Re: [RFC] ext4: use kmem_cache for short fname allocation in readdir

["David Wang" <00107082@163.com>]

At 2025-05-30 05:44:26, "Andreas Dilger" <adilger@dilger.ca> wrote:
>On May 29, 2025, at 8:42 AM, David Wang <00107082@163.com> wrote:
>> 
>> When searching files, ext4_readdir would kzalloc() a fname
>> object for each entry. It would be faster if a dedicated
>> kmem_cache is used for fname.
>> 
>> But fnames are of variable length.
>> 
>> This patch suggests using kmem_cache for fname with short
>> length, and resorting to kzalloc when fname needs larger buffer.
>> Assuming long file names are not very common.
>
>It may be reasonable to have a cache, but I suspect that 127 bytes
>is too large for most common workloads.  Statistics that I've seen
>show 99-percentile filename length is <= 48 bytes on most filesystems,
>so allocating 128 bytes is probably sub-optimal (most is wasted).
>
>That said, kmalloc() is mostly a wrapper for the kmalloc-* slabs, so
>creating a 128-byte slab for this doesn't seem super useful?

Thanks for the feedback~!

Yes, there would be memory wasted.  I choose 127 is just for testing on my system, it can be
adjusted based on empirical data.

kmalloc is globally used,  while a dedicated/isolcated fname-slab is only accessed by
ext4-dir. Adding a new slab can avoid the contention between ext4-dir and other users of 
kmalloc, especially when alloc frequency is high. (When searching in a "large" dir, the
memory allocation frequency would be very high, a new slab can reduce the impact on other users
of kmalloc )


Thanks
David

>
>> Profiling when searching files in kernel code base, with following
>> command:
>> 	# perf record -g -e cpu-clock --freq=max bash -c \
>> 	"for i in {1..100}; do find ./linux -name notfoundatall > /dev/null; done"
>> And using sample counts as indicator of performance improvement.
>> (The faster, the less samples collected. And the tests are carried out
>> when system is under no memory pressure.)
>> 
>> Before without the change:
>> 	1232868--ext4_readdir
>> 		 |
>> 		 |--839085--ext4_htree_fill_tree
>> 		 |          |
>> 		 |           --829223--htree_dirblock_to_tree
>> 		 |                     |
>> 		 |                     |--365869--ext4_htree_store_dirent
>> 		 |                     |          |
>> 		 |                     |          |--43169--0xffffffffa7f8d094
>> 		 |                     |          |
>> 		 |                     |           --21947--0xffffffffa7f8d0f7
>> 		 |                     |
>> 		 |                     |--213124--ext4fs_dirhash
>> 		 |                     |          |
>> 		 |                     |           --86339--str2hashbuf_signed
>> 		 |                     |
>> 		 |                     |--145839--__ext4_read_dirblock
>> 
>> and with the change, ~3% less samples:
>> 	1202922--ext4_readdir
>
>> 		 |
>> 		 |--805105--ext4_htree_fill_tree
>> 		 |          |
>> 		 |           --795055--htree_dirblock_to_tree
>> 		 |                     |
>> 		 |                     |--328876--ext4_htree_store_dirent
>> 		 |                     |          |
>> 		 |                     |          |--123207--kmem_cache_alloc_noprof
>> 		 |                     |          |          |
>> 		 |                     |          |          |--26453--__alloc_tagging_slab_alloc_hook
>> 		 |                     |          |          |
>> 		 |                     |          |           --20413--__slab_alloc.isra.0
>> 		 |                     |          |
>> 		 |                     |           --31566--rb_insert_color
>> 		 |                     |
>> 		 |                     |--212915--ext4fs_dirhash
>> 		 |                     |          |
>> 		 |                     |           --86004--str2hashbuf_signed
>> 		 |                     |
>> 		 |                     |--149146--__ext4_read_dirblock
>> 
>> readdir() would have sigfinicant improvement, but the overall
>> improvements for searching files is only ~0.5%, might be more
>> sigfinicant if the system is under some memory pressures.
>> 
>> The slab stats after the test:
>> ext4_dir_fname      1242   1242    176   23    1 : tunables    0    0    0 : slabdata     54     54      0
>> 
>> Signed-off-by: David Wang <00107082@163.com>
>> ---
>> fs/ext4/dir.c   | 47 ++++++++++++++++++++++++++++++++++++++++++++---
>> fs/ext4/ext4.h  |  4 ++++
>> fs/ext4/super.c |  3 +++
>> 3 files changed, 51 insertions(+), 3 deletions(-)
>> 
>> diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
>> index d4164c507a90..3adfa0d038cd 100644
>> --- a/fs/ext4/dir.c
>> +++ b/fs/ext4/dir.c
>> @@ -424,6 +424,48 @@ struct fname {
>> 	char		name[] __counted_by(name_len);
>> };
>> 
>> +#define EXT4_DIR_FNAME_SHORT_LENGTH 127
>> +static struct kmem_cache *ext4_dir_fname_cachep;
>> +
>> +void __init ext4_init_dir(void)
>> +{
>> +	ext4_dir_fname_cachep = kmem_cache_create_usercopy("ext4_dir_fname",
>> +			struct_size_t(struct fname, name,  EXT4_DIR_FNAME_SHORT_LENGTH + 1),
>> +			0, SLAB_RECLAIM_ACCOUNT,
>> +			offsetof(struct fname, name), EXT4_DIR_FNAME_SHORT_LENGTH + 1,
>> +			NULL);
>> +}
>> +
>> +void ext4_exit_dir(void)
>> +{
>> +	if (ext4_dir_fname_cachep)
>> +		kmem_cache_destroy(ext4_dir_fname_cachep);
>> +}
>> +
>> +static struct fname *rb_node_fname_zalloc(__u8 name_len)
>> +{
>> +	struct fname *p;
>> +	if (ext4_dir_fname_cachep && name_len <= EXT4_DIR_FNAME_SHORT_LENGTH)
>> +		p = kmem_cache_alloc(ext4_dir_fname_cachep, GFP_KERNEL);
>> +	else
>> +		p = kmalloc(struct_size(p, name, name_len + 1), GFP_KERNEL);
>> +	if (p) {
>> +		/* no need to fill name with zeroes*/
>> +		memset(p, 0, offsetof(struct fname, name));
>> +		p->name[name_len] = 0;
>> +	}
>> +	return p;
>> +}
>> +
>> +static void rb_node_fname_free(struct fname *p) {
>> +	if (!p)
>> +		return;
>> +	if (ext4_dir_fname_cachep && p->name_len <= EXT4_DIR_FNAME_SHORT_LENGTH)
>> +		kmem_cache_free(ext4_dir_fname_cachep, p);
>> +	else
>> +		kfree(p);
>> +}
>> +
>> /*
>>  * This function implements a non-recursive way of freeing all of the
>>  * nodes in the red-black tree.
>> @@ -436,7 +478,7 @@ static void free_rb_tree_fname(struct rb_root *root)
>> 		while (fname) {
>> 			struct fname *old = fname;
>> 			fname = fname->next;
>> -			kfree(old);
>> +			rb_node_fname_free(old);
>> 		}
>> 
>> 	*root = RB_ROOT;
>> @@ -479,8 +521,7 @@ int ext4_htree_store_dirent(struct file *dir_file, __u32 hash,
>> 	p = &info->root.rb_node;
>> 
>> 	/* Create and allocate the fname structure */
>> -	new_fn = kzalloc(struct_size(new_fn, name, ent_name->len + 1),
>> -			 GFP_KERNEL);
>> +	new_fn = rb_node_fname_zalloc(ent_name->len);
>> 	if (!new_fn)
>> 		return -ENOMEM;
>> 	new_fn->hash = hash;
>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
>> index 5a20e9cd7184..33ab97143000 100644
>> --- a/fs/ext4/ext4.h
>> +++ b/fs/ext4/ext4.h
>> @@ -3795,6 +3795,10 @@ extern void ext4_orphan_file_block_trigger(
>> 				struct buffer_head *bh,
>> 				void *data, size_t size);
>> 
>> +/* dir.c */
>> +extern void __init ext4_init_dir(void);
>> +extern void ext4_exit_dir(void);
>> +
>> /*
>>  * Add new method to test whether block and inode bitmaps are properly
>>  * initialized. With uninit_bg reading the block from disk is not enough
>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>> index 181934499624..21ce3d78912a 100644
>> --- a/fs/ext4/super.c
>> +++ b/fs/ext4/super.c
>> @@ -7457,6 +7457,8 @@ static int __init ext4_init_fs(void)
>> 	if (err)
>> 		goto out;
>> 
>> +	ext4_init_dir();
>> +
>> 	return 0;
>> out:
>> 	unregister_as_ext2();
>> @@ -7497,6 +7499,7 @@ static void __exit ext4_exit_fs(void)
>> 	ext4_exit_post_read_processing();
>> 	ext4_exit_es();
>> 	ext4_exit_pending();
>> +	ext4_exit_dir();
>> }
>> 
>> MODULE_AUTHOR("Remy Card, Stephen Tweedie, Andrew Morton, Andreas Dilger, Theodore Ts'o and others");
>> --
>> 2.39.2
>> 
>
>
>Cheers, Andreas
>
>
>
>
>