[PATCH v2] dax/pmem: fix unaligned compound devmap initialization

[Muchun Song <songmuchun@bytedance.com>]

Static pmem Device DAX namespaces reserve space for namespace metadata
before the user-visible data range, and may also include padding needed
to satisfy the Device DAX alignment. __dax_pmem_probe() adjusts the
dax_region range to start at the data range, but the dev_pagemap range
still starts at the beginning of the namespace.

That is not valid for compound devmap mappings whose memmap is stored in
system RAM. Compound page initialization requires the start PFN to be
naturally aligned to the compound page size, because the head and tail
struct pages are initialized according to that geometry. If the pgmap
starts before the aligned data range, memmap_init_zone_device() can build
compound page metadata from an unaligned PFN.

The resulting struct page layout is inconsistent with the Device DAX
mapping. A later mmap fault can then resolve the user-visible PFN through
invalid compound page metadata and hit a kernel panic while inserting the
DAX folio.

Fix this by rounding the pgmap start up to the Device DAX alignment for
non-altmap mappings. This gives compound mappings the required naturally
aligned start while retaining full-section vmemmap savings when the
original start is already aligned. The altmap path remains unchanged
because memmap_init_zone_device() skips the altmap reservation itself.

Fixes: 14606001efb4 ("device-dax: compound devmap support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Reported-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Closes: https://lore.kernel.org/r/wlrkqqvg.ritesh.list@gmail.com
---
v2:
- Round the pgmap start up to the Device DAX alignment to retain
  full-section vmemmap savings (suggested by Alison Schofield)
- Update the commit message to describe the alignment approach

v1: https://lore.kernel.org/20261016054914.2601510-1-songmuchun@bytedance.com/
---
 drivers/dax/pmem.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/dax/pmem.c b/drivers/dax/pmem.c
index bee93066a849..d54d23f1cc58 100644
--- a/drivers/dax/pmem.c
+++ b/drivers/dax/pmem.c
@@ -51,6 +51,12 @@ static struct dev_dax *__dax_pmem_probe(struct device *dev)
 	/* adjust the dax_region range to the start of data */
 	range = pgmap.range;
 	range.start += offset;
+	/*
+	 * When the memmap is stored in RAM, compound page initialization
+	 * must start at a naturally aligned address.
+	 */
+	if (!pgmap_altmap(&pgmap))
+		pgmap.range.start = ALIGN(pgmap.range.start, nd_pfn->align);
 	dax_region = alloc_dax_region(dev, region_id, &range,
 			nd_region->target_node, le32_to_cpu(pfn_sb->align),
 			IORESOURCE_DAX_STATIC);

base-commit: a91337fb5e8e0127c8b5be87be9f3027f95b48bc
-- 
2.54.0