Merge tag 'v6.6-rc3.vfs.ctime.revert' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull finegrained timestamp reverts from Christian Brauner:
 "Earlier this week we sent a few minor fixes for the multi-grained
  timestamp work in [1]. While we were polishing those up after Linus
  realized that there might be a nicer way to fix them we received a
  regression report in [2] that fine grained timestamps break gnulib
  tests and thus possibly other tools.

  The kernel will elide fine-grain timestamp updates when no one is
  actively querying for them to avoid performance impacts. So a sequence
  like write(f1) stat(f2) write(f2) stat(f2) write(f1) stat(f1) may
  result in timestamp f1 to be older than the final f2 timestamp even
  though f1 was last written too but the second write didn't update the
  timestamp.

  Such plotholes can lead to subtle bugs when programs compare
  timestamps. For example, the nap() function in [2] will estimate that
  it needs to wait one ns on a fine-grain timestamp enabled filesytem
  between subsequent calls to observe a timestamp change. But in general
  we don't update timestamps with more than one jiffie if we think that
  no one is actively querying for fine-grain timestamps to avoid
  performance impacts.

  While discussing various fixes the decision was to go back to the
  drawing board and ultimately to explore a solution that involves only
  exposing such fine-grained timestamps to nfs internally and never to
  userspace.

  As there are multiple solutions discussed the honest thing to do here
  is not to fix this up or disable it but to cleanly revert. The general
  infrastructure will probably come back but there is no reason to keep
  this code in mainline.

  The general changes to timestamp handling are valid and a good cleanup
  that will stay. The revert is fully bisectable"

Link: https://lore.kernel.org/all/20230918-hirte-neuzugang-4c2324e7bae3@brauner [1]
Link: https://lore.kernel.org/all/bf0524debb976627693e12ad23690094e4514303.camel@linuxfromscratch.org [2]

* tag 'v6.6-rc3.vfs.ctime.revert' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  Revert "fs: add infrastructure for multigrain timestamps"
  Revert "btrfs: convert to multigrain timestamps"
  Revert "ext4: switch to multigrain timestamps"
  Revert "xfs: switch to multigrain timestamps"
  Revert "tmpfs: add support for multigrain timestamps"

2 files changed, 22 insertions, 7 deletions

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 87a5c128f716..361535c71c0f 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1106,6 +1106,25 @@ void btrfs_check_nocow_unlock(struct btrfs_inode *inode)
 	btrfs_drew_write_unlock(&inode->root->snapshot_lock);
 }
 
+static void update_time_for_write(struct inode *inode)
+{
+	struct timespec64 now, ctime;
+
+	if (IS_NOCMTIME(inode))
+		return;
+
+	now = current_time(inode);
+	if (!timespec64_equal(&inode->i_mtime, &now))
+		inode->i_mtime = now;
+
+	ctime = inode_get_ctime(inode);
+	if (!timespec64_equal(&ctime, &now))
+		inode_set_ctime_to_ts(inode, now);
+
+	if (IS_I_VERSION(inode))
+		inode_inc_iversion(inode);
+}
+
 static int btrfs_write_check(struct kiocb *iocb, struct iov_iter *from,
 			     size_t count)
 {
@@ -1137,10 +1156,7 @@ static int btrfs_write_check(struct kiocb *iocb, struct iov_iter *from,
 	 * need to start yet another transaction to update the inode as we will
 	 * update the inode when we finish writing whatever data we write.
 	 */
-	if (!IS_NOCMTIME(inode)) {
-		inode->i_mtime = inode_set_ctime_current(inode);
-		inode_inc_iversion(inode);
-	}
+	update_time_for_write(inode);
 
 	start_pos = round_down(pos, fs_info->sectorsize);
 	oldsize = i_size_read(inode);
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 09bfe68d2ea3..cffdd6f7f8e8 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2150,7 +2150,7 @@ static struct file_system_type btrfs_fs_type = {
 	.name		= "btrfs",
 	.mount		= btrfs_mount,
 	.kill_sb	= btrfs_kill_super,
-	.fs_flags	= FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA | FS_MGTIME,
+	.fs_flags	= FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA,
 };
 
 static struct file_system_type btrfs_root_fs_type = {
@@ -2158,8 +2158,7 @@ static struct file_system_type btrfs_root_fs_type = {
 	.name		= "btrfs",
 	.mount		= btrfs_mount_root,
 	.kill_sb	= btrfs_kill_super,
-	.fs_flags	= FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA |
-			  FS_ALLOW_IDMAP | FS_MGTIME,
+	.fs_flags	= FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA | FS_ALLOW_IDMAP,
 };
 
 MODULE_ALIAS_FS("btrfs");