linux-kernel - [PATCH v2 2/3] kernfs: Rearrange kernfs_node fields to reduce its size on 64bit

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Message-ID: <ZZ7hwA18nfmFjYpj@slm.duckdns.org>
Date: Wed, 10 Jan 2024 08:28:16 -1000
From: Tejun Heo <tj@...nel.org>
To: gregkh@...uxfoundation.org
Cc: andrea.righi@...onical.com, ast@...nel.org,
	linux-kernel@...r.kernel.org, geert@...ux-m68k.org
Subject: [PATCH v2 2/3] kernfs: Rearrange kernfs_node fields to reduce its
 size on 64bit

From: Tejun Heo <tj@...nel.org>
Subject: kernfs: Rearrange kernfs_node fields to reduce its size on 64bit

Moving .flags and .mode right below .hash makes kernfs_node smaller by 8
bytes on 64bit. To avoid creating a hole from 8 bytes alignment on 32bit
archs, .priv is moved below so that there are two 32bit pointers after the
64bit .id field.

v2: Updated to avoid size increase on 32bit noticed by Geert.

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
---
The third patch applies fine without update.

 include/linux/kernfs.h |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -206,22 +206,22 @@ 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;
 		struct kernfs_elem_attr		attr;
 	};
 
-	void			*priv;
-
 	/*
 	 * 64bit unique ID.  On 64bit ino setups, id is the ino.  On 32bit,
 	 * the low 32bits are ino and upper generation.
 	 */
 	u64			id;
 
-	unsigned short		flags;
-	umode_t			mode;
+	void			*priv;
 	struct kernfs_iattrs	*iattr;
 };