linux-kernel - Re: [RFC PATCH V3 2/7] ext4: specify the free pointer offset for ext4_inode

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Message-ID: <CAJuCfpGo=m9vdRQCqa-2MtAb9GBNsF4+6YXm7vzGFYtqsOzq1A@mail.gmail.com>
Date: Tue, 28 Oct 2025 10:22:24 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: akpm@...ux-foundation.org, vbabka@...e.cz, andreyknvl@...il.com, 
	cl@...ux.com, 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, vincenzo.frascino@....com, yeoreum.yun@....com, 
	tytso@....edu, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V3 2/7] ext4: specify the free pointer offset for ext4_inode_cache

On Mon, Oct 27, 2025 at 5:29 AM Harry Yoo <harry.yoo@...cle.com> 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.
> 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 699c15db28a8..2860e0ee913f 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1474,12 +1474,20 @@ 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,
> +               .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),

Hi Harry,
AFAIK freeptr_offset can be used only with SLAB_TYPESAFE_BY_RCU caches
(see https://elixir.bootlin.com/linux/v6.18-rc3/source/include/linux/slab.h#L302)
and check at https://elixir.bootlin.com/linux/v6.18-rc3/source/mm/slab_common.c#L234
should fail otherwise. The cache you are changing does not seem to
have this flag set.
Thanks,
Suren.

> +               .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;
> --
> 2.43.0
>