linux-kernel - Re: [PATCH 2/3] kernfs: Rearrange kernfs_node fields to reduce its size on 64bit

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Date: Wed, 10 Jan 2024 16:18:36 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Tejun Heo <tj@...nel.org>
Cc: gregkh@...uxfoundation.org, andrea.righi@...onical.com, ast@...nel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] kernfs: Rearrange kernfs_node fields to reduce its
 size on 64bit

Hi Tejun,

On Tue, Jan 9, 2024 at 10:49 PM Tejun Heo <tj@...nel.org> wrote:
> Moving .flags and .mode right below .hash makes kernfs_node smaller by 8
> bytes on 64bit.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>

Thanks for your patch!

> --- a/include/linux/kernfs.h
> +++ b/include/linux/kernfs.h
> @@ -206,6 +206,9 @@ struct kernfs_node {
>
>         const void              *ns;    /* namespace tag */
>         unsigned int            hash;   /* ns + name hash */
> +       unsigned short          flags;
> +       umode_t                 mode;
> +
>         union {
>                 struct kernfs_elem_dir          dir;
>                 struct kernfs_elem_symlink      symlink;
> @@ -220,8 +223,6 @@ struct kernfs_node {
>          */
>         u64                     id;
>
> -       unsigned short          flags;
> -       umode_t                 mode;
>         struct kernfs_iattrs    *iattr;

Note that there is now a hole at the end of the structure on 32-bit
architectures
where the alignment of u64 is 8 bytes.

Hence, sizeof(struct kernfs_node) grew from 104 to 112 bytes on (at
least) arm32 and rv32.
It did shrink by 8 bytes on amd64, arm64, mips64, and rv64.
Size is unchanged on ia32, m68k and sh.

I didn't check any other architectures.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68korg

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds