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

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Message-ID: <20250321180731.568460-1-gourry@gourry.net>
Date: Fri, 21 Mar 2025 14:07:31 -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
Subject: [PATCH] DAX: warn when kmem regions are truncated for memory block alignment.

Device capacity intended for use as system ram should be aligned to the
architecture-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.

Example:
 [...] kmem dax0.0: dax region truncated 2684354560 bytes - alignment
 [...] kmem dax1.0: dax region truncated 1610612736 bytes - alignment

Signed-off-by: Gregory Price <gourry@...rry.net>
---
 drivers/dax/kmem.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
index e97d47f42ee2..15b6807b703d 100644
--- a/drivers/dax/kmem.c
+++ b/drivers/dax/kmem.c
@@ -28,7 +28,8 @@ static const char *kmem_name;
 /* Set if any memory will remain added when the driver will be unloaded. */
 static bool any_hotremove_failed;
 
-static int dax_kmem_range(struct dev_dax *dev_dax, int i, struct range *r)
+static int dax_kmem_range(struct dev_dax *dev_dax, int i, struct range *r,
+			  unsigned long *truncated)
 {
 	struct dev_dax_range *dax_range = &dev_dax->ranges[i];
 	struct range *range = &dax_range->range;
@@ -41,6 +42,9 @@ static int dax_kmem_range(struct dev_dax *dev_dax, int i, struct range *r)
 		r->end = range->end;
 		return -ENOSPC;
 	}
+
+	if (truncated && (r->start != range->start || r->end != range->end))
+		*truncated = (r->start - range->start) + (range->end - r->end);
 	return 0;
 }
 
@@ -75,6 +79,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 	mhp_t mhp_flags;
 	int numa_node;
 	int adist = MEMTIER_DEFAULT_DAX_ADISTANCE;
+	unsigned long ttl_trunc = 0;
 
 	/*
 	 * Ensure good NUMA information for the persistent memory.
@@ -97,7 +102,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 	for (i = 0; i < dev_dax->nr_range; i++) {
 		struct range range;
 
-		rc = dax_kmem_range(dev_dax, i, &range);
+		rc = dax_kmem_range(dev_dax, i, &range, NULL);
 		if (rc) {
 			dev_info(dev, "mapping%d: %#llx-%#llx too small after alignment\n",
 					i, range.start, range.end);
@@ -130,8 +135,9 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 	for (i = 0; i < dev_dax->nr_range; i++) {
 		struct resource *res;
 		struct range range;
+		unsigned long truncated = 0;
 
-		rc = dax_kmem_range(dev_dax, i, &range);
+		rc = dax_kmem_range(dev_dax, i, &range, &truncated);
 		if (rc)
 			continue;
 
@@ -180,8 +186,12 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 				continue;
 			goto err_request_mem;
 		}
+
+		ttl_trunc += truncated;
 		mapped++;
 	}
+	if (ttl_trunc)
+		dev_warn(dev, "dax region truncated %ld bytes - alignment\n", ttl_trunc);
 
 	dev_set_drvdata(dev, data);
 
@@ -216,7 +226,7 @@ static void dev_dax_kmem_remove(struct dev_dax *dev_dax)
 		struct range range;
 		int rc;
 
-		rc = dax_kmem_range(dev_dax, i, &range);
+		rc = dax_kmem_range(dev_dax, i, &range, NULL);
 		if (rc)
 			continue;
 
-- 
2.48.1