeared overnight following another LLVM upgrade. Then for a while everything was back to normal and presumed fixed until the brand new LLVM-22 arrived and the bug instantly reappeared with it.

I'm happy to provide any more info I can but getting to the bottom of this proved beyond me I'm afraid and I really, really tried... (this is why I'm not a developer)

[StormBytePP]

Bingo! Yes, I confirm. In my case I use Gentoo and gentoo-kernel, and everything in system (including as you say other external kernel modules) compiles just fine with clang/LLVM except for ZFS, all using clang21.

[StormBytePP]

config.log
build.log.kabi

[comrade-meowski]

Haha, welcome fellow sufferer!

Can you emerge LLVM-19 on your system and try to build ZFS with that toolchain version? If your Gentoo box ends up behaving exactly the same as my Ubuntu ones that would go a long way towards ruling out me just having messed up something locally.

[StormBytePP]

Haha, welcome fellow sufferer!

Can you emerge LLVM-19 on your system and try to build ZFS with that toolchain version? If your Gentoo box ends up behaving exactly the same as my Ubuntu ones that would go a long way towards ruling out me just having messed up something locally.

LLVM-19 is confirmed to pass through that configure stage (and also compiles fine).

[StormBytePP]

Ran more tests and it only seems to fail llvm21 for me, so updated title accordingly.

[comrade-meowski]

That prompted me to re-install and re-test LLVM-20 (only had 19, 21 and 22 to hand thanks to dist-upgrade) and my results match yours. LLVM-20 also works fine here, as does LLVM-19. 21/22 both fail.

There is a pattern then - when I first hit this bug 6+ months ago LLVM-19 was the stable version and 20/21 were testing/dev. When LLVM version bumped a month or so back LLVM-20 became stable and 21/22 testing/dev.

So only the non-stable versions of LLVM trigger this bug, and only when building the ZFS kernel module.

[behlendorf]

@StormBytePP I'm not able to grab the logs you posted. Can you either post the relevant section from the log, or directly attach them to this issue.

[comrade-meowski]

Ubuntu 25.04 x86_64 host. Running kernel 6.16.3 (built with LLVM-19).

git clone https://github.com/openzfs/zfs.git && cd zfs
git checkout zfs-2.3.4
sh autogen.sh
LLVM=1 ./configure
LLVM=1 make deb

Successful configure and build of zfs-2.3.4 with LLVM-20:

config.log:
https://pastefy.app/np8V60Oe

build.log.kabi:
https://pastefy.app/NbAVhFBb

Failed test of zfs-2.3.4 with LLVM-21:

config.log:
https://pastefy.app/gShfZF6w

build.log.kabi:
https://pastefy.app/zOIPJlMt