linux-kernel - Re: [PATCH V5 3/8] ext4: specify the free pointer offset for ext4_inode

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Message-ID: <e28c08e4-5048-429b-97a0-8d51e494efcd@suse.cz>
Date: Wed, 7 Jan 2026 14:54:15 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Harry Yoo <harry.yoo@...cle.com>, akpm@...ux-foundation.org
Cc: andreyknvl@...il.com, cl@...two.org, dvyukov@...gle.com,
 glider@...gle.com, hannes@...xchg.org, linux-mm@...ck.org,
 mhocko@...nel.org, muchun.song@...ux.dev, rientjes@...gle.com,
 roman.gushchin@...ux.dev, ryabinin.a.a@...il.com, shakeel.butt@...ux.dev,
 surenb@...gle.com, vincenzo.frascino@....com, yeoreum.yun@....com,
 tytso@....edu, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, hao.li@...ux.dev
Subject: Re: [PATCH V5 3/8] ext4: specify the free pointer offset for
 ext4_inode_cache

On 1/5/26 09:02, Harry Yoo wrote:
> Convert ext4_inode_cache to use the kmem_cache_args interface and
> specify a free pointer offset.
> 
> Since ext4_inode_cache uses a constructor, the free pointer would be
> placed after the object to overwriting fields used by the constructor.

                             ^ prevent?

> However, some fields such as ->i_flags are not used by the constructor
> and can safely be repurposed for the free pointer.
> 
> Specify the free pointer offset at i_flags to reduce the object size.
> 
> Signed-off-by: Harry Yoo <harry.yoo@...cle.com>
> ---
>  fs/ext4/super.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 87205660c5d0..42580643a466 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1491,12 +1491,20 @@ static void init_once(void *foo)
>  
>  static int __init init_inodecache(void)
>  {
> -	ext4_inode_cachep = kmem_cache_create_usercopy("ext4_inode_cache",
> -				sizeof(struct ext4_inode_info), 0,
> -				SLAB_RECLAIM_ACCOUNT | SLAB_ACCOUNT,
> -				offsetof(struct ext4_inode_info, i_data),
> -				sizeof_field(struct ext4_inode_info, i_data),
> -				init_once);
> +	struct kmem_cache_args args = {
> +		.align = 0,

Nit: it's implicit so not necessary.

> +		.useroffset = offsetof(struct ext4_inode_info, i_data),
> +		.usersize = sizeof_field(struct ext4_inode_info, i_data),
> +		.use_freeptr_offset = true,
> +		.freeptr_offset = offsetof(struct ext4_inode_info, i_flags),
> +		.ctor = init_once,
> +	};
> +
> +	ext4_inode_cachep = kmem_cache_create("ext4_inode_cache",
> +				sizeof(struct ext4_inode_info),
> +				&args,
> +				SLAB_RECLAIM_ACCOUNT | SLAB_ACCOUNT);
> +
>  	if (ext4_inode_cachep == NULL)
>  		return -ENOMEM;
>  	return 0;