On Fri, May 22, 2009 at 1:29 AM, Andre Noll <maan@xxxxxxxxxxxxxxx> wrote:
> 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.
Yes, it would be safer if we did not trust the caller to get this right.
>
>> + 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}.
Yeah, that is a leftover comment from the original implementation,
I'll change it to be more informative.
Thanks,
Dan
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