Re: [PATCH 2/4] btrfs: simplify tree block checksumming loop

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2020/2/28 上午4:00, David Sterba wrote:
> Thw whole point of csum_tree_block is to iterate over all extent buffer
> pages and pass it to checksumming functions. The bytes where checksum is
> stored must be skipped, thus map_private_extent_buffer. This complicates
> further offset calculations.
> 
> As the first page will be always present, checksum the relevant bytes
> unconditionally and then do a simple iteration over the remaining pages.

The new operation looks much better.

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> 
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  fs/btrfs/disk-io.c | 32 ++++++++------------------------
>  1 file changed, 8 insertions(+), 24 deletions(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 3952e4a2f3d7..5f74eb69f2fe 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -259,38 +259,22 @@ struct extent_map *btree_get_extent(struct btrfs_inode *inode,
>  static int csum_tree_block(struct extent_buffer *buf, u8 *result)
>  {
>  	struct btrfs_fs_info *fs_info = buf->fs_info;
> +	const int num_pages = fs_info->nodesize >> PAGE_SHIFT;
>  	SHASH_DESC_ON_STACK(shash, fs_info->csum_shash);
> -	unsigned long len;
> -	unsigned long cur_len;
> -	unsigned long offset = BTRFS_CSUM_SIZE;
>  	char *kaddr;
> -	unsigned long map_start;
> -	unsigned long map_len;
> -	int err;
> +	int i;
>  
>  	shash->tfm = fs_info->csum_shash;
>  	crypto_shash_init(shash);
> +	kaddr = page_address(buf->pages[0]);
> +	crypto_shash_update(shash, kaddr + BTRFS_CSUM_SIZE,
> +			    PAGE_SIZE - BTRFS_CSUM_SIZE);
>  
> -	len = buf->len - offset;
> -
> -	while (len > 0) {
> -		/*
> -		 * Note: we don't need to check for the err == 1 case here, as
> -		 * with the given combination of 'start = BTRFS_CSUM_SIZE (32)'
> -		 * and 'min_len = 32' and the currently implemented mapping
> -		 * algorithm we cannot cross a page boundary.
> -		 */
> -		err = map_private_extent_buffer(buf, offset, 32,
> -					&kaddr, &map_start, &map_len);
> -		if (WARN_ON(err))
> -			return err;
> -		cur_len = min(len, map_len - (offset - map_start));
> -		crypto_shash_update(shash, kaddr + offset - map_start, cur_len);
> -		len -= cur_len;
> -		offset += cur_len;
> +	for (i = 1; i < num_pages; i++) {
> +		kaddr = page_address(buf->pages[i]);
> +		crypto_shash_update(shash, kaddr, PAGE_SIZE);
>  	}
>  	memset(result, 0, BTRFS_CSUM_SIZE);
> -
>  	crypto_shash_final(shash, result);
>  
>  	return 0;
> 


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