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>

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 84d7ca1..009bcf7 100644 (file)
--- 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;
+       }
 }
 
 /*