Re: [PATCH v2 09/11] async_tx: add support for asynchronous RAID6 recovery operations

[Andre Noll <maan@systemlinux.org>]

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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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