mm/slab: reduce slab accounting memory overhead by allocating slabobj_ext metadata within unused slab space

[RFC PATCH V3 2/7] ext4: specify the free pointer offset for ext4_inode_cache

Posted by Harry Yoo 3 months, 2 weeks ago

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@oracle.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),
+		.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

Re: [RFC PATCH V3 2/7] ext4: specify the free pointer offset for ext4_inode_cache

Posted by Suren Baghdasaryan 3 months, 1 week ago

On Mon, Oct 27, 2025 at 5:29 AM Harry Yoo <harry.yoo@oracle.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@oracle.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
>

Re: [RFC PATCH V3 2/7] ext4: specify the free pointer offset for ext4_inode_cache

Posted by Suren Baghdasaryan 3 months, 1 week ago

On Tue, Oct 28, 2025 at 10:22 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Mon, Oct 27, 2025 at 5:29 AM Harry Yoo <harry.yoo@oracle.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@oracle.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.

Oh, sorry, your patches got reordered in my mailbox and I missed the
first one where you are removing this limitation. Let me review that
first. Sorry for the noise.

> 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
> >

[RFC PATCH V3 1/7] mm/slab: allow specifying freepointer offset when using constructor
[RFC PATCH V3 2/7] ext4: specify the free pointer offset for ext4_inode_cache
[RFC PATCH V3 3/7] mm/slab: abstract slabobj_ext access via new slab_obj_ext() helper
[RFC PATCH V3 4/7] mm/slab: use stride to access slabobj_ext
[RFC PATCH V3 5/7] mm/memcontrol,alloc_tag: handle slabobj_ext access under KASAN poison
[RFC PATCH V3 6/7] mm/slab: save memory by allocating slabobj_ext array from leftover
[RFC PATCH V3 7/7] mm/slab: place slabobj_ext metadata in unused space within s->size