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>

1 files changed, 53 insertions, 29 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 53026806ae9e..4aedbff36b8f 100644
--- 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;