Re: [PATCH 1/2] ext4: use module parameters instead of debugfs for mballoc_debug

[Eric Sandeen <sandeen@redhat.com>]

On 2/9/13 7:42 PM, Theodore Ts'o wrote:
> There are multiple reasons to move away from debugfs.  First of all,
> we are only using it for a single parameter, and it is much more
> complicated to set up (some 30 lines of code compared to 3), and one
> more thing that might fail while loading the ext4 module.
> 
> Secondly, as a module paramter it can be specified as a boot option if
> ext4 is built into the kernel, or as a parameter when the module is
> loaded, and it can also be manipulated dynamically under
> /sys/module/ext4/parameters/mballoc_debug.  So it is more flexible.
> 
> Ultimately we want to move away from using mb_debug() towards
> tracepoints, but for now this is still a useful simplification of the
> code base.
> 
> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

Good idea.

My only suggestion would be to add parameter information to the descriptions -
default value and permissible value range?  I guess nothing enforces that
like for proc entries, right?

 "Debugging level for ext4's mballoc: 0/1 (default 0)"

(hm, we never pass anything but "1" o_O)

It doesn't seem like the values need sanity checking at module load time,
but maybe it'd be worth it just out of paranoia.

Updating Documentation/filesystems/ext4.txt would be good too.

Thanks,
-Eric

-Eric

> ---
>  fs/ext4/mballoc.c | 47 +++++++++--------------------------------------
>  fs/ext4/mballoc.h |  4 ++--
>  2 files changed, 11 insertions(+), 40 deletions(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index e350885..6540ebe 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -23,11 +23,18 @@
>  
>  #include "ext4_jbd2.h"
>  #include "mballoc.h"
> -#include <linux/debugfs.h>
>  #include <linux/log2.h>
> +#include <linux/module.h>
>  #include <linux/slab.h>
>  #include <trace/events/ext4.h>
>  
> +#ifdef CONFIG_EXT4_DEBUG
> +ushort ext4_mballoc_debug __read_mostly;
> +
> +module_param_named(mballoc_debug, ext4_mballoc_debug, ushort, 0644);
> +MODULE_PARM_DESC(mballoc_debug, "Debugging level for ext4's mballoc");
> +#endif
> +
>  /*
>   * MUSTDO:
>   *   - test ext4_ext_search_left() and ext4_ext_search_right()
> @@ -2660,40 +2667,6 @@ static void ext4_free_data_callback(struct super_block *sb,
>  	mb_debug(1, "freed %u blocks in %u structures\n", count, count2);
>  }
>  
> -#ifdef CONFIG_EXT4_DEBUG
> -u8 mb_enable_debug __read_mostly;
> -
> -static struct dentry *debugfs_dir;
> -static struct dentry *debugfs_debug;
> -
> -static void __init ext4_create_debugfs_entry(void)
> -{
> -	debugfs_dir = debugfs_create_dir("ext4", NULL);
> -	if (debugfs_dir)
> -		debugfs_debug = debugfs_create_u8("mballoc-debug",
> -						  S_IRUGO | S_IWUSR,
> -						  debugfs_dir,
> -						  &mb_enable_debug);
> -}
> -
> -static void ext4_remove_debugfs_entry(void)
> -{
> -	debugfs_remove(debugfs_debug);
> -	debugfs_remove(debugfs_dir);
> -}
> -
> -#else
> -
> -static void __init ext4_create_debugfs_entry(void)
> -{
> -}
> -
> -static void ext4_remove_debugfs_entry(void)
> -{
> -}
> -
> -#endif
> -
>  int __init ext4_init_mballoc(void)
>  {
>  	ext4_pspace_cachep = KMEM_CACHE(ext4_prealloc_space,
> @@ -2715,7 +2688,6 @@ int __init ext4_init_mballoc(void)
>  		kmem_cache_destroy(ext4_ac_cachep);
>  		return -ENOMEM;
>  	}
> -	ext4_create_debugfs_entry();
>  	return 0;
>  }
>  
> @@ -2730,7 +2702,6 @@ void ext4_exit_mballoc(void)
>  	kmem_cache_destroy(ext4_ac_cachep);
>  	kmem_cache_destroy(ext4_free_data_cachep);
>  	ext4_groupinfo_destroy_slabs();
> -	ext4_remove_debugfs_entry();
>  }
>  
>  
> @@ -3876,7 +3847,7 @@ static void ext4_mb_show_ac(struct ext4_allocation_context *ac)
>  	struct super_block *sb = ac->ac_sb;
>  	ext4_group_t ngroups, i;
>  
> -	if (!mb_enable_debug ||
> +	if (!ext4_mballoc_debug ||
>  	    (EXT4_SB(sb)->s_mount_flags & EXT4_MF_FS_ABORTED))
>  		return;
>  
> diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
> index 3ccd889..08481ee 100644
> --- a/fs/ext4/mballoc.h
> +++ b/fs/ext4/mballoc.h
> @@ -37,11 +37,11 @@
>  /*
>   */
>  #ifdef CONFIG_EXT4_DEBUG
> -extern u8 mb_enable_debug;
> +extern ushort ext4_mballoc_debug;
>  
>  #define mb_debug(n, fmt, a...)	                                        \
>  	do {								\
> -		if ((n) <= mb_enable_debug) {		        	\
> +		if ((n) <= ext4_mballoc_debug) {		        \
>  			printk(KERN_DEBUG "(%s, %d): %s: ",		\
>  			       __FILE__, __LINE__, __func__);		\
>  			printk(fmt, ## a);				\
>