[PATCH 1/3] mm/memory_hotplug: Allow an override for the memmap_on_memory param

[Vishal Verma <vishal.l.verma@intel.com>]

For memory hotplug to consider MHP_MEMMAP_ON_MEMORY behavior, the
'memmap_on_memory' module parameter was a hard requirement.

In preparation for the dax/kmem driver to use memmap_on_memory
semantics, arrange for the module parameter check to be bypassed via the
appropriate mhp_flag.

Recall that the kmem driver could contribute huge amounts of hotplugged
memory originating from special purposes devices such as CXL memory
expanders. In some cases memmap_on_memory may be the /only/ way this new
memory can be hotplugged. Hence it makes sense for kmem to have a way to
force memmap_on_memory without depending on a module param, if all the
other conditions for it are met.

The only other user of this interface is acpi/acpi_memoryhotplug.c,
which only enables the mhp_flag if an initial
mhp_supports_memmap_on_memory() test passes. Maintain the existing
behavior and semantics for this by performing the initial check from
acpi without the MHP_MEMMAP_ON_MEMORY flag, so its decision falls back
to the module parameter.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Huang Ying <ying.huang@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
---
 include/linux/memory_hotplug.h |  2 +-
 drivers/acpi/acpi_memhotplug.c |  2 +-
 mm/memory_hotplug.c            | 24 ++++++++++++++++--------
 3 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 9fcbf5706595..c9ddcd3cad70 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -358,7 +358,7 @@ extern struct zone *zone_for_pfn_range(int online_type, int nid,
 extern int arch_create_linear_mapping(int nid, u64 start, u64 size,
 				      struct mhp_params *params);
 void arch_remove_linear_mapping(u64 start, u64 size);
-extern bool mhp_supports_memmap_on_memory(unsigned long size);
+extern bool mhp_supports_memmap_on_memory(unsigned long size, mhp_t mhp_flags);
 #endif /* CONFIG_MEMORY_HOTPLUG */
 
 #endif /* __LINUX_MEMORY_HOTPLUG_H */
diff --git a/drivers/acpi/acpi_memhotplug.c b/drivers/acpi/acpi_memhotplug.c
index 24f662d8bd39..119d3bb49753 100644
--- a/drivers/acpi/acpi_memhotplug.c
+++ b/drivers/acpi/acpi_memhotplug.c
@@ -211,7 +211,7 @@ static int acpi_memory_enable_device(struct acpi_memory_device *mem_device)
 		if (!info->length)
 			continue;
 
-		if (mhp_supports_memmap_on_memory(info->length))
+		if (mhp_supports_memmap_on_memory(info->length, 0))
 			mhp_flags |= MHP_MEMMAP_ON_MEMORY;
 		result = __add_memory(mgid, info->start_addr, info->length,
 				      mhp_flags);
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 8e0fa209d533..bb3845830922 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1283,15 +1283,21 @@ static int online_memory_block(struct memory_block *mem, void *arg)
 	return device_online(&mem->dev);
 }
 
-bool mhp_supports_memmap_on_memory(unsigned long size)
+bool mhp_supports_memmap_on_memory(unsigned long size, mhp_t mhp_flags)
 {
 	unsigned long nr_vmemmap_pages = size / PAGE_SIZE;
 	unsigned long vmemmap_size = nr_vmemmap_pages * sizeof(struct page);
 	unsigned long remaining_size = size - vmemmap_size;
 
 	/*
-	 * Besides having arch support and the feature enabled at runtime, we
-	 * need a few more assumptions to hold true:
+	 * The MHP_MEMMAP_ON_MEMORY flag indicates a caller that wants to force
+	 * memmap_on_memory (if other conditions are met), regardless of the
+	 * module parameter. drivers/dax/kmem.c is an example, where large
+	 * amounts of hotplug memory may come from, and the only option to
+	 * successfully online all of it is to place the memmap on this memory.
+	 *
+	 * Besides having arch support and the feature enabled at runtime or
+	 * via the mhp_flag, we need a few more assumptions to hold true:
 	 *
 	 * a) We span a single memory block: memory onlining/offlinin;g happens
 	 *    in memory block granularity. We don't want the vmemmap of online
@@ -1315,10 +1321,12 @@ bool mhp_supports_memmap_on_memory(unsigned long size)
 	 *       altmap as an alternative source of memory, and we do not exactly
 	 *       populate a single PMD.
 	 */
-	return mhp_memmap_on_memory() &&
-	       size == memory_block_size_bytes() &&
-	       IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
-	       IS_ALIGNED(remaining_size, (pageblock_nr_pages << PAGE_SHIFT));
+
+	if ((mhp_flags & MHP_MEMMAP_ON_MEMORY) || mhp_memmap_on_memory())
+		return size == memory_block_size_bytes() &&
+		       IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
+		       IS_ALIGNED(remaining_size, (pageblock_nr_pages << PAGE_SHIFT));
+	return false;
 }
 
 /*
@@ -1375,7 +1383,7 @@ int __ref add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
 	 * Self hosted memmap array
 	 */
 	if (mhp_flags & MHP_MEMMAP_ON_MEMORY) {
-		if (!mhp_supports_memmap_on_memory(size)) {
+		if (!mhp_supports_memmap_on_memory(size, mhp_flags)) {
 			ret = -EINVAL;
 			goto error;
 		}

-- 
2.40.1