Re: [PATCH RFC v2 01/12] btrfs: introduce a pure data checksum checking helper

[Christoph Hellwig <hch@infradead.org>]

> +/*
> + * Just verify one sector of csum, without any extra handling.
> + *
> + * Functions like btrfs_verify_data_csum() have extra handling like setting
> + * page flags which is not suitable for call sites like direct IO.
> + *
> + * This pure csum checking allows us to utilize it for call sites which need
> + * to handle page for both buffered and direct IO.
> + */

I think this is way too much historic context that's probably soon
rather irrelevant.

I'd simplify it down to something like:

/*
 * Verify the checksum for a single sector without any extra action that
 * depend on the type of I/O.
 */

> +int btrfs_check_data_sector(struct btrfs_fs_info *fs_info, struct page *page,
> +			    u32 pgoff, u8 *csum_expected)
> +{
> +	SHASH_DESC_ON_STACK(shash, fs_info->csum_shash);
> +	char *kaddr;
> +	const u32 len = fs_info->sectorsize;
> +	const u32 csum_size = fs_info->csum_size;
> +	u8 csum[BTRFS_CSUM_SIZE];
> +
> +	ASSERT(pgoff + len <= PAGE_SIZE);
> +
> +	kaddr = kmap_atomic(page);

Please switch to kmap_local_page while you touch this.