linux-ext4 - [PATCH 01/10] fs: turn inode ctime fields into a single ktime

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Message-Id: <20240626-mgtime-v1-1-a189352d0f8f@kernel.org>
Date: Wed, 26 Jun 2024 21:00:21 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Alexander Viro <viro@...iv.linux.org.uk>, 
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
 Steven Rostedt <rostedt@...dmis.org>, 
 Masami Hiramatsu <mhiramat@...nel.org>, 
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
 Chandan Babu R <chandan.babu@...cle.com>, 
 "Darrick J. Wong" <djwong@...nel.org>, Theodore Ts'o <tytso@....edu>, 
 Andreas Dilger <adilger.kernel@...ger.ca>, Chris Mason <clm@...com>, 
 Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>, 
 Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: kernel-team@...com, linux-fsdevel@...r.kernel.org, 
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
 linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org, 
 linux-btrfs@...r.kernel.org, linux-mm@...ck.org, linux-nfs@...r.kernel.org, 
 Jeff Layton <jlayton@...nel.org>
Subject: [PATCH 01/10] fs: turn inode ctime fields into a single ktime_t

The ctime is not settable to arbitrary values. It always comes from the
system clock, so we'll never stamp an inode with a value that can't be
represented there. If we disregard people setting their system clock
past the year 2262, there is no reason we can't replace the ctime fields
with a ktime_t.

Switch the ctime fields to a single ktime_t. Move the i_generation down
above i_fsnotify_mask and then move the i_version into the resulting 8
byte hole. This shrinks struct inode by 8 bytes total, and should
improve the cache footprint as the i_version and ctime are usually
updated together.

The one downside I can see to switching to a ktime_t is that if someone
has a filesystem with files on it that has ctimes outside the ktime_t
range (before ~1678 AD or after ~2262 AD), we won't be able to display
them properly in stat() without some special treatment in the
filesystem. The operating assumption here is that that is not a
practical problem.

Signed-off-by: Jeff Layton <jlayton@...nel.org>
---
 include/linux/fs.h | 26 +++++++++++---------------
 1 file changed, 11 insertions(+), 15 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 5ff362277834..5139dec085f2 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -662,11 +662,10 @@ struct inode {
 	loff_t			i_size;
 	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;
+	ktime_t			__i_ctime;
+	atomic64_t		i_version;
 	spinlock_t		i_lock;	/* i_blocks, i_bytes, maybe i_size */
 	unsigned short          i_bytes;
 	u8			i_blkbits;
@@ -701,7 +700,6 @@ struct inode {
 		struct hlist_head	i_dentry;
 		struct rcu_head		i_rcu;
 	};
-	atomic64_t		i_version;
 	atomic64_t		i_sequence; /* see futex */
 	atomic_t		i_count;
 	atomic_t		i_dio_count;
@@ -724,6 +722,8 @@ struct inode {
 	};

+	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 */
@@ -1608,29 +1608,25 @@ static inline struct timespec64 inode_set_mtime(struct inode *inode,
 	return inode_set_mtime_to_ts(inode, ts);
 }

-static inline time64_t inode_get_ctime_sec(const struct inode *inode)
+static inline struct timespec64 inode_get_ctime(const struct inode *inode)
 {
-	return inode->i_ctime_sec;
+	return ktime_to_timespec64(inode->__i_ctime);
 }

-static inline long inode_get_ctime_nsec(const struct inode *inode)
+static inline time64_t inode_get_ctime_sec(const struct inode *inode)
 {
-	return inode->i_ctime_nsec;
+	return inode_get_ctime(inode).tv_sec;
 }

-static inline struct timespec64 inode_get_ctime(const struct inode *inode)
+static inline long inode_get_ctime_nsec(const struct inode *inode)
 {
-	struct timespec64 ts = { .tv_sec  = inode_get_ctime_sec(inode),
-				 .tv_nsec = inode_get_ctime_nsec(inode) };
-
-	return ts;
+	return inode_get_ctime(inode).tv_nsec;
 }

 static inline struct timespec64 inode_set_ctime_to_ts(struct inode *inode,
 						      struct timespec64 ts)
 {
-	inode->i_ctime_sec = ts.tv_sec;
-	inode->i_ctime_nsec = ts.tv_nsec;
+	inode->__i_ctime = ktime_set(ts.tv_sec, ts.tv_nsec);
 	return ts;
 }

-- 
2.45.2