btrfs: move eb subpage preallocation out of the loop

Initially we preallocate btrfs_subpage structure in the main loop of
alloc_extent_buffer().

But later commit fbca46eb ("btrfs: make nodesize >= PAGE_SIZE case
to reuse the non-subpage routine") has made sure we only go subpage
routine if our nodesize is smaller than PAGE_SIZE.

This means for that case, we only need to allocate the subpage structure
once anyway.

So this patch would make the preallocation out of the main loop.  This
would slightly reduce the workload when we hold the page lock, and make
code a little easier to read.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -3559,6 +3559,7 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 	struct extent_buffer *exists = NULL;
 	struct page *p;
 	struct address_space *mapping = fs_info->btree_inode->i_mapping;
+	struct btrfs_subpage *prealloc = NULL;
 	u64 lockdep_owner = owner_root;
 	int uptodate = 1;
 	int ret;
@@ -3595,36 +3596,30 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 	btrfs_set_buffer_lockdep_class(lockdep_owner, eb, level);
 
 	num_pages = num_extent_pages(eb);
-	for (i = 0; i < num_pages; i++, index++) {
-		struct btrfs_subpage *prealloc = NULL;
-
-		p = find_or_create_page(mapping, index, GFP_NOFS|__GFP_NOFAIL);
-		if (!p) {
-			exists = ERR_PTR(-ENOMEM);
-			goto free_eb;
-		}
 
 	/*
 	 * Preallocate page->private for subpage case, so that we won't
-		 * allocate memory with private_lock hold.  The memory will be
-		 * freed by attach_extent_buffer_page() or freed manually if
-		 * we exit earlier.
+	 * allocate memory with private_lock nor page lock hold.
 	 *
-		 * Although we have ensured one subpage eb can only have one
-		 * page, but it may change in the future for 16K page size
-		 * support, so we still preallocate the memory in the loop.
+	 * The memory will be freed by attach_extent_buffer_page() or freed
+	 * manually if we exit earlier.
 	 */
 	if (fs_info->nodesize < PAGE_SIZE) {
 		prealloc = btrfs_alloc_subpage(fs_info, BTRFS_SUBPAGE_METADATA);
 		if (IS_ERR(prealloc)) {
-				ret = PTR_ERR(prealloc);
-				unlock_page(p);
-				put_page(p);
-				exists = ERR_PTR(ret);
+			exists = ERR_CAST(prealloc);
 			goto free_eb;
 		}
 	}
 
+	for (i = 0; i < num_pages; i++, index++) {
+		p = find_or_create_page(mapping, index, GFP_NOFS|__GFP_NOFAIL);
+		if (!p) {
+			exists = ERR_PTR(-ENOMEM);
+			btrfs_free_subpage(prealloc);
+			goto free_eb;
+		}
+
 		spin_lock(&mapping->private_lock);
 		exists = grab_extent_buffer(fs_info, p);
 		if (exists) {