linux-ext4 - [PATCH 06/10] fuse2fs: use coarse timestamps for iomap mode

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Message-ID: <175279461828.716436.6399243218938406982.stgit@frogsfrogsfrogs>
Date: Thu, 17 Jul 2025 16:46:57 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: tytso@....edu
Cc: joannelkoong@...il.com, miklos@...redi.hu, John@...ves.net,
 linux-fsdevel@...r.kernel.org, bernd@...ernd.com, linux-ext4@...r.kernel.org,
 neal@...pa.dev
Subject: [PATCH 06/10] fuse2fs: use coarse timestamps for iomap mode

From: Darrick J. Wong <djwong@...nel.org>

In iomap mode, the kernel is responsible for maintaining timestamps
because file writes don't upcall to fuse2fs.  The kernel's predicate for
deciding if [cm]time should be updated bases its decisions off [cm]time
being an exact match for the coarse clock (instead of checking that
[cm]time < coarse_clock) which means that fuse2fs setting a fine-grained
timestamp that is slightly ahead of the coarse clock can result in
timestamps appearing to go backwards.  generic/423 doesn't like seeing
btime > ctime from statx, so we'll use the coarse clock in iomap mode.

Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 misc/fuse2fs.c |   34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)


diff --git a/misc/fuse2fs.c b/misc/fuse2fs.c
index ddc647f32c5df6..54f501b36d808b 100644
--- a/misc/fuse2fs.c
+++ b/misc/fuse2fs.c
@@ -575,8 +575,24 @@ static inline void fuse2fs_dump_extents(struct fuse2fs *ff, ext2_ino_t ino,
 	ext2fs_extent_free(extents);
 }
 
-static void get_now(struct timespec *now)
+static void fuse2fs_get_now(struct fuse2fs *ff, struct timespec *now)
 {
+#ifdef CLOCK_REALTIME_COARSE
+	/*
+	 * In iomap mode, the kernel is responsible for maintaining timestamps
+	 * because file writes don't upcall to fuse2fs.  The kernel's predicate
+	 * for deciding if [cm]time should be updated bases its decisions off
+	 * [cm]time being an exact match for the coarse clock (instead of
+	 * checking that [cm]time < coarse_clock) which means that fuse2fs
+	 * setting a fine-grained timestamp that is slightly ahead of the
+	 * coarse clock can result in timestamps appearing to go backwards.
+	 * generic/423 doesn't like seeing btime > ctime from statx, so we'll
+	 * use the coarse clock in iomap mode.
+	 */
+	if (fuse2fs_iomap_does_fileio(ff) &&
+	    !clock_gettime(CLOCK_REALTIME_COARSE, now))
+		return;
+#endif
 #ifdef CLOCK_REALTIME
 	if (!clock_gettime(CLOCK_REALTIME, now))
 		return;
@@ -604,7 +620,7 @@ static void fuse2fs_init_timestamps(struct fuse2fs *ff, ext2_ino_t ino,
 {
 	struct timespec now;
 
-	get_now(&now);
+	fuse2fs_get_now(ff, &now);
 	EXT4_INODE_SET_XTIME(i_atime, &now, inode);
 	EXT4_INODE_SET_XTIME(i_ctime, &now, inode);
 	EXT4_INODE_SET_XTIME(i_mtime, &now, inode);
@@ -623,7 +639,7 @@ static int fuse2fs_update_ctime(struct fuse2fs *ff, ext2_ino_t ino,
 	struct timespec now;
 	struct ext2_inode_large inode;
 
-	get_now(&now);
+	fuse2fs_get_now(ff, &now);
 
 	/* If user already has a inode buffer, just update that */
 	if (pinode) {
@@ -669,7 +685,7 @@ static int fuse2fs_update_atime(struct fuse2fs *ff, ext2_ino_t ino)
 	pinode = &inode;
 	EXT4_INODE_GET_XTIME(i_atime, &atime, pinode);
 	EXT4_INODE_GET_XTIME(i_mtime, &mtime, pinode);
-	get_now(&now);
+	fuse2fs_get_now(ff, &now);
 
 	datime = atime.tv_sec + ((double)atime.tv_nsec / NSEC_PER_SEC);
 	dmtime = mtime.tv_sec + ((double)mtime.tv_nsec / NSEC_PER_SEC);
@@ -704,7 +720,7 @@ static int fuse2fs_update_mtime(struct fuse2fs *ff, ext2_ino_t ino,
 	struct timespec now;
 
 	if (pinode) {
-		get_now(&now);
+		fuse2fs_get_now(ff, &now);
 		EXT4_INODE_SET_XTIME(i_mtime, &now, pinode);
 		EXT4_INODE_SET_XTIME(i_ctime, &now, pinode);
 		increment_version(pinode);
@@ -719,7 +735,7 @@ static int fuse2fs_update_mtime(struct fuse2fs *ff, ext2_ino_t ino,
 	if (err)
 		return translate_error(fs, ino, err);
 
-	get_now(&now);
+	fuse2fs_get_now(ff, &now);
 	EXT4_INODE_SET_XTIME(i_mtime, &now, &inode);
 	EXT4_INODE_SET_XTIME(i_ctime, &now, &inode);
 	increment_version(&inode);
@@ -4380,9 +4396,9 @@ static int op_utimens(const char *path, const struct timespec ctv[2]
 	tv[1] = ctv[1];
 #ifdef UTIME_NOW
 	if (tv[0].tv_nsec == UTIME_NOW)
-		get_now(tv);
+		fuse2fs_get_now(ff, tv);
 	if (tv[1].tv_nsec == UTIME_NOW)
-		get_now(tv + 1);
+		fuse2fs_get_now(ff, tv + 1);
 #endif /* UTIME_NOW */
 #ifdef UTIME_OMIT
 	if (tv[0].tv_nsec != UTIME_OMIT)
@@ -6917,7 +6933,7 @@ static int __translate_error(ext2_filsys fs, ext2_ino_t ino, errcode_t err,
 			error_message(err), func, line);
 
 	/* Make a note in the error log */
-	get_now(&now);
+	fuse2fs_get_now(ff, &now);
 	ext2fs_set_tstamp(fs->super, s_last_error_time, now.tv_sec);
 	fs->super->s_last_error_ino = ino;
 	fs->super->s_last_error_line = line;