Btrfs: reduce lock contention during extent insertion

We're spending huge amounts of time on lock contention during
end_io processing because we unconditionally assume we are overwriting
an existing extent in the file for each IO.

This checks to see if we are outside i_size, and if so, it uses a
less expensive readonly search of the btree to look for existing
extents.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index d83260d..53bf2d7 100644 (file)
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -567,6 +567,7 @@ int btrfs_drop_extents(struct btrfs_trans_handle *trans, struct inode *inode,
        int extent_type;
        int recow;
        int ret;
+       int modify_tree = -1;
 
        if (drop_cache)
                btrfs_drop_extent_cache(inode, start, end - 1, 0);
@@ -575,10 +576,13 @@ int btrfs_drop_extents(struct btrfs_trans_handle *trans, struct inode *inode,
        if (!path)
                return -ENOMEM;
 
+       if (start >= BTRFS_I(inode)->disk_i_size)
+               modify_tree = 0;
+
        while (1) {
                recow = 0;
                ret = btrfs_lookup_file_extent(trans, root, path, ino,
-                                              search_start, -1);
+                                              search_start, modify_tree);
                if (ret < 0)
                        break;
                if (ret > 0 && path->slots[0] > 0 && search_start == start) {
@@ -634,7 +638,8 @@ next_slot:
                }
 
                search_start = max(key.offset, start);
-               if (recow) {
+               if (recow || !modify_tree) {
+                       modify_tree = -1;
                        btrfs_release_path(path);
                        continue;
                }