btrfs: scrub: try to fix super block errors

[BUG]
The following script shows that, although scrub can detect super block
errors, it never tries to fix it:

	mkfs.btrfs -f -d raid1 -m raid1 $dev1 $dev2
	xfs_io -c "pwrite 67108864 4k" $dev2

	mount $dev1 $mnt
	btrfs scrub start -B $dev2
	btrfs scrub start -Br $dev2
	umount $mnt

The first scrub reports the super error correctly:

  scrub done for f3289218-abd3-41ac-a630-202f766c0859
  Scrub started:    Tue Aug  2 14:44:11 2022
  Status:           finished
  Duration:         0:00:00
  Total to scrub:   1.26GiB
  Rate:             0.00B/s
  Error summary:    super=1
    Corrected:      0
    Uncorrectable:  0
    Unverified:     0

But the second read-only scrub still reports the same super error:

  Scrub started:    Tue Aug  2 14:44:11 2022
  Status:           finished
  Duration:         0:00:00
  Total to scrub:   1.26GiB
  Rate:             0.00B/s
  Error summary:    super=1
    Corrected:      0
    Uncorrectable:  0
    Unverified:     0

[CAUSE]
The comments already shows that super block can be easily fixed by
committing a transaction:

	/*
	 * If we find an error in a super block, we just report it.
	 * They will get written with the next transaction commit
	 * anyway
	 */

But the truth is, such assumption is not always true, and since scrub
should try to repair every error it found (except for read-only scrub),
we should really actively commit a transaction to fix this.

[FIX]
Just commit a transaction if we found any super block errors, after
everything else is done.

We cannot do this just after scrub_supers(), as
btrfs_commit_transaction() will try to pause and wait for the running
scrub, thus we can not call it with scrub_lock hold.

Signed-off-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/scrub.c b/fs/btrfs/scrub.c
index 0fe7c48..7d9b09e 100644 (file)
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -4093,6 +4093,7 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
        int ret;
        struct btrfs_device *dev;
        unsigned int nofs_flag;
+       bool need_commit = false;
 
        if (btrfs_fs_closing(fs_info))
                return -EAGAIN;
@@ -4196,6 +4197,12 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
         */
        nofs_flag = memalloc_nofs_save();
        if (!is_dev_replace) {
+               u64 old_super_errors;
+
+               spin_lock(&sctx->stat_lock);
+               old_super_errors = sctx->stat.super_errors;
+               spin_unlock(&sctx->stat_lock);
+
                btrfs_info(fs_info, "scrub: started on devid %llu", devid);
                /*
                 * by holding device list mutex, we can
@@ -4204,6 +4211,16 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
                mutex_lock(&fs_info->fs_devices->device_list_mutex);
                ret = scrub_supers(sctx, dev);
                mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+
+               spin_lock(&sctx->stat_lock);
+               /*
+                * Super block errors found, but we can not commit transaction
+                * at current context, since btrfs_commit_transaction() needs
+                * to pause the current running scrub (hold by ourselves).
+                */
+               if (sctx->stat.super_errors > old_super_errors && !sctx->readonly)
+                       need_commit = true;
+               spin_unlock(&sctx->stat_lock);
        }
 
        if (!ret)
@@ -4230,6 +4247,25 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
        scrub_workers_put(fs_info);
        scrub_put_ctx(sctx);
 
+       /*
+        * We found some super block errors before, now try to force a
+        * transaction commit, as scrub has finished.
+        */
+       if (need_commit) {
+               struct btrfs_trans_handle *trans;
+
+               trans = btrfs_start_transaction(fs_info->tree_root, 0);
+               if (IS_ERR(trans)) {
+                       ret = PTR_ERR(trans);
+                       btrfs_err(fs_info,
+       "scrub: failed to start transaction to fix super block errors: %d", ret);
+                       return ret;
+               }
+               ret = btrfs_commit_transaction(trans);
+               if (ret < 0)
+                       btrfs_err(fs_info,
+       "scrub: failed to commit transaction to fix super block errors: %d", ret);
+       }
        return ret;
 out:
        scrub_workers_put(fs_info);