Is it a file system code bug?

[Zhihui Zhang]

In routine ffs_balloc(), after we have determined that the block is
already there, we use the following statement to read the block in:

        if (flags & B_CLRBUF) {
                error = bread(vp, lbn, (int)fs->fs_bsize, NOCRED, &nbp);
                if (error) {
                        brelse(nbp);
                        goto fail;
                }
        } else {
                nbp = getblk(vp, lbn, fs->fs_bsize, 0, 0);
                nbp->b_blkno = fsbtodb(fs, nb);
        }

The semantics of B_CLRBUF is confusing here. Even if it is allowed to be
twisted a little bit, what about the case when we are writing a FULL
block?  Do we still have to read its old contents back which is going to
be erased entirely?  Note that ufs_write() always sets B_CLRBUF.

Thanks for any enlightenment.

-Zhihui


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