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2006-02-01

ext3: magic, more magic.


ext3 htree code in Linux kernel implements peculiar version of balanced tree used to efficiently handle large directories.

htree directory consists of block sized nodes. Some of them (leaf nodes) contain directory entries in the same format as ext2. Other nodes contain index: they are filled with hashes and pointers to other nodes.

When new file is created in a directory, a directory entry is inserted in one of leaf nodes. When leaf node has not enough space for new entry, new node is appended to the tree, and part of directory entries is moved there. This process is known as a split. Pointer to new node is then inserted into some index node, and new node can overflow at this point, causing another split and so on.

If splits occur whole way up to the root of the tree, new root has to be added (tree grows).

It's obvious that in the worst case (extremely rare in practice) insertion of a new entry may require a new block on each tree level, plus new root, right? Now, looking at the ext3_dx_add_entry() function we see something strange:

                 } else {
                         dxtrace(printk("Creating second level index...\n"));
                         memcpy((char *) entries2, (char *) entries,
                                icount * sizeof(struct dx_entry));
                         dx_set_limit(entries2, dx_node_limit(dir));
 
                         /* Set up root */
                         dx_set_count(entries, 1);
                         dx_set_block(entries + 0, newblock);
                         ((struct dx_root *) frames[0].bh->b_data)->info.indirect_levels = 1;
 
                         /* Add new access path frame */
                         frame = frames + 1;
                         frame->at = at = at - entries + entries2;
                         frame->entries = entries = entries2;
                         frame->bh = bh2;
                         err = ext3_journal_get_write_access(handle,
                                                              frame->bh);
                         if (err)
                                 goto journal_error;
                 }

At this moment entries points to the already existing full node and entries2 to the newly created one. As one can see, contents of entries is shifted into entries2, and entries is declared to be new root of the tree. So now tree has a root node with a single pointer to the index node that... still has not enough free space (remember entries2 got everything entries had). Omitted code that follows proceeds with splitting leaf node, assuming that its parent has enough space to insert a pointer to the new leaf. So how this is supposed to work? Or, does this work at all? That's tricky part and the curious reader is invited to try to infer what's going on without looking at the rest of this post.

[full text follows]

The answer is simple: by ext3 htree design, capacity of the root node is smaller than that of non-root index one. This is a byproduct of binary compatibility between htree and old ext2 format: root node is always the first block in the directory and it always contains dot and dotdot directory entries. As a result, when contents of old root is copied into new node, that node ends up having enough space for two additional entries.

This is obviously one of the worst hacks and least documented at that. Shame.

Thanks to Alex Tomas for clearing this mystery for me. As he says: "Htree code is simple to understand: it only takes to tune yourself to Daniel Phillips brain-waves frequency".

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