Commit d8d5f3e1 authored by Chris Mason's avatar Chris Mason

Btrfs: Add lowest key information to back refs for extent tree blocks as well.

Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
parent 7bb86316
......@@ -210,7 +210,7 @@ static int btree_writepages(struct address_space *mapping,
{
struct extent_map_tree *tree;
tree = &BTRFS_I(mapping->host)->extent_tree;
if (0 && wbc->sync_mode == WB_SYNC_NONE) {
if (wbc->sync_mode == WB_SYNC_NONE) {
u64 num_dirty;
u64 start = 0;
unsigned long thresh = 96 * 1024 * 1024;
......
......@@ -457,6 +457,94 @@ static int lookup_extent_backref(struct btrfs_trans_handle *trans,
return ret;
}
/*
* Back reference rules. Back refs have three main goals:
*
* 1) differentiate between all holders of references to an extent so that
* when a reference is dropped we can make sure it was a valid reference
* before freeing the extent.
*
* 2) Provide enough information to quickly find the holders of an extent
* if we notice a given block is corrupted or bad.
*
* 3) Make it easy to migrate blocks for FS shrinking or storage pool
* maintenance. This is actually the same as #2, but with a slightly
* different use case.
*
* File extents can be referenced by:
*
* - multiple snapshots, subvolumes, or different generations in one subvol
* - different files inside a single subvolume (in theory, not implemented yet)
* - different offsets inside a file (bookend extents in file.c)
*
* The extent ref structure has fields for:
*
* - Objectid of the subvolume root
* - Generation number of the tree holding the reference
* - objectid of the file holding the reference
* - offset in the file corresponding to the key holding the reference
*
* When a file extent is allocated the fields are filled in:
* (root_key.objectid, trans->transid, inode objectid, offset in file)
*
* When a leaf is cow'd new references are added for every file extent found
* in the leaf. It looks the same as the create case, but trans->transid
* will be different when the block is cow'd.
*
* (root_key.objectid, trans->transid, inode objectid, offset in file)
*
* When a file extent is removed either during snapshot deletion or file
* truncation, the corresponding back reference is found
* by searching for:
*
* (btrfs_header_owner(leaf), btrfs_header_generation(leaf),
* inode objectid, offset in file)
*
* Btree extents can be referenced by:
*
* - Different subvolumes
* - Different generations of the same subvolume
*
* Storing sufficient information for a full reverse mapping of a btree
* block would require storing the lowest key of the block in the backref,
* and it would require updating that lowest key either before write out or
* every time it changed. Instead, the objectid of the lowest key is stored
* along with the level of the tree block. This provides a hint
* about where in the btree the block can be found. Searches through the
* btree only need to look for a pointer to that block, so they stop one
* level higher than the level recorded in the backref.
*
* Some btrees do not do reference counting on their extents. These
* include the extent tree and the tree of tree roots. Backrefs for these
* trees always have a generation of zero.
*
* When a tree block is created, back references are inserted:
*
* (root->root_key.objectid, trans->transid or zero, lowest_key_objectid, level)
*
* When a tree block is cow'd in a reference counted root,
* new back references are added for all the blocks it points to.
* These are of the form (trans->transid will have increased since creation):
*
* (root->root_key.objectid, trans->transid, lowest_key_objectid, level)
*
* Because the lowest_key_objectid and the level are just hints
* they are not used when backrefs are deleted. When a backref is deleted:
*
* if backref was for a tree root:
* root_objectid = root->root_key.objectid
* else
* root_objectid = btrfs_header_owner(parent)
*
* (root_objectid, btrfs_header_generation(parent) or zero, 0, 0)
*
* Back Reference Key hashing:
*
* Back references have four fields, each 64 bits long. Unfortunately,
* This is hashed into a single 64 bit number and placed into the key offset.
* The key objectid corresponds to the first byte in the extent, and the
* key type is set to BTRFS_EXTENT_REF_KEY
*/
int btrfs_insert_extent_backref(struct btrfs_trans_handle *trans,
struct btrfs_root *root,
struct btrfs_path *path, u64 bytenr,
......@@ -939,10 +1027,13 @@ static int finish_current_insert(struct btrfs_trans_handle *trans, struct
u64 start;
u64 end;
struct btrfs_fs_info *info = extent_root->fs_info;
struct extent_buffer *eb;
struct btrfs_path *path;
struct btrfs_key ins;
struct btrfs_disk_key first;
struct btrfs_extent_item extent_item;
int ret;
int level;
int err = 0;
btrfs_set_stack_extent_refs(&extent_item, 1);
......@@ -961,10 +1052,19 @@ static int finish_current_insert(struct btrfs_trans_handle *trans, struct
&extent_item, sizeof(extent_item));
clear_extent_bits(&info->extent_ins, start, end, EXTENT_LOCKED,
GFP_NOFS);
eb = read_tree_block(extent_root, ins.objectid, ins.offset);
level = btrfs_header_level(eb);
if (level == 0) {
btrfs_item_key(eb, &first, 0);
} else {
btrfs_node_key(eb, &first, 0);
}
err = btrfs_insert_extent_backref(trans, extent_root, path,
start, extent_root->root_key.objectid,
0, 0, 0);
0, btrfs_disk_key_objectid(&first),
level);
BUG_ON(err);
free_extent_buffer(eb);
}
btrfs_free_path(path);
return 0;
......
......@@ -545,13 +545,10 @@ int btrfs_drop_extents(struct btrfs_trans_handle *trans,
u64 disk_num_bytes = 0;
u64 extent_num_bytes = 0;
u64 root_gen;
u64 root_owner;
if (leaf != root->node) {
root_gen =
btrfs_header_generation(path->nodes[1]);
} else {
root_gen = btrfs_header_generation(leaf);
}
root_gen = btrfs_header_generation(leaf);
root_owner = btrfs_header_owner(leaf);
if (found_extent) {
disk_bytenr =
btrfs_file_extent_disk_bytenr(leaf,
......@@ -575,7 +572,7 @@ int btrfs_drop_extents(struct btrfs_trans_handle *trans,
ret = btrfs_free_extent(trans, root,
disk_bytenr,
disk_num_bytes,
root->root_key.objectid,
root_owner,
root_gen, inode->i_ino,
key.offset, 0);
}
......
......@@ -563,6 +563,7 @@ static int btrfs_truncate_in_trans(struct btrfs_trans_handle *trans,
u64 extent_num_bytes = 0;
u64 item_end = 0;
u64 root_gen = 0;
u64 root_owner = 0;
int found_extent;
int del_item;
int extent_type = -1;
......@@ -673,15 +674,8 @@ static int btrfs_truncate_in_trans(struct btrfs_trans_handle *trans,
found_extent = 1;
inode->i_blocks -= num_dec;
}
if (leaf == root->node) {
root_gen =
btrfs_header_generation(leaf);
} else {
struct extent_buffer *parent;
parent = path->nodes[1];
root_gen =
btrfs_header_generation(parent);
}
root_gen = btrfs_header_generation(leaf);
root_owner = btrfs_header_owner(leaf);
}
} else if (extent_type == BTRFS_FILE_EXTENT_INLINE &&
!del_item) {
......@@ -703,7 +697,7 @@ static int btrfs_truncate_in_trans(struct btrfs_trans_handle *trans,
if (found_extent) {
ret = btrfs_free_extent(trans, root, extent_start,
extent_num_bytes,
root->root_key.objectid,
root_owner,
root_gen, inode->i_ino,
found_key.offset, 0);
BUG_ON(ret);
......
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