Re: [RFC PATCH v3 7/7] btrfs: enable swap file support

[David Sterba]

On Tue, Dec 09, 2014 at 05:45:48PM -0800, Omar Sandoval wrote:
> +static void __clear_swapfile_extents(struct inode *inode)
> +{
> +	u64 isize = inode->i_size;
> +	struct extent_map *em;
> +	u64 start, len;
> +
> +	start = 0;
> +	while (start < isize) {
> +		len = isize - start;
> +		em = btrfs_get_extent(inode, NULL, 0, start, len, 0);
> +		if (IS_ERR(em))
> +			return;

This could transiently fail if there's no memory to allocate the em, and
would leak the following extents.

> +
> +		clear_bit(EXTENT_FLAG_SWAPFILE, &em->flags);
> +
> +		start = extent_map_end(em);
> +		free_extent_map(em);
> +	}
> +}
> +
> +static int btrfs_swap_activate(struct swap_info_struct *sis, struct file *file,
> +			       sector_t *span)
> +{
> +	struct inode *inode = file_inode(file);
> +	struct btrfs_fs_info *fs_info = BTRFS_I(inode)->root->fs_info;
> +	struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
> +	int ret = 0;
> +	u64 isize = inode->i_size;
> +	struct extent_state *cached_state = NULL;
> +	struct extent_map *em;
> +	u64 start, len;
> +
> +	if (BTRFS_I(inode)->flags & BTRFS_INODE_COMPRESS) {
> +		/* Can't do direct I/O on a compressed file. */
> +		btrfs_err(fs_info, "swapfile is compressed");
> +		return -EINVAL;
> +	}
> +	if (!(BTRFS_I(inode)->flags & BTRFS_INODE_NODATACOW)) {
> +		/*
> +		 * Going through the copy-on-write path while swapping pages
> +		 * in/out and doing a bunch of allocations could stress the
> +		 * memory management code that got us there in the first place,
> +		 * and that's sure to be a bad time.
> +		 */
> +		btrfs_err(fs_info, "swapfile is copy-on-write");
> +		return -EINVAL;
> +	}
> +
> +	lock_extent_bits(io_tree, 0, isize - 1, 0, &cached_state);
> +
> +	/*
> +	 * All of the extents must be allocated and support direct I/O. Inline
> +	 * extents and compressed extents fall back to buffered I/O, so those
> +	 * are no good. Additionally, all of the extents must be safe for nocow.
> +	 */
> +	atomic_inc(&BTRFS_I(inode)->root->nr_swapfiles);
> +	start = 0;
> +	while (start < isize) {
> +		len = isize - start;
> +		em = btrfs_get_extent(inode, NULL, 0, start, len, 0);
> +		if (IS_ERR(em)) {

		IS_ERR_OR_NULL(em)

>From now on the em is valid and has to be free_extent_map()ed ...

> +			ret = PTR_ERR(em);
> +			goto out;
> +		}
> +
> +		if (test_bit(EXTENT_FLAG_VACANCY, &em->flags) ||
> +		    em->block_start == EXTENT_MAP_HOLE) {
> +			btrfs_err(fs_info, "swapfile has holes");
> +			ret = -EINVAL;

... and all the error branches would miss it.

> +			goto out;
> +		}
> +		if (em->block_start == EXTENT_MAP_INLINE) {
> +			/*
> +			 * It's unlikely we'll ever actually find ourselves
> +			 * here, as a file small enough to fit inline won't be
> +			 * big enough to store more than the swap header, but in
> +			 * case something changes in the future, let's catch it
> +			 * here rather than later.
> +			 */
> +			btrfs_err(fs_info, "swapfile is inline");
> +			ret = -EINVAL;

here

> +			goto out;
> +		}
> +		if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {
> +			btrfs_err(fs_info, "swapfile is compresed");
> +			ret = -EINVAL;

here

> +			goto out;
> +		}
> +		ret = can_nocow_extent(inode, start, &len, NULL, NULL, NULL);
> +		if (ret < 0) {

here

> +			goto out;
> +		} else if (ret == 1) {
> +			ret = 0;
> +		} else {
> +			btrfs_err(fs_info, "swapfile has extent requiring COW (%llu-%llu)",
> +				  start, start + len - 1);
> +			ret = -EINVAL;

here

> +			goto out;
> +		}
> +
> +		set_bit(EXTENT_FLAG_SWAPFILE, &em->flags);
> +
> +		start = extent_map_end(em);
> +		free_extent_map(em);
> +	}
> +
> +out:
> +	if (ret) {

should be fixed by:

		if (!IS_ERR_OR_NULL(em))
			free_extent_map(em);

> +		__clear_swapfile_extents(inode);
> +		atomic_dec(&BTRFS_I(inode)->root->nr_swapfiles);
> +	}
> +	unlock_extent_cached(io_tree, 0, isize - 1, &cached_state, GFP_NOFS);
> +	return ret;
> +}
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