Btrfs: search for an allocation hint while filling file COW

The allocator has some nice knobs for sending hints about where
to try and allocate new blocks, but when we're doing file allocations
we're not sending any hint at all.

This commit adds a simple extent map search to see if we can
quickly and easily find a hint for the allocator.

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

1 files changed, 9 insertions, 1 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 941f1b71cd22..81ba6654c332 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -726,6 +726,15 @@ static noinline int cow_file_range(struct inode *inode,
         BUG_ON(disk_num_bytes >
                btrfs_super_total_bytes(&root->fs_info->super_copy));
 
+
+        read_lock(&BTRFS_I(inode)->extent_tree.lock);
+        em = search_extent_mapping(&BTRFS_I(inode)->extent_tree,
+                                   start, num_bytes);
+        if (em) {
+                alloc_hint = em->block_start;
+                free_extent_map(em);
+        }
+        read_unlock(&BTRFS_I(inode)->extent_tree.lock);
         btrfs_drop_extent_cache(inode, start, start + num_bytes - 1, 0);
 
         while (disk_num_bytes > 0) {
@@ -738,7 +747,6 @@ static noinline int cow_file_range(struct inode *inode,
                 em = alloc_extent_map(GFP_NOFS);
                 em->start = start;
                 em->orig_start = em->start;
-
                 ram_size = ins.offset;
                 em->len = ins.offset;