Btrfs: Add mount option to turn off data cow

A number of workloads do not require copy on write data or checksumming.
mount -o nodatasum to disable checksums and -o nodatacow to disable
both copy on write and checksumming.

In nodatacow mode, copy on write is still performed when a given extent
is under snapshot.

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

1 files changed, 98 insertions, 3 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 00414836e963..1412d556313f 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -679,6 +679,104 @@ out:
         return 0;
 }
 
+u32 btrfs_count_snapshots_in_path(struct btrfs_root *root,
+                                  struct btrfs_path *count_path,
+                                  u64 first_extent)
+{
+        struct btrfs_root *extent_root = root->fs_info->extent_root;
+        struct btrfs_path *path;
+        u64 bytenr;
+        u64 found_objectid;
+        u64 root_objectid = 0;
+        u32 total_count = 0;
+        u32 cur_count;
+        u32 refs;
+        u32 nritems;
+        int ret;
+        struct btrfs_key key;
+        struct btrfs_key found_key;
+        struct extent_buffer *l;
+        struct btrfs_extent_item *item;
+        struct btrfs_extent_ref *ref_item;
+        int level = -1;
+
+        path = btrfs_alloc_path();
+again:
+        if (level == -1)
+                bytenr = first_extent;
+        else
+                bytenr = count_path->nodes[level]->start;
+
+        cur_count = 0;
+        key.objectid = bytenr;
+        key.offset = 0;
+
+        btrfs_set_key_type(&key, BTRFS_EXTENT_ITEM_KEY);
+        ret = btrfs_search_slot(NULL, extent_root, &key, path, 0, 0);
+        if (ret < 0)
+                goto out;
+        BUG_ON(ret == 0);
+
+        l = path->nodes[0];
+        btrfs_item_key_to_cpu(l, &found_key, path->slots[0]);
+
+        if (found_key.objectid != bytenr ||
+            found_key.type != BTRFS_EXTENT_ITEM_KEY) {
+                goto out;
+        }
+
+        item = btrfs_item_ptr(l, path->slots[0], struct btrfs_extent_item);
+        refs = btrfs_extent_refs(l, item);
+        while (1) {
+                nritems = btrfs_header_nritems(l);
+                if (path->slots[0] >= nritems) {
+                        ret = btrfs_next_leaf(extent_root, path);
+                        if (ret == 0)
+                                continue;
+                        break;
+                }
+                btrfs_item_key_to_cpu(l, &found_key, path->slots[0]);
+                if (found_key.objectid != bytenr)
+                        break;
+                if (found_key.type != BTRFS_EXTENT_REF_KEY) {
+                        path->slots[0]++;
+                        continue;
+                }
+
+                cur_count++;
+                ref_item = btrfs_item_ptr(l, path->slots[0],
+                                          struct btrfs_extent_ref);
+                found_objectid = btrfs_ref_root(l, ref_item);
+
+                if (found_objectid != root_objectid)
+                        total_count++;
+
+                if (total_count > 1)
+                        goto out;
+
+                if (root_objectid == 0)
+                        root_objectid = found_objectid;
+
+                path->slots[0]++;
+        }
+        if (cur_count == 0) {
+                total_count = 0;
+                goto out;
+        }
+        if (total_count > 1)
+                goto out;
+        if (level >= 0 && root->node == count_path->nodes[level])
+                goto out;
+        level++;
+        btrfs_release_path(root, path);
+        goto again;
+
+out:
+        btrfs_free_path(path);
+        return total_count;
+
+}
+
 int btrfs_inc_root_ref(struct btrfs_trans_handle *trans,
                        struct btrfs_root *root, u64 owner_objectid)
 {
@@ -1127,9 +1225,6 @@ static int __free_extent(struct btrfs_trans_handle *trans, struct btrfs_root
         if (!path)
                 return -ENOMEM;
 
-        if (ref_generation && owner_objectid == 0 && root_objectid == 3) {
-//printk("drop backref root %Lu gen %Lu byte %Lu\n", root_objectid, ref_generation, bytenr );
-        }
         ret = lookup_extent_backref(trans, extent_root, path,
                                     bytenr, root_objectid,
                                     ref_generation,