On Fri, Jun 12, 2015 at 11:00:15AM +0800, Qu Wenruo wrote:
> Introduce the new partial csum mechanism for tree block.
>
> [Old tree block csum]
> 0 4 8 12 16 20 24 28 32
> -------------------------------------------------
> |csum | unused, all 0 |
> -------------------------------------------------
> Csum is the crc32 of the whole tree block data.
>
> [New tree block csum]
> -------------------------------------------------
> |csum0|csum1|csum2|csum3|csum4|csum5|csum6|csum7|
> -------------------------------------------------
> Where csum0 is the same as the old one, crc32 of the whole tree block
> data.
>
> But csum1~csum7 will restore crc32 of each eighth part.
> Take example of 16K leafsize, then:
> csum1: crc32 of BTRFS_CSUM_SIZE~4K
> csum2: crc32 of 4K~6K
> ...
> csum7: crc32 of 14K~16K
>
> This provides the ability for btrfs not only to detect corruption but
> also to know where corruption is.
> Further improve the robustness of btrfs.
What of the use to know where corruption locates? It still turns to
find a good copy of this block (if there is) and recovery from it,
doesn't it?
Can you eleborate more on how to improve the robustness?
>
> Although the best practise is to introduce new csum type and put every
> eighth crc32 into corresponding place, but the benefit is not worthy to
> break the backward compatibility.
> So keep csum0 and modify csum1 range to keep backward compatibility.
>
> Signed-off-by: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
> ---
> TODO:
> Add scrub support to use partial csum to rebuild metadata.
> This is a little hard as current scrub don't treat tree block as a whole.
> Needs a lot of other improvement for scrub.
> ---
> fs/btrfs/disk-io.c | 75 ++++++++++++++++++++++++++++++++++++------------------
> 1 file changed, 50 insertions(+), 25 deletions(-)
>
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 7f83778..54052d3 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -271,47 +271,76 @@ void btrfs_csum_final(u32 crc, char *result)
> }
>
> /*
> - * compute the csum for a btree block, and either verify it or write it
> - * into the csum field of the block.
> + * Calcuate partial crc32 for each part.
> + *
> + * Part should be in [0, 7].
> + * Part 0 is the old crc32 of the whole leaf/node.
> + * Part 1 is the crc32 of 32~ 2/8 of leaf/node.
> + * Part 2 is the crc32 of 3/8 of leaf/node.
> + * Part 3 is the crc32 of 4/8 of lean/node and so on.
typo here.
Thanks,
-liubo
> */
> -static int csum_tree_block(struct btrfs_fs_info *fs_info,
> - struct extent_buffer *buf,
> - int verify)
> +static int csum_tree_block_part(struct extent_buffer *buf,
> + char *result, int part)
> {
> - u16 csum_size = btrfs_super_csum_size(fs_info->super_copy);
> - char *result = NULL;
> + int offset;
> + int err;
> 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;
> + char *kaddr;
> u32 crc = ~(u32)0;
> - unsigned long inline_result;
>
> - len = buf->len - offset;
> + BUG_ON(part >= 8 || part < 0);
> + BUG_ON(ALIGN(buf->len, 8) != buf->len);
> +
> + if (part == 0) {
> + offset = BTRFS_CSUM_SIZE;
> + len = buf->len - offset;
> + } else if (part == 0) {
> + offset = BTRFS_CSUM_SIZE;
> + len = buf->len * 2 / 8 - offset;
> + } else {
> + offset = part * buf->len / 8;
> + len = buf->len / 8;
> + }
> +
> while (len > 0) {
> err = map_private_extent_buffer(buf, offset, 32,
> &kaddr, &map_start, &map_len);
> if (err)
> - return 1;
> + return err;
> cur_len = min(len, map_len - (offset - map_start));
> crc = btrfs_csum_data(kaddr + offset - map_start,
> crc, cur_len);
> len -= cur_len;
> offset += cur_len;
> }
> - if (csum_size > sizeof(inline_result)) {
> - result = kzalloc(csum_size, GFP_NOFS);
> - if (!result)
> + btrfs_csum_final(crc, result + BTRFS_CSUM_SIZE * part/ 8);
> + return 0;
> +}
> +
> +/*
> + * compute the csum for a btree block, and either verify it or write it
> + * into the csum field of the block.
> + */
> +static int csum_tree_block(struct btrfs_fs_info *fs_info,
> + struct extent_buffer *buf,
> + int verify)
> +{
> + u16 csum_size = btrfs_super_csum_size(fs_info->super_copy);
> + char result[BTRFS_CSUM_SIZE] = {0};
> + int err;
> + u32 crc = ~(u32)0;
> + int index = 0;
> +
> + /* get every part csum */
> + for (index = 0; index < 8; index++) {
> + err = csum_tree_block_part(buf, result, index);
> + if (err)
> return 1;
> - } else {
> - result = (char *)&inline_result;
> }
>
> - btrfs_csum_final(crc, result);
> -
> if (verify) {
> if (memcmp_extent_buffer(buf, result, 0, csum_size)) {
> u32 val;
> @@ -324,15 +353,11 @@ static int csum_tree_block(struct btrfs_fs_info *fs_info,
> "level %d\n",
> fs_info->sb->s_id, buf->start,
> val, found, btrfs_header_level(buf));
> - if (result != (char *)&inline_result)
> - kfree(result);
> return 1;
> }
> } else {
> - write_extent_buffer(buf, result, 0, csum_size);
> + write_extent_buffer(buf, result, 0, BTRFS_CSUM_SIZE);
> }
> - if (result != (char *)&inline_result)
> - kfree(result);
> return 0;
> }
>
> --
> 2.4.2
>
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