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

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Message-ID: <20250402015920.819077-1-gourry@gourry.net>
Date: Tue,  1 Apr 2025 21:59:20 -0400
From: Gregory Price <gourry@...rry.net>
To: 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,
	David Hildenbrand <david@...hat.com>
Subject: [PATCH v2] DAX: warn when kmem regions are truncated for memory block alignment.

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);
 	}
 
 	init_node_memory_type(numa_node, mtype);
-- 
2.47.1