linux-kernel - Re: [PATCH v2] DAX: warn when kmem regions are truncated for memory block alignment.

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Message-ID: <1789e1cf-ab35-4730-8c75-8ea037e590f6@redhat.com>
Date: Wed, 2 Apr 2025 09:58:45 +0200
From: David Hildenbrand <david@...hat.com>
To: Gregory Price <gourry@...rry.net>, linux-cxl@...r.kernel.org
Cc: nvdimm@...ts.linux.dev, linux-kernel@...r.kernel.org,
 kernel-team@...a.com, dan.j.williams@...el.com, vishal.l.verma@...el.com,
 dave.jiang@...el.com
Subject: Re: [PATCH v2] DAX: warn when kmem regions are truncated for memory
 block alignment.

On 02.04.25 03:59, Gregory Price wrote:
> Device capacity intended for use as system ram should be aligned to the
> archite-defined memory block size or that capacity will be silently
> truncated and capacity stranded.
> 
> As hotplug dax memory becomes more prevelant, the memory block size
> alignment becomes more important for platform and device vendors to
> pay attention to - so this truncation should not be silent.
> 
> This issue is particularly relevant for CXL Dynamic Capacity devices,
> whose capacity may arrive in spec-aligned but block-misaligned chunks.
> 
> Suggested-by: David Hildenbrand <david@...hat.com>
> Suggested-by: Dan Williams <dan.j.williams@...el.com>
> Signed-off-by: Gregory Price <gourry@...rry.net>
> ---
>   drivers/dax/kmem.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
> index e97d47f42ee2..32fe3215e11e 100644
> --- a/drivers/dax/kmem.c
> +++ b/drivers/dax/kmem.c
> @@ -13,6 +13,7 @@
>   #include <linux/mman.h>
>   #include <linux/memory-tiers.h>
>   #include <linux/memory_hotplug.h>
> +#include <linux/string_helpers.h>
>   #include "dax-private.h"
>   #include "bus.h"
>   
> @@ -68,7 +69,7 @@ static void kmem_put_memory_types(void)
>   static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
>   {
>   	struct device *dev = &dev_dax->dev;
> -	unsigned long total_len = 0;
> +	unsigned long total_len = 0, orig_len = 0;
>   	struct dax_kmem_data *data;
>   	struct memory_dev_type *mtype;
>   	int i, rc, mapped = 0;
> @@ -97,6 +98,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
>   	for (i = 0; i < dev_dax->nr_range; i++) {
>   		struct range range;
>   
> +		orig_len += range_len(&dev_dax->ranges[i].range);
>   		rc = dax_kmem_range(dev_dax, i, &range);
>   		if (rc) {
>   			dev_info(dev, "mapping%d: %#llx-%#llx too small after alignment\n",
> @@ -109,6 +111,12 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
>   	if (!total_len) {
>   		dev_warn(dev, "rejecting DAX region without any memory after alignment\n");
>   		return -EINVAL;
> +	} else if (total_len != orig_len) {
> +		char buf[16];
> +
> +		string_get_size((orig_len - total_len), 1, STRING_UNITS_2,
> +				buf, sizeof(buf));
> +		dev_warn(dev, "DAX region truncated by %s due to alignment\n", buf);

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb