linux-kernel - Re: [PATCH v2] raid6: fix the input of raid6 algorithm

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Date:   Sun, 28 Aug 2016 14:08:27 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     liuzhengyuan@...inos.cn
CC:     shli@...nel.org, linux-raid@...r.kernel.org, fenghua.yu@...el.com,
        linux-kernel@...r.kernel.org, liuzhengyuang521@...il.com
Subject: Re: [PATCH v2] raid6: fix the input of raid6 algorithm

On August 27, 2016 11:51:04 PM PDT, liuzhengyuan@...inos.cn wrote:
>From: ZhengYuan Liu <liuzhengyuan@...inos.cn>
>
>To test and choose the best algorithm for raid6, disks number
>and disks data must be offered. These input depend on page
>size and gfmul table at current time. It would cause the disk
>number less than 4 when the page size is more than 64KB.This
>patch would support arbitrarily page size by defining a macro
>for disks number and using a PRNG based on linear congruential
>bit to fill the disks data.
>
>Signed-off-by: ZhengYuan Liu <liuzhengyuan@...inos.cn>
>---
> lib/raid6/algos.c | 39 +++++++++++++++++++++++++--------------
> 1 file changed, 25 insertions(+), 14 deletions(-)
>
>diff --git a/lib/raid6/algos.c b/lib/raid6/algos.c
>index 975c6e0..3991ac6 100644
>--- a/lib/raid6/algos.c
>+++ b/lib/raid6/algos.c
>@@ -30,6 +30,8 @@ EXPORT_SYMBOL(raid6_empty_zero_page);
> #endif
> #endif
> 
>+#define RAID6_DISKS	8
>+
> struct raid6_calls raid6_call;
> EXPORT_SYMBOL_GPL(raid6_call);
> 
>@@ -129,7 +131,7 @@ static inline const struct raid6_recov_calls
>*raid6_choose_recov(void)
> }
> 
> static inline const struct raid6_calls *raid6_choose_gen(
>-	void *(*const dptrs)[(65536/PAGE_SIZE)+2], const int disks)
>+	void *(*const dptrs)[RAID6_DISKS], const int disks)
> {
> 	unsigned long perf, bestgenperf, bestxorperf, j0, j1;
>	int start = (disks>>1)-1, stop = disks-3;	/* work on the second half
>of the disks */
>@@ -206,27 +208,36 @@ static inline const struct raid6_calls
>*raid6_choose_gen(
> 
> int __init raid6_select_algo(void)
> {
>-	const int disks = (65536/PAGE_SIZE)+2;
>+	const int disks = RAID6_DISKS;
> 
> 	const struct raid6_calls *gen_best;
> 	const struct raid6_recov_calls *rec_best;
>-	char *syndromes;
>-	void *dptrs[(65536/PAGE_SIZE)+2];
>-	int i;
>-
>-	for (i = 0; i < disks-2; i++)
>-		dptrs[i] = ((char *)raid6_gfmul) + PAGE_SIZE*i;
>+	char *disk_ptr;
>+	void *dptrs[RAID6_DISKS];
>+	int32_t seed;
>+	int i, j;
> 
>-	/* Normal code - use a 2-page allocation to avoid D$ conflict */
>-	syndromes = (void *) __get_free_pages(GFP_KERNEL, 1);
>+	/* use a 8-page allocation, The first 6 pages for disks
>+	   and the last 2 pages for syndromes */
>+	disk_ptr = (void *) __get_free_pages(GFP_KERNEL, 3);
> 
>-	if (!syndromes) {
>+	if (!disk_ptr) {
> 		pr_err("raid6: Yikes!  No memory available.\n");
> 		return -ENOMEM;
> 	}
> 
>-	dptrs[disks-2] = syndromes;
>-	dptrs[disks-1] = syndromes + PAGE_SIZE;
>+	/*use a PRNG based on LCB to fill the disks*/
>+	seed = 1;
>+	for (i = 0; i < disks-2; i++) {
>+		dptrs[i] = disk_ptr + PAGE_SIZE*i;
>+		for (j = 0; j < PAGE_SIZE; j = j + 4) {
>+			seed = ((seed * 1103515245) + 12345) & 0x7fffffff;
>+			*(int32_t *)(dptrs[i]+j) = seed;
>+		}
>+	}
>+
>+	dptrs[disks-2] = disk_ptr + PAGE_SIZE*(disks-2);
>+	dptrs[disks-1] = disk_ptr + PAGE_SIZE*(disks-1);
> 
> 	/* select raid gen_syndrome function */
> 	gen_best = raid6_choose_gen(&dptrs, disks);
>@@ -234,7 +245,7 @@ int __init raid6_select_algo(void)
> 	/* select raid recover functions */
> 	rec_best = raid6_choose_recov();
> 
>-	free_pages((unsigned long)syndromes, 1);
>+	free_pages((unsigned long)disk_ptr, 3);
> 
> 	return gen_best && rec_best ? 0 : -EINVAL;
> }

Linear congruential exactly what we don't want...
-- 
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