Enable multigrain timestamps, which should ensure that there is an
apparent change to the timestamp whenever it has been written after
being actively observed via getattr.

Beyond enabling the FS_MGTIME flag, this patch eliminates
update_time_for_write, which goes to great pains to avoid in-memory
stores. Just have it overwrite the timestamps unconditionally.

Note that this also drops the IS_I_VERSION check and unconditionally
bumps the change attribute, since SB_I_VERSION is always set on btrfs.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/btrfs/file.c  | 25 ++++---------------------
 fs/btrfs/super.c |  3 ++-
 2 files changed, 6 insertions(+), 22 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 2aeb8116549c..1656ad7498b8 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1120,26 +1120,6 @@ 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, ts;
-
-	if (IS_NOCMTIME(inode))
-		return;
-
-	now = current_time(inode);
-	ts = inode_get_mtime(inode);
-	if (!timespec64_equal(&ts, &now))
-		inode_set_mtime_to_ts(inode, now);
-
-	ts = inode_get_ctime(inode);
-	if (!timespec64_equal(&ts, &now))
-		inode_set_ctime_to_ts(inode, now);
-
-	if (IS_I_VERSION(inode))
-		inode_inc_iversion(inode);
-}
-
 int btrfs_write_check(struct kiocb *iocb, struct iov_iter *from, size_t count)
 {
 	struct file *file = iocb->ki_filp;
@@ -1170,7 +1150,10 @@ int btrfs_write_check(struct kiocb *iocb, struct iov_iter *from, size_t count)
 	 * need to start yet another transaction to update the inode as we will
 	 * update the inode when we finish writing whatever data we write.
 	 */
-	update_time_for_write(inode);
+	if (!IS_NOCMTIME(inode)) {
+		inode_set_mtime_to_ts(inode, inode_set_ctime_current(inode));
+		inode_inc_iversion(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 98fa0f382480..d423acfe11d0 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2198,7 +2198,8 @@ static struct file_system_type btrfs_fs_type = {
 	.init_fs_context	= btrfs_init_fs_context,
 	.parameters		= btrfs_fs_parameters,
 	.kill_sb		= btrfs_kill_super,
-	.fs_flags		= FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA | FS_ALLOW_IDMAP,
+	.fs_flags		= FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA |
+				  FS_ALLOW_IDMAP | FS_MGTIME,
  };
 
 MODULE_ALIAS_FS("btrfs");

-- 
2.46.2