[PATCH] btrfs: print error if primary super block write fails

[Howard McLauchlan]

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 <hmclauch...@fb.com>
---
 fs/btrfs/disk-io.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 5da18ebc9222..8f96e1e4c613 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3293,11 +3293,13 @@ static int write_dev_supers(struct btrfs_device *device,
  * Return number of errors when buffer head is not found or not marked up to
  * date.
  */
-static int wait_dev_supers(struct btrfs_device *device, int max_mirrors)
+static int wait_dev_supers(struct btrfs_fs_info *fs_info,
+                          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)
@@ -3314,11 +3316,14 @@ static int wait_dev_supers(struct btrfs_device *device, 
int max_mirrors)
                                      BTRFS_SUPER_INFO_SIZE);
                if (!bh) {
                        errors++;
+                       primary_failed = (i == 0) || primary_failed;
                        continue;
                }
                wait_on_buffer(bh);
-               if (!buffer_uptodate(bh))
+               if (!buffer_uptodate(bh)) {
                        errors++;
+                       primary_failed = (i == 0) || primary_failed;
+               }
 
                /* drop our reference */
                brelse(bh);
@@ -3327,6 +3332,12 @@ static int wait_dev_supers(struct btrfs_device *device, 
int max_mirrors)
                brelse(bh);
        }
 
+       /* log error, force error return */
+       if (primary_failed) {
+               btrfs_err(fs_info, "error encountered writing primary super 
block");
+               return -1;
+       }
+
        return errors < i ? 0 : -1;
 }
 
@@ -3557,7 +3568,7 @@ int write_all_supers(struct btrfs_fs_info *fs_info, int 
max_mirrors)
                if (!dev->in_fs_metadata || !dev->writeable)
                        continue;
 
-               ret = wait_dev_supers(dev, max_mirrors);
+               ret = wait_dev_supers(fs_info, dev, max_mirrors);
                if (ret)
                        total_errors++;
        }
-- 
2.14.1

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