Re: [PATCH] ext4: convert legacy ext4_debug() to standard pr_debug()

[Baokun Li]

On 2026/5/21 15:46, lirongqing wrote:
> From: Li RongQing <lirongqing@xxxxxxxxx>
>
> The ext4 file system historically implemented its own debug logging macro
> ext4_debug() via EXT4FS_DEBUG conditional compilation. This legacy
> implementation suffers from two major drawbacks:
>
> 1. It makes two consecutive un-serialized printk() calls, which can
>    lead to severe log interleaving and corruption under multi-core
>    concurrent workloads.
> 2. It completely bypasses the standard modern kernel dynamic debug
>    (CONFIG_DYNAMIC_DEBUG) infrastructure.
>
> Clean up the legacy implementation by leveraging pr_debug(). This squashes
> the multiple printk() calls into a single atomic execution, ensuring
> log integrity, while seamlessly hooking ext4 into the kernel's native
> dynamic debug framework.
>
> The redundant __FILE__ and __LINE__ macros are intentionally removed from
> the string format because the dynamic debug infrastructure can already
> append them automatically at runtime (via the '+fl' flags) if desired.
> This avoids redundancy and double-logging in modern production/debugging
> environments while keeping the macro clean and robust against dangling
> comma compiler errors.
>
> Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>

Thanks for cleaning this up.

> ---
>  fs/ext4/ext4.h | 20 ++------------------
>  1 file changed, 2 insertions(+), 18 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 94283a9..39e86ff 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -62,24 +62,8 @@
>   */
>  #define DOUBLE_CHECK__
>  
> -/*
> - * Define EXT4FS_DEBUG to produce debug messages
> - */
> -#undef EXT4FS_DEBUG
> -

This looks like it's just disabling EXT4FS_DEBUG, but it's actually
the only toggle point — enabling debug required manually flipping this
to #define EXT4FS_DEBUG in the source.

Since balloc.c, ialloc.c, and page-io.c still have #ifdef EXT4FS_DEBUG
blocks that weren't cleaned up here, it would be cleaner to replace
those with CONFIG_EXT4_DEBUG so they fall under a single
Kconfig-controlled umbrella.

> -/*
> - * Debug code
> - */
> -#ifdef EXT4FS_DEBUG
> -#define ext4_debug(f, a...)						\
> -	do {								\
> -		printk(KERN_DEBUG "EXT4-fs DEBUG (%s, %d): %s:",	\
> -			__FILE__, __LINE__, __func__);			\
> -		printk(KERN_DEBUG f, ## a);				\
> -	} while (0)
> -#else
> -#define ext4_debug(fmt, ...)	no_printk(fmt, ##__VA_ARGS__)
> -#endif
> +#define ext4_debug(fmt, ...)						\
> +	pr_debug("EXT4-fs DEBUG %s: " fmt, __func__,  ##__VA_ARGS__)

Nit: an extra space between __func__, and ##__VA_ARGS__.

>  
>   /*
>    * Turn on EXT_DEBUG to enable ext4_ext_show_path/leaf/move in extents.c

Also, could ext4_debug be moved next to ext_debug and placed under
#ifdef CONFIG_EXT4_DEBUG together, for a cleaner layout?


Cheers,
Baokun