btrfs: print error if primary super block write fails

Presently, failing a primary super block write but succeeding in at
least one super block write in general will appear to users as if
nothing important went wrong. However, upon unmounting and re-mounting,
the file system will be in a rolled back state. This was discovered
with a BCC program that uses bpf_override_return() to fail super block
writes.

This patch outputs an error clarifying that the primary super block
write has failed, so users can expect potentially erroneous behaviour.
It also forces wait_dev_supers() to return an error to its caller if
the primary super block write fails.

Signed-off-by: Howard McLauchlan <hmclauchlan@fb.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 21f34ad..9f7ed43 100644 (file)
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3290,6 +3290,7 @@ static int wait_dev_supers(struct btrfs_device *device, int max_mirrors)
        struct buffer_head *bh;
        int i;
        int errors = 0;
+       bool primary_failed = false;
        u64 bytenr;
 
        if (max_mirrors == 0)
@@ -3306,11 +3307,16 @@ static int wait_dev_supers(struct btrfs_device *device, int max_mirrors)
                                      BTRFS_SUPER_INFO_SIZE);
                if (!bh) {
                        errors++;
+                       if (i == 0)
+                               primary_failed = true;
                        continue;
                }
                wait_on_buffer(bh);
-               if (!buffer_uptodate(bh))
+               if (!buffer_uptodate(bh)) {
                        errors++;
+                       if (i == 0)
+                               primary_failed = true;
+               }
 
                /* drop our reference */
                brelse(bh);
@@ -3319,6 +3325,13 @@ static int wait_dev_supers(struct btrfs_device *device, int max_mirrors)
                brelse(bh);
        }
 
+       /* log error, force error return */
+       if (primary_failed) {
+               btrfs_err(device->fs_info, "error writing primary super block to device %llu",
+                         device->devid);
+               return -1;
+       }
+
        return errors < i ? 0 : -1;
 }