Btrfs: optimize back reference update during btrfs_drop_snapshot

This patch reading level 0 tree blocks that already use full backrefs.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 5302680..4aedbff 100644 (file)
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -4911,6 +4911,7 @@ static noinline void reada_walk_down(struct btrfs_trans_handle *trans,
        u64 bytenr;
        u64 generation;
        u64 refs;
+       u64 flags;
        u64 last = 0;
        u32 nritems;
        u32 blocksize;
@@ -4948,15 +4949,19 @@ static noinline void reada_walk_down(struct btrfs_trans_handle *trans,
                    generation <= root->root_key.offset)
                        continue;
 
+               /* We don't lock the tree block, it's OK to be racy here */
+               ret = btrfs_lookup_extent_info(trans, root, bytenr, blocksize,
+                                              &refs, &flags);
+               BUG_ON(ret);
+               BUG_ON(refs == 0);
+
                if (wc->stage == DROP_REFERENCE) {
-                       ret = btrfs_lookup_extent_info(trans, root,
-                                               bytenr, blocksize,
-                                               &refs, NULL);
-                       BUG_ON(ret);
-                       BUG_ON(refs == 0);
                        if (refs == 1)
                                goto reada;
 
+                       if (wc->level == 1 &&
+                           (flags & BTRFS_BLOCK_FLAG_FULL_BACKREF))
+                               continue;
                        if (!wc->update_ref ||
                            generation <= root->root_key.offset)
                                continue;
@@ -4965,6 +4970,10 @@ static noinline void reada_walk_down(struct btrfs_trans_handle *trans,
                                                  &wc->update_progress);
                        if (ret < 0)
                                continue;
+               } else {
+                       if (wc->level == 1 &&
+                           (flags & BTRFS_BLOCK_FLAG_FULL_BACKREF))
+                               continue;
                }
 reada:
                ret = readahead_tree_block(root, bytenr, blocksize,
@@ -4988,7 +4997,7 @@ reada:
 static noinline int walk_down_proc(struct btrfs_trans_handle *trans,
                                   struct btrfs_root *root,
                                   struct btrfs_path *path,
-                                  struct walk_control *wc)
+                                  struct walk_control *wc, int lookup_info)
 {
        int level = wc->level;
        struct extent_buffer *eb = path->nodes[level];
@@ -5003,8 +5012,9 @@ static noinline int walk_down_proc(struct btrfs_trans_handle *trans,
         * when reference count of tree block is 1, it won't increase
         * again. once full backref flag is set, we never clear it.
         */
-       if ((wc->stage == DROP_REFERENCE && wc->refs[level] != 1) ||
-           (wc->stage == UPDATE_BACKREF && !(wc->flags[level] & flag))) {
+       if (lookup_info &&
+           ((wc->stage == DROP_REFERENCE && wc->refs[level] != 1) ||
+            (wc->stage == UPDATE_BACKREF && !(wc->flags[level] & flag)))) {
                BUG_ON(!path->locks[level]);
                ret = btrfs_lookup_extent_info(trans, root,
                                               eb->start, eb->len,
@@ -5065,7 +5075,7 @@ static noinline int walk_down_proc(struct btrfs_trans_handle *trans,
 static noinline int do_walk_down(struct btrfs_trans_handle *trans,
                                 struct btrfs_root *root,
                                 struct btrfs_path *path,
-                                struct walk_control *wc)
+                                struct walk_control *wc, int *lookup_info)
 {
        u64 bytenr;
        u64 generation;
@@ -5085,8 +5095,10 @@ static noinline int do_walk_down(struct btrfs_trans_handle *trans,
         * for the subtree
         */
        if (wc->stage == UPDATE_BACKREF &&
-           generation <= root->root_key.offset)
+           generation <= root->root_key.offset) {
+               *lookup_info = 1;
                return 1;
+       }
 
        bytenr = btrfs_node_blockptr(path->nodes[level], path->slots[level]);
        blocksize = btrfs_level_size(root, level - 1);
@@ -5099,14 +5111,19 @@ static noinline int do_walk_down(struct btrfs_trans_handle *trans,
        btrfs_tree_lock(next);
        btrfs_set_lock_blocking(next);
 
-       if (wc->stage == DROP_REFERENCE) {
-               ret = btrfs_lookup_extent_info(trans, root, bytenr, blocksize,
-                                              &wc->refs[level - 1],
-                                              &wc->flags[level - 1]);
-               BUG_ON(ret);
-               BUG_ON(wc->refs[level - 1] == 0);
+       ret = btrfs_lookup_extent_info(trans, root, bytenr, blocksize,
+                                      &wc->refs[level - 1],
+                                      &wc->flags[level - 1]);
+       BUG_ON(ret);
+       BUG_ON(wc->refs[level - 1] == 0);
+       *lookup_info = 0;
 
+       if (wc->stage == DROP_REFERENCE) {
                if (wc->refs[level - 1] > 1) {
+                       if (level == 1 &&
+                           (wc->flags[0] & BTRFS_BLOCK_FLAG_FULL_BACKREF))
+                               goto skip;
+
                        if (!wc->update_ref ||
                            generation <= root->root_key.offset)
                                goto skip;
@@ -5120,12 +5137,17 @@ static noinline int do_walk_down(struct btrfs_trans_handle *trans,
                        wc->stage = UPDATE_BACKREF;
                        wc->shared_level = level - 1;
                }
+       } else {
+               if (level == 1 &&
+                   (wc->flags[0] & BTRFS_BLOCK_FLAG_FULL_BACKREF))
+                       goto skip;
        }
 
        if (!btrfs_buffer_uptodate(next, generation)) {
                btrfs_tree_unlock(next);
                free_extent_buffer(next);
                next = NULL;
+               *lookup_info = 1;
        }
 
        if (!next) {
@@ -5148,21 +5170,22 @@ static noinline int do_walk_down(struct btrfs_trans_handle *trans,
 skip:
        wc->refs[level - 1] = 0;
        wc->flags[level - 1] = 0;
+       if (wc->stage == DROP_REFERENCE) {
+               if (wc->flags[level] & BTRFS_BLOCK_FLAG_FULL_BACKREF) {
+                       parent = path->nodes[level]->start;
+               } else {
+                       BUG_ON(root->root_key.objectid !=
+                              btrfs_header_owner(path->nodes[level]));
+                       parent = 0;
+               }
 
-       if (wc->flags[level] & BTRFS_BLOCK_FLAG_FULL_BACKREF) {
-               parent = path->nodes[level]->start;
-       } else {
-               BUG_ON(root->root_key.objectid !=
-                      btrfs_header_owner(path->nodes[level]));
-               parent = 0;
+               ret = btrfs_free_extent(trans, root, bytenr, blocksize, parent,
+                                       root->root_key.objectid, level - 1, 0);
+               BUG_ON(ret);
        }
-
-       ret = btrfs_free_extent(trans, root, bytenr, blocksize, parent,
-                               root->root_key.objectid, level - 1, 0);
-       BUG_ON(ret);
-
        btrfs_tree_unlock(next);
        free_extent_buffer(next);
+       *lookup_info = 1;
        return 1;
 }
 
@@ -5276,6 +5299,7 @@ static noinline int walk_down_tree(struct btrfs_trans_handle *trans,
                                   struct walk_control *wc)
 {
        int level = wc->level;
+       int lookup_info = 1;
        int ret;
 
        while (level >= 0) {
@@ -5283,14 +5307,14 @@ static noinline int walk_down_tree(struct btrfs_trans_handle *trans,
                    btrfs_header_nritems(path->nodes[level]))
                        break;
 
-               ret = walk_down_proc(trans, root, path, wc);
+               ret = walk_down_proc(trans, root, path, wc, lookup_info);
                if (ret > 0)
                        break;
 
                if (level == 0)
                        break;
 
-               ret = do_walk_down(trans, root, path, wc);
+               ret = do_walk_down(trans, root, path, wc, &lookup_info);
                if (ret > 0) {
                        path->slots[level]++;
                        continue;