/cc @brendangregg

The tool is checking to see if that symbol exists in the kernel, as a means to determine if you have the ext4 compiled or loaded so as to ensure that those kprobes can attach successfully to the necessary functions.

The ext4_file_operations is a struct and should show up as a symbol of type 'd'. In all likelihood you need to compile the kernel with CONFIG_KALLSYMS_ALL option enabled to ensure that symbol table contains more than just text type symbols.

Indeed, CONFIG_KALLSYMS_ALL is disabled:

$ zcat /proc/config.gz | grep CONFIG_KALLSYMS_ALL
# CONFIG_KALLSYMS_ALL is not set

I believe it worth adding a corresponding hint to the script:

diff --git a/home/eax/temp/ext4slower.orig b/usr/share/bcc/tools/ext4slower
index 3b5e7ae..09e8572 100755
--- a/home/eax/temp/ext4slower.orig
+++ b/usr/share/bcc/tools/ext4slower
@@ -262,6 +262,7 @@ with open(kallsyms) as syms:
             break
     if ops == '':
         print("ERROR: no ext4_file_operations in /proc/kallsyms. Exiting.")
+        print("HINT: the kernel should be built with CONFIG_KALLSYMS_ALL.")
         exit()
     bpf_text = bpf_text.replace('EXT4_FILE_OPERATIONS', ops)
 if min_ms == 0:

Could you submit a formal patch for this? I think this is indeed helpful.
cc @brendangregg

Done.

For the record - also I've contacted Tobias Powalowski, the maintainer of the linux package for Arch Linux, and asked him if it's possible to enable CONFIG_KALLSYMS_ALL in this package.

For the record - updated PR is here #1408