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>
---
V3: Rewrote boolean setting logic for clarity
V2: Added devid to output, removed unnecessary fs_info parameter

 fs/btrfs/disk-io.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 5da18ebc9222..6ef32eb94669 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3298,6 +3298,7 @@ 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)
@@ -3314,11 +3315,16 @@ 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);
@@ -3327,6 +3333,13 @@ 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;
 }
 
-- 
2.14.1

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