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 08:26:49 -1000
From: Tejun Heo <tj@...nel.org>
To: Geert Uytterhoeven <geert@...ux-m68k.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

On Wed, Jan 10, 2024 at 04:18:36PM +0100, Geert Uytterhoeven wrote:
> 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.

Ah, thanks. I'll shuffle things a bit more so that the size doesn't increase
for 32bits.

-- 
tejun