Btrfs: map the node block when looking for readahead targets

If we have particularly full nodes, we could call btrfs_node_blockptr up to 32
times, which is 32 pairs of kmap/kunmap, which _sucks_.  So go ahead and map the
extent buffer while we look for readahead targets.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

1 files changed, 21 insertions, 2 deletions

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 84d7ca1fe0ba..009bcf7f1e4b 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -1229,6 +1229,7 @@ static void reada_for_search(struct btrfs_root *root,
         u64 search;
         u64 target;
         u64 nread = 0;
+        u64 gen;
         int direction = path->reada;
         struct extent_buffer *eb;
         u32 nr;
@@ -1256,6 +1257,15 @@ static void reada_for_search(struct btrfs_root *root,
         nritems = btrfs_header_nritems(node);
         nr = slot;
         while (1) {
+                if (!node->map_token) {
+                        unsigned long offset = btrfs_node_key_ptr_offset(nr);
+                        map_private_extent_buffer(node, offset,
+                                                  sizeof(struct btrfs_key_ptr),
+                                                  &node->map_token,
+                                                  &node->kaddr,
+                                                  &node->map_start,
+                                                  &node->map_len, KM_USER1);
+                }
                 if (direction < 0) {
                         if (nr == 0)
                                 break;
@@ -1273,14 +1283,23 @@ static void reada_for_search(struct btrfs_root *root,
                 search = btrfs_node_blockptr(node, nr);
                 if ((search <= target && target - search <= 65536) ||
                     (search > target && search - target <= 65536)) {
-                        readahead_tree_block(root, search, blocksize,
-                                     btrfs_node_ptr_generation(node, nr));
+                        gen = btrfs_node_ptr_generation(node, nr);
+                        if (node->map_token) {
+                                unmap_extent_buffer(node, node->map_token,
+                                                    KM_USER1);
+                                node->map_token = NULL;
+                        }
+                        readahead_tree_block(root, search, blocksize, gen);
                         nread += blocksize;
                 }
                 nscan++;
                 if ((nread > 65536 || nscan > 32))
                         break;
         }
+        if (node->map_token) {
+                unmap_extent_buffer(node, node->map_token, KM_USER1);
+                node->map_token = NULL;
+        }
 }
 
 /*