Hi,

I'm looking into a strange behavior that we can't break
bio_for_each_segment_all() after commit 6dc4f100c175 ("block: allow
bio_for_each_segment_all() to iterate over multi-page bvec").

It's screwing up all bio_for_each_segment_all() call with error out branch.


There is one relatively easy to trigger setup.

Here is my kernel branch, which is based on David Sterb's misc-next branch:
https://github.com/adam900710/linux/tree/tree_checker_testing

It's just two commits ahead.

Then some debug diff:

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index c2c0640aea55..05c880a5254b 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -851,16 +851,22 @@ static blk_status_t btree_csum_one_bio(struct bio
*bio)
        struct bio_vec *bvec;
        struct btrfs_root *root;
        int i, ret = 0;
+       int err = 0;
        struct bvec_iter_all iter_all;

        ASSERT(!bio_flagged(bio, BIO_CLONED));
        bio_for_each_segment_all(bvec, bio, i, iter_all) {
                root = BTRFS_I(bvec->bv_page->mapping->host)->root;
                ret = csum_dirty_buffer(root->fs_info, bvec->bv_page);
-               if (ret)
+               if (ret) {
+                       err = ret;
+                       pr_info("breaking out with ret=%d\n", ret);
                        break;
+               }
        }

+       if (err)
+               pr_info("err=%d out, but ret=%d\n",err, ret);
        return errno_to_blk_status(ret);
 }

Straightforward, if we break, we should have err == ret.

Then run fstests btrfs/151, which will trigger a false alert in
tree-checker:

  BTRFS critical (device dm-1): corrupt leaf: root=3 block=570572800
slot=1 devid=1 invalid total bytes: have 0
  BTRFS error (device dm-1): block=570572800 write time tree block
corruption detected
  breaking out with ret=-117
  err=-117 out, but ret=0

So it looks like the break line doens't really break, but continue
executing.

Thanks,
Qu