linux-kernel - [PATCH] fs: switch timespec64 fields in inode to discrete integers

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Message-Id: <20240517-amtime-v1-1-7b804ca4be8f@kernel.org>
Date: Fri, 17 May 2024 20:08:40 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Christian Brauner <brauner@...nel.org>, 
 Alexander Viro <viro@...iv.linux.org.uk>
Cc: Jan Kara <jack@...e.cz>, Linus Torvalds <torvalds@...ux-foundation.org>, 
 linux-fsdevel@...r.kernel.org, Amir Goldstein <amir73il@...il.com>, 
 linux-kernel@...r.kernel.org, Jeff Layton <jlayton@...nel.org>
Subject: [PATCH] fs: switch timespec64 fields in inode to discrete integers

Adjacent struct timespec64's pack poorly. Switch them to discrete
integer fields for the seconds and nanoseconds. This shaves 8 bytes off
struct inode with a garden-variety Fedora Kconfig on x86_64, but that
also moves the i_lock into the previous cacheline, away from the fields
it protects.

To remedy that, move i_generation above the i_lock, which moves the new
4-byte hole to just after the i_fsnotify_mask in my setup. Amir has
plans to use that to expand the i_fsnotify_mask, so add a comment to
that effect as well.

Cc: Amir Goldstein <amir73il@...il.com>
Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Jeff Layton <jlayton@...nel.org>
---
For reference (according to pahole):

    Before:	/* size: 624, cachelines: 10, members: 53 */
    After: 	/* size: 616, cachelines: 10, members: 56 */

I've done some lightweight testing with this and it seems fine, but I
wouldn't mind seeing it sit in -next for a bit to make sure I haven't
missed anything.
---
 include/linux/fs.h | 48 ++++++++++++++++++++++++++++++++----------------
 1 file changed, 32 insertions(+), 16 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index b7b9b3b79acc..45e8766de7d8 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -660,9 +660,13 @@ struct inode {
 	};
 	dev_t			i_rdev;
 	loff_t			i_size;
-	struct timespec64	__i_atime;
-	struct timespec64	__i_mtime;
-	struct timespec64	__i_ctime; /* use inode_*_ctime accessors! */
+	time64_t		i_atime_sec;
+	time64_t		i_mtime_sec;
+	time64_t		i_ctime_sec;
+	u32			i_atime_nsec;
+	u32			i_mtime_nsec;
+	u32			i_ctime_nsec;
+	u32			i_generation;
 	spinlock_t		i_lock;	/* i_blocks, i_bytes, maybe i_size */
 	unsigned short          i_bytes;
 	u8			i_blkbits;
@@ -719,10 +723,10 @@ struct inode {
 		unsigned		i_dir_seq;
 	};
 
-	__u32			i_generation;
 
 #ifdef CONFIG_FSNOTIFY
 	__u32			i_fsnotify_mask; /* all events this inode cares about */
+	/* 32-bit hole reserved for expanding i_fsnotify_mask */
 	struct fsnotify_mark_connector __rcu	*i_fsnotify_marks;
 #endif
 
@@ -1544,23 +1548,27 @@ struct timespec64 inode_set_ctime_current(struct inode *inode);
 
 static inline time64_t inode_get_atime_sec(const struct inode *inode)
 {
-	return inode->__i_atime.tv_sec;
+	return inode->i_atime_sec;
 }
 
 static inline long inode_get_atime_nsec(const struct inode *inode)
 {
-	return inode->__i_atime.tv_nsec;
+	return inode->i_atime_nsec;
 }
 
 static inline struct timespec64 inode_get_atime(const struct inode *inode)
 {
-	return inode->__i_atime;
+	struct timespec64 ts = { .tv_sec  = inode_get_atime_sec(inode),
+				 .tv_nsec = inode_get_atime_nsec(inode) };
+
+	return ts;
 }
 
 static inline struct timespec64 inode_set_atime_to_ts(struct inode *inode,
 						      struct timespec64 ts)
 {
-	inode->__i_atime = ts;
+	inode->i_atime_sec = ts.tv_sec;
+	inode->i_atime_nsec = ts.tv_nsec;
 	return ts;
 }
 
@@ -1569,28 +1577,32 @@ static inline struct timespec64 inode_set_atime(struct inode *inode,
 {
 	struct timespec64 ts = { .tv_sec  = sec,
 				 .tv_nsec = nsec };
+
 	return inode_set_atime_to_ts(inode, ts);
 }
 
 static inline time64_t inode_get_mtime_sec(const struct inode *inode)
 {
-	return inode->__i_mtime.tv_sec;
+	return inode->i_mtime_sec;
 }
 
 static inline long inode_get_mtime_nsec(const struct inode *inode)
 {
-	return inode->__i_mtime.tv_nsec;
+	return inode->i_mtime_nsec;
 }
 
 static inline struct timespec64 inode_get_mtime(const struct inode *inode)
 {
-	return inode->__i_mtime;
+	struct timespec64 ts = { .tv_sec  = inode_get_mtime_sec(inode),
+				 .tv_nsec = inode_get_mtime_nsec(inode) };
+	return ts;
 }
 
 static inline struct timespec64 inode_set_mtime_to_ts(struct inode *inode,
 						      struct timespec64 ts)
 {
-	inode->__i_mtime = ts;
+	inode->i_mtime_sec = ts.tv_sec;
+	inode->i_mtime_nsec = ts.tv_nsec;
 	return ts;
 }
 
@@ -1604,23 +1616,27 @@ static inline struct timespec64 inode_set_mtime(struct inode *inode,
 
 static inline time64_t inode_get_ctime_sec(const struct inode *inode)
 {
-	return inode->__i_ctime.tv_sec;
+	return inode->i_ctime_sec;
 }
 
 static inline long inode_get_ctime_nsec(const struct inode *inode)
 {
-	return inode->__i_ctime.tv_nsec;
+	return inode->i_ctime_nsec;
 }
 
 static inline struct timespec64 inode_get_ctime(const struct inode *inode)
 {
-	return inode->__i_ctime;
+	struct timespec64 ts = { .tv_sec  = inode_get_ctime_sec(inode),
+				 .tv_nsec = inode_get_ctime_nsec(inode) };
+
+	return ts;
 }
 
 static inline struct timespec64 inode_set_ctime_to_ts(struct inode *inode,
 						      struct timespec64 ts)
 {
-	inode->__i_ctime = ts;
+	inode->i_ctime_sec = ts.tv_sec;
+	inode->i_ctime_nsec = ts.tv_nsec;
 	return ts;
 }
 

---
base-commit: 7ee332c9f12bc5b380e36919cd7d056592a7073f
change-id: 20240517-amtime-68a7ebc76f45

Best regards,
-- 
Jeff Layton <jlayton@...nel.org>