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@@ -457,6 +457,94 @@ out:
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return ret;
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}
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+/*
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+ * Back reference rules. Back refs have three main goals:
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+ *
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+ * 1) differentiate between all holders of references to an extent so that
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+ * when a reference is dropped we can make sure it was a valid reference
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+ * before freeing the extent.
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+ *
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+ * 2) Provide enough information to quickly find the holders of an extent
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+ * if we notice a given block is corrupted or bad.
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+ *
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+ * 3) Make it easy to migrate blocks for FS shrinking or storage pool
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+ * maintenance. This is actually the same as #2, but with a slightly
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+ * different use case.
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+ *
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+ * File extents can be referenced by:
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+ *
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+ * - multiple snapshots, subvolumes, or different generations in one subvol
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+ * - different files inside a single subvolume (in theory, not implemented yet)
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+ * - different offsets inside a file (bookend extents in file.c)
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+ *
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+ * The extent ref structure has fields for:
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+ *
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+ * - Objectid of the subvolume root
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+ * - Generation number of the tree holding the reference
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+ * - objectid of the file holding the reference
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+ * - offset in the file corresponding to the key holding the reference
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+ *
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+ * When a file extent is allocated the fields are filled in:
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+ * (root_key.objectid, trans->transid, inode objectid, offset in file)
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+ *
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+ * When a leaf is cow'd new references are added for every file extent found
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+ * in the leaf. It looks the same as the create case, but trans->transid
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+ * will be different when the block is cow'd.
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+ *
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+ * (root_key.objectid, trans->transid, inode objectid, offset in file)
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+ *
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+ * When a file extent is removed either during snapshot deletion or file
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+ * truncation, the corresponding back reference is found
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+ * by searching for:
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+ *
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+ * (btrfs_header_owner(leaf), btrfs_header_generation(leaf),
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+ * inode objectid, offset in file)
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+ *
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+ * Btree extents can be referenced by:
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+ *
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+ * - Different subvolumes
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+ * - Different generations of the same subvolume
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+ *
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+ * Storing sufficient information for a full reverse mapping of a btree
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+ * block would require storing the lowest key of the block in the backref,
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+ * and it would require updating that lowest key either before write out or
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+ * every time it changed. Instead, the objectid of the lowest key is stored
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+ * along with the level of the tree block. This provides a hint
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+ * about where in the btree the block can be found. Searches through the
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+ * btree only need to look for a pointer to that block, so they stop one
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+ * level higher than the level recorded in the backref.
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+ *
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+ * Some btrees do not do reference counting on their extents. These
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+ * include the extent tree and the tree of tree roots. Backrefs for these
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+ * trees always have a generation of zero.
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+ *
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+ * When a tree block is created, back references are inserted:
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+ *
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+ * (root->root_key.objectid, trans->transid or zero, lowest_key_objectid, level)
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+ *
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+ * When a tree block is cow'd in a reference counted root,
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+ * new back references are added for all the blocks it points to.
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+ * These are of the form (trans->transid will have increased since creation):
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+ *
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+ * (root->root_key.objectid, trans->transid, lowest_key_objectid, level)
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+ *
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+ * Because the lowest_key_objectid and the level are just hints
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+ * they are not used when backrefs are deleted. When a backref is deleted:
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+ *
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+ * if backref was for a tree root:
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+ * root_objectid = root->root_key.objectid
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+ * else
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+ * root_objectid = btrfs_header_owner(parent)
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+ *
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+ * (root_objectid, btrfs_header_generation(parent) or zero, 0, 0)
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+ *
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+ * Back Reference Key hashing:
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+ *
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+ * Back references have four fields, each 64 bits long. Unfortunately,
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+ * This is hashed into a single 64 bit number and placed into the key offset.
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+ * The key objectid corresponds to the first byte in the extent, and the
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+ * key type is set to BTRFS_EXTENT_REF_KEY
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+ */
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int btrfs_insert_extent_backref(struct btrfs_trans_handle *trans,
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struct btrfs_root *root,
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struct btrfs_path *path, u64 bytenr,
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@@ -939,10 +1027,13 @@ static int finish_current_insert(struct btrfs_trans_handle *trans, struct
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u64 start;
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u64 end;
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struct btrfs_fs_info *info = extent_root->fs_info;
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+ struct extent_buffer *eb;
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struct btrfs_path *path;
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struct btrfs_key ins;
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+ struct btrfs_disk_key first;
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struct btrfs_extent_item extent_item;
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int ret;
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+ int level;
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int err = 0;
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btrfs_set_stack_extent_refs(&extent_item, 1);
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@@ -961,10 +1052,19 @@ static int finish_current_insert(struct btrfs_trans_handle *trans, struct
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&extent_item, sizeof(extent_item));
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clear_extent_bits(&info->extent_ins, start, end, EXTENT_LOCKED,
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GFP_NOFS);
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+ eb = read_tree_block(extent_root, ins.objectid, ins.offset);
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+ level = btrfs_header_level(eb);
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+ if (level == 0) {
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+ btrfs_item_key(eb, &first, 0);
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+ } else {
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+ btrfs_node_key(eb, &first, 0);
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+ }
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err = btrfs_insert_extent_backref(trans, extent_root, path,
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start, extent_root->root_key.objectid,
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- 0, 0, 0);
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+ 0, btrfs_disk_key_objectid(&first),
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+ level);
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BUG_ON(err);
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+ free_extent_buffer(eb);
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}
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btrfs_free_path(path);
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return 0;
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Chris Mason