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

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Message-ID: <aV9ZQP6nMk8rmXy0@hyeyoo>
Date: Thu, 8 Jan 2026 16:14:08 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: akpm@...ux-foundation.org, 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 Wed, Jan 07, 2026 at 02:54:15PM +0100, Vlastimil Babka wrote:
> 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?

Oops, right.
Will fix.

> > 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.

Right, will do.

> > +		.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;

-- 
Cheers,
Harry / Hyeonggon