btrfs: make reflinks respect O_SYNC O_DSYNC and S_SYNC flags

If we reflink to or from a file opened with O_SYNC/O_DSYNC or to/from a
file that has the S_SYNC attribute set, we totally ignore that and do not
durably persist the reflink changes. Since a reflink can change the data
readable from a file (and mtime/ctime, or a file size), it makes sense to
durably persist (fsync) the source and destination files/ranges.

This was previously discussed at:

https://lore.kernel.org/linux-btrfs/20200903035225.GJ6090@magnolia/

The recently introduced test case generic/628, from fstests, exercises
these scenarios and currently fails without this change.

So make sure we fsync the source and destination files/ranges when either
of them was opened with O_SYNC/O_DSYNC or has the S_SYNC attribute set,
just like XFS already does.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/reflink.c b/fs/btrfs/reflink.c
index 6746460..f4ec06b 100644 (file)
--- a/fs/btrfs/reflink.c
+++ b/fs/btrfs/reflink.c
@@ -834,6 +834,16 @@ static int btrfs_remap_file_range_prep(struct file *file_in, loff_t pos_in,
                                            len, remap_flags);
 }
 
+static bool file_sync_write(const struct file *file)
+{
+       if (file->f_flags & (__O_SYNC | O_DSYNC))
+               return true;
+       if (IS_SYNC(file_inode(file)))
+               return true;
+
+       return false;
+}
+
 loff_t btrfs_remap_file_range(struct file *src_file, loff_t off,
                struct file *dst_file, loff_t destoff, loff_t len,
                unsigned int remap_flags)
@@ -871,5 +881,20 @@ out_unlock:
                unlock_two_nondirectories(src_inode, dst_inode);
        }
 
+       /*
+        * If either the source or the destination file was opened with O_SYNC,
+        * O_DSYNC or has the S_SYNC attribute, fsync both the destination and
+        * source files/ranges, so that after a successful return (0) followed
+        * by a power failure results in the reflinked data to be readable from
+        * both files/ranges.
+        */
+       if (ret == 0 && len > 0 &&
+           (file_sync_write(src_file) || file_sync_write(dst_file))) {
+               ret = btrfs_sync_file(src_file, off, off + len - 1, 0);
+               if (ret == 0)
+                       ret = btrfs_sync_file(dst_file, destoff,
+                                             destoff + len - 1, 0);
+       }
+
        return ret < 0 ? ret : len;
 }