Btrfs: create a pinned em when writing to a prealloc range in DIO

Wade Cline reported a problem where he was getting garbage and warnings when
writing to a preallocated range via O_DIRECT.  This is because we weren't
creating our normal pinned extent_map for the range we were writing to,
which was causing all sorts of issues.  This patch fixes the problem and
makes his testcase much happier.  Thanks,

Reported-by: Wade Cline <clinew@linux.vnet.ibm.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/inode.c

@@ -5898,6 +5898,48 @@ static int lock_extent_direct(struct inode *inode, u64 lockstart, u64 lockend,
 	return ret;
 }
 
+static struct extent_map *create_pinned_em(struct inode *inode, u64 start,
+					   u64 len, u64 orig_start,
+					   u64 block_start, u64 block_len,
+					   int type)
+{
+	struct extent_map_tree *em_tree;
+	struct extent_map *em;
+	struct btrfs_root *root = BTRFS_I(inode)->root;
+	int ret;
+
+	em_tree = &BTRFS_I(inode)->extent_tree;
+	em = alloc_extent_map();
+	if (!em)
+		return ERR_PTR(-ENOMEM);
+
+	em->start = start;
+	em->orig_start = orig_start;
+	em->len = len;
+	em->block_len = block_len;
+	em->block_start = block_start;
+	em->bdev = root->fs_info->fs_devices->latest_bdev;
+	set_bit(EXTENT_FLAG_PINNED, &em->flags);
+	if (type == BTRFS_ORDERED_PREALLOC)
+		set_bit(EXTENT_FLAG_PREALLOC, &em->flags);
+
+	do {
+		btrfs_drop_extent_cache(inode, em->start,
+				em->start + em->len - 1, 0);
+		write_lock(&em_tree->lock);
+		ret = add_extent_mapping(em_tree, em);
+		write_unlock(&em_tree->lock);
+	} while (ret == -EEXIST);
+
+	if (ret) {
+		free_extent_map(em);
+		return ERR_PTR(ret);
+	}
+
+	return em;
+}
+
+
 static int btrfs_get_blocks_direct(struct inode *inode, sector_t iblock,
 				   struct buffer_head *bh_result, int create)
 {
@@ -6012,6 +6054,19 @@ static int btrfs_get_blocks_direct(struct inode *inode, sector_t iblock,
 			goto must_cow;
 
 		if (can_nocow_odirect(trans, inode, start, len) == 1) {
+			u64 orig_start = em->start;
+
+			if (type == BTRFS_ORDERED_PREALLOC) {
+				free_extent_map(em);
+				em = create_pinned_em(inode, start, len,
+						       orig_start,
+						       block_start, len, type);
+				if (IS_ERR(em)) {
+					btrfs_end_transaction(trans, root);
+					goto unlock_err;
+				}
+			}
+
 			ret = btrfs_add_ordered_extent_dio(inode, start,
 					   block_start, len, len, type);
 			btrfs_end_transaction(trans, root);