Messages in this thread | | | From | Andreas Dilger <> | Subject | Re: [Ext2-devel] [UPDATE] Directory index for ext2 | Date | Thu, 31 May 2001 13:44:18 -0600 (MDT) |
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Daniel, you write: > - Fall back to linear search in case of corrupted index
OK, I have _some_ of the code needed to do this, but in one case I'm not sure of what your intention was - in dx_probe() you check for "unused_flags & 1" to signal a bad/unsupported index. Why only check the low bit instead of the whole thing? I currently have:
if (dir->i_size < dir->i_sb->s_blocksize * 2) goto fail; // bad EXT2_INDEX_FL set, clear? if (IS_ERR(bh = ext2_bread (dir, 0, 0))) goto fail; // FIXME error message? root = (struct dx_root *) bh->b_data;
// use the following for production once hash_version is 1 // if (root->info.hash_version & 3 == 0 || root->info.unused_flags) if (root->info.hash_version > 0 || root->info.unused_flags & 1) goto fail2; // unsupported hash format if ((indirect = root->info.indirect_levels) > 1) goto fail2; // unsupported hash format if (root->info.reserved_zero.v || root->info.info_length < sizeof(root->info)) goto fail2; // bad root node data ... if (dx_get_limit(entries) != dx_root_limit(dir, root->info.info_length)) goto fail2; // bad root node data ... if (dx_get_limit(entries) != dx_node_limit (dir)) goto fail3; // bad index node data
On lookup it is OK to fall back to linear search, but if we add an entry via linear we would then overwrite the root index. We probably want extra logic so that if we have a bad interior node we overwrite that (or another leaf instead of killing the root index). We probably also want to make a distinction between I/O errors and bad data (currently I just return NULL for both). I think Al's idea of doing the validation once on the initial read is a good one.
Instead of keeping reserved_zero as zero and using it to detect if an inode number was written there, we could write a magic number there to signal a valid hash. Alternately, instead of using hash_version to mark the hash type, we could leave that unused and make the above magic number the hash value, which is the hash of some fixed string, e.g.
HASH_V0 := dx_hack_hash("Daniel Phillips", 15) // constant value HASH_V1 := dx_new_hash("Daniel Phillips", 15) // constant value
struct dx_root { struct fake_dirent dot; char dot_name[4]; struct fake_dirent dotdot; char dotdot_name[4]; struct dx_root_info { le_u32 hash_magic; u8 unused; u8 info_length; /* 8 */ u8 indirect_levels; u8 unused_flags; } info; struct dx_entry entries[0]; };
/* * Hash value depends on existing hash type, so it is calculated here. * For new directories we never use this function, and simply pick the * default hash function when creating the new dx_root. */ static inline dx_frame *dx_probe(inode *dir, dentry *dentry, u32 *hash) dx_frame *frame) { struct dx_root *root;
if (IS_ERR(bh = ext2_bread (dir, 0, 0))) goto fail; // return error code root = (struct dx_root *) bh->b_data;
switch (le32_to_cpu(root->info.hash_magic.v)) { case HASH_V0: // hash-specific dx_root validation here *hash = dx_hack_hash(dentry->d_name.name, dentry->d_name.len); return dx_probe_hash(dir, hash, frame, bh); case HASH_V1: // hash-specific dx_root validation here *hash = dx_new_hash(dentry->d_name.name, dentry->d_name.len); return dx_probe_hash(dir, hash, frame, bh); default: printk("unsupported hash %u in directory %lu\n", root->info.hash_magic, dir->i_ino); brelse(bh); *hash = 0; } fail: return NULL; }
> - Finalize hash function
I noticed something interesting when running "mongo" with debugging on. It is adding filenames which are only sequential numbers, and the hash code is basically adding to only two blocks until those blocks are full, at which point (I guess) the blocks split and we add to two other blocks.
I haven't looked at it closely, but for _example_ it something like:
65531 to block 113 65532 to block 51 65533 to block 51 65534 to block 113 65535 to block 113 (repeats) 65600 to block 21 65601 to block 96 65602 to block 96 65603 to block 21 65604 to block 21 (repeats)
I will have to recompile and run with debugging on again to get actual output.
To me this would _seem_ bad, as it indicates the hash is not uniformly distributing the files across the hash space. However, skewed hashing may not necessarily be the bad for performance. It may even be that because we never have to rebalance the hash index structure that as long as we don't get large numbers of identical hashes it is just fine if similar filenames produce similar hash values. We just keep splitting the leaf blocks until the hash moves over to a different "range". For a balanced tree-based structure a skewed hash would be bad, because you would have to do full-tree rebalancing very often then.
> No known bugs, please test, thanks in advance.
Running mongo has shown up another bug, I see, but haven't had a chance to look into yet. It involves not being able to delete files from an indexed directory:
rm: cannot remove `/mnt/tmp/testdir1-0-0/d0/d1/d2/d3/509.r': Input/output error
This is after the files had been renamed (.r suffix). Do we re-hash directory entries if the file is renamed? If not, then that would explain this problem. It _looks_ like we do the right thing, but the mongo testing wipes out the filesystem after each test, and the above message is from a logfile only.
Cheers, Andreas -- Andreas Dilger \ "If a man ate a pound of pasta and a pound of antipasto, \ would they cancel out, leaving him still hungry?" http://www-mddsp.enel.ucalgary.ca/People/adilger/ -- Dogbert - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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