linux-kernel - Re: [RFC PATCH 1/1] cxl/edac: Fix the min_scrub

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Message-ID: <aDn4o8Fw91vQ9D-D@aschofie-mobl2.lan>
Date: Fri, 30 May 2025 11:27:47 -0700
From: Alison Schofield <alison.schofield@...el.com>
To: Li Ming <ming.li@...omail.com>
CC: <dave@...olabs.net>, <jonathan.cameron@...wei.com>,
	<dave.jiang@...el.com>, <vishal.l.verma@...el.com>, <ira.weiny@...el.com>,
	<dan.j.williams@...el.com>, <shiju.jose@...wei.com>,
	<linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/1] cxl/edac: Fix the min_scrub_cycle of a region
 miscalculation

On Fri, May 30, 2025 at 08:28:52PM +0800, Li Ming wrote:
> When trying to update the scrub_cycle value of a cxl region, which means
> updating the scrub_cycle value of each memdev under a cxl region. cxl
> driver needs to guarantee the new scrub_cycle value is greater than the
> min_scrub_cycle value of a memdev, otherwise the updating operation will
> fail(Per Table 8-223 in CXL r3.2 section 8.2.10.9.11.1).
> 
> Current implementation logic of getting the min_scrub_cycle value of a
> cxl region is that getting the min_scrub_cycle value of each memdevs
> under the cxl region, then using the minimum min_scrub_cycle value as
> the region's min_scrub_cycle. Checking if the new scrub_cycle value is
> greater than this value. If yes, updating the new scrub_cycle value to
> each memdevs. The issue is that the new scrub_cycle value is possibly
> greater than the minimum min_scrub_cycle value of all memdevs but less
> than the maximum min_scrub_cycle value of all memdevs if memdevs have
> a different min_scrub_cycle value. The updating operation will always
> fail on these memdevs which have a greater min_scrub_cycle than the new
> scrub_cycle.
> 
> The correct implementation logic is to get the maximum value of these
> memdevs' min_scrub_cycle, check if the new scrub_cycle value is greater
> than the value. If yes, the new scrub_cycle value is fit for the region.
> 
> The change also impacts the result of
> cxl_patrol_scrub_get_min_scrub_cycle(), the interface returned the
> minimum min_scrub_cycle value among all memdevs under the region before
> the change. The interface will return the maximum min_scrub_cycle value
> among all memdevs under the region with the change.
> 
> Signed-off-by: Li Ming <ming.li@...omail.com>
> ---
> I made this change based on my understanding on the SPEC and current CXL
> EDAC code, but I am not sure if it is a bug or it is designed this way.

The attribute is defined to show (per Documentation/ABI/testing/sysfs-edac-scrub)
   "Supported minimum scrub cycle duration in seconds by the memory scrubber."

Your fix, making the min the max of the mins, looks needed.

I took a look at the max attribute. If the min is the max on the mins, then
the max should be the max of the maxes. But, not true. We do this:

instead: *max = U8_MAX * 3600; /* Max set by register size */

The comment isn't helping me, esp since the sysfs description doesn't
explain that we are using a constant max.


> 
> base-commit: 9f153b7fb5ae45c7d426851f896487927f40e501 cxl/next
> ---
>  drivers/cxl/core/edac.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cxl/core/edac.c b/drivers/cxl/core/edac.c
> index 2cbc664e5d62..ad243cfe00e7 100644
> --- a/drivers/cxl/core/edac.c
> +++ b/drivers/cxl/core/edac.c
> @@ -103,10 +103,10 @@ static int cxl_scrub_get_attrbs(struct cxl_patrol_scrub_context *cxl_ps_ctx,
>  				u8 *cap, u16 *cycle, u8 *flags, u8 *min_cycle)
>  {
>  	struct cxl_mailbox *cxl_mbox;
> -	u8 min_scrub_cycle = U8_MAX;
>  	struct cxl_region_params *p;
>  	struct cxl_memdev *cxlmd;
>  	struct cxl_region *cxlr;
> +	u8 min_scrub_cycle = 0;
>  	int i, ret;
>  
>  	if (!cxl_ps_ctx->cxlr) {
> @@ -133,8 +133,12 @@ static int cxl_scrub_get_attrbs(struct cxl_patrol_scrub_context *cxl_ps_ctx,
>  		if (ret)
>  			return ret;
>  
> +		/*
> +		 * The min_scrub_cycle of a region is the maximum value among
> +		 * the min_scrub_cycle of all the memdevs under the region.
> +		 */
>  		if (min_cycle)
> -			min_scrub_cycle = min(*min_cycle, min_scrub_cycle);
> +			min_scrub_cycle = max(*min_cycle, min_scrub_cycle);
>  	}
>  
>  	if (min_cycle)
> -- 
> 2.34.1
>