On Mon, May 18, 2009 at 06:00:12PM -0700, Dan Williams wrote:
> +/**
> + * async_raid6_2data_recov - asynchronously calculate two missing data blocks
> + * @disks: number of disks in the RAID-6 array
> + * @bytes: block size
> + * @faila: first failed drive index
> + * @failb: second failed drive index
> + * @blocks: array of source pointers where the last two entries are p and q
> + * @submit: submission/completion modifiers
> + */
> +struct dma_async_tx_descriptor *
> +async_raid6_2data_recov(int disks, size_t bytes, int faila, int failb,
> + struct page **blocks, struct async_submit_ctl *submit)
> +{
[...]
> + /* Dx = A*(P+Pxy) + B*(Q+Qxy) */
> + srcs[0] = dp;
> + srcs[1] = dq;
> + coef[0] = raid6_gfexi[failb-faila];
Here it's essential that faila < failb. This should either be clearly
documented and checked for, or (better) the function should swap
faila and failb if they are in the wrong order.
> + p = blocks[disks-2];
> + q = blocks[disks-1];
> +
> + /* Compute syndrome with zero for the missing data page
> + Use the dead data page as temporary storage for delta q */
> + dq = blocks[faila];
> + blocks[faila] = (void *)raid6_empty_zero_page;
> + blocks[disks-1] = dq;
> +
> + submit->flags &= ~ASYNC_TX_ACK;
> + submit->cb_fn = NULL;
> + submit->cb_param = NULL;
> + tx = async_gen_syndrome(blocks, 0, disks, bytes, submit);
> + submit->depend_tx = tx;
> +
> + /* Restore pointer table */
> + blocks[faila] = dq;
> + blocks[disks-1] = q;
> +
> + /* Now, pick the proper data tables */
> + coef = raid6_gfinv[raid6_gfexp[faila]];
Pick data tables? The formula for recovering the data block is
(q + dq) / g^faila
(with g being the generator). What this line actually does is computing
g^{-faila}.
Regards
Andre
--
The only person who always got his work done by Friday was Robinson Crusoe
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