Re: [PATCH 1/2] btrfs: remove conditional path allocation from read_locked_inode, add path allocation to iget

David Sterba
2024-08-13T22:21:56Z

On Tue, Aug 13, 2024 at 01:27:33PM -0700, Leo Martins wrote:
> Move the path allocation from inside btrfs_read_locked_inode to
> btrfs_iget. This makes the code easier to reason about as it is clear
> where the allocation occurs and who is in charge of freeing the path.
> I have investigated all of the callers of btrfs_iget_path to make sure
> that it is never called with a null path with the expectation
> of a path allocation. All of the null calls seem to come from btrfs_iget
> so it makes sense to do the allocation within btrfs_iget.

This looks like a good cleanup. The path argument was added to fix some
problem in the past, 4222ea7100c0e3 ("Btrfs: fix deadlock on tree root
leaf when finding free extent") but doing the allocation in the caller
makes things more clear.

> ---
> fs/btrfs/inode.c | 23 ++++++++++-------------
> 1 file changed, 10 insertions(+), 13 deletions(-)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 07858d63378f..b89b4b1bd3da 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -3790,10 +3790,9 @@ static int btrfs_init_file_extent_tree(struct btrfs_inode *inode)
> * read an inode from the btree into the in-memory inode
> */
> static int btrfs_read_locked_inode(struct inode *inode,
> - struct btrfs_path *in_path)
> + struct btrfs_path *path)
> {
> struct btrfs_fs_info *fs_info = inode_to_fs_info(inode);
> - struct btrfs_path *path = in_path;
> struct extent_buffer *leaf;
> struct btrfs_inode_item *inode_item;
> struct btrfs_root *root = BTRFS_I(inode)->root;
> @@ -3813,18 +3812,10 @@ static int btrfs_read_locked_inode(struct inode *inode,
> if (!ret)
> filled = true;

You can add an ASSERT(path)

>
> - if (!path) {
> - path = btrfs_alloc_path();
> - if (!path)
> - return -ENOMEM;
> - }
> -
> btrfs_get_inode_key(BTRFS_I(inode), &location);
>
> ret = btrfs_lookup_inode(NULL, root, path, &location, 0);
> if (ret) {
> - if (path != in_path)
> - btrfs_free_path(path);
> return ret;
> }
>
> @@ -3960,8 +3951,6 @@ static int btrfs_read_locked_inode(struct inode *inode,
> btrfs_ino(BTRFS_I(inode)),
> btrfs_root_id(root), ret);
> }
> - if (path != in_path)
> - btrfs_free_path(path);
>
> if (!maybe_acls)
> cache_no_acl(inode);
> @@ -5631,7 +5620,15 @@ struct inode *btrfs_iget_path(u64 ino, struct btrfs_root *root,
>
> struct inode *btrfs_iget(u64 ino, struct btrfs_root *root)
> {
> - return btrfs_iget_path(ino, root, NULL);
> + struct btrfs_path *path = btrfs_alloc_path();
> +
> + if (!path)
> + return ERR_PTR(-ENOMEM);
> +
> + struct inode *inode = btrfs_iget_path(ino, root, path);

In kernel the declaration block should be contiguous, ie. not mixed with
statements like that. Also, we don't do allocatios (and other
non-trivial assignments that require error handling) in the declaration
block. This may different in other parts of kernel, in btrfs it's the
preferred way.