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.

Signed-off-by: Jeff Layton <jlay...@kernel.org>
---
 fs/btrfs/file.c  | 24 ++++--------------------
 fs/btrfs/super.c |  5 +++--
 2 files changed, 7 insertions(+), 22 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index d7a9ece7a40b..b9e75c9f95ac 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1106,25 +1106,6 @@ 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, 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)
 {
@@ -1156,7 +1137,10 @@ 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.
         */
-       update_time_for_write(inode);
+       if (!IS_NOCMTIME(inode)) {
+               inode->i_mtime = 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 f1dd172d8d5b..8eda51b095c9 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2144,7 +2144,7 @@ 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_flags       = FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA | FS_MGTIME,
 };
 
 static struct file_system_type btrfs_root_fs_type = {
@@ -2152,7 +2152,8 @@ 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_flags       = FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA |
+                         FS_ALLOW_IDMAP | FS_MGTIME,
 };
 
 MODULE_ALIAS_FS("btrfs");

-- 
2.41.0

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