mm/memblock: Add reserved memory release function

Add reserve_mem_release_by_name() to release a reserved memory region
with a given name. This allows us to release reserved memory which is
defined by kernel cmdline, after boot.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: linux-mm@kvack.org
Link: https://lore.kernel.org/173989133862.230693.14094993331347437600.stgit@devnote2
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: mhiramat

include/linux/mm.h

@@ -4123,6 +4123,7 @@ void vma_pgtable_walk_begin(struct vm_area_struct *vma);
 void vma_pgtable_walk_end(struct vm_area_struct *vma);
 
 int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *size);
+int reserve_mem_release_by_name(const char *name);
 
 #ifdef CONFIG_64BIT
 int do_mseal(unsigned long start, size_t len_in, unsigned long flags);

mm/memblock.c

@@ -16,6 +16,7 @@
 #include <linux/kmemleak.h>
 #include <linux/seq_file.h>
 #include <linux/memblock.h>
+#include <linux/mutex.h>
 
 #include <asm/sections.h>
 #include <linux/io.h>
@@ -2283,6 +2284,7 @@ struct reserve_mem_table {
 };
 static struct reserve_mem_table reserved_mem_table[RESERVE_MEM_MAX_ENTRIES];
 static int reserved_mem_count;
+static DEFINE_MUTEX(reserve_mem_lock);
 
 /* Add wildcard region with a lookup name */
 static void __init reserved_mem_add(phys_addr_t start, phys_addr_t size,
@@ -2296,6 +2298,21 @@ static void __init reserved_mem_add(phys_addr_t start, phys_addr_t size,
 	strscpy(map->name, name);
 }
 
+static struct reserve_mem_table *reserve_mem_find_by_name_nolock(const char *name)
+{
+	struct reserve_mem_table *map;
+	int i;
+
+	for (i = 0; i < reserved_mem_count; i++) {
+		map = &reserved_mem_table[i];
+		if (!map->size)
+			continue;
+		if (strcmp(name, map->name) == 0)
+			return map;
+	}
+	return NULL;
+}
+
 /**
  * reserve_mem_find_by_name - Find reserved memory region with a given name
  * @name: The name that is attached to a reserved memory region
@@ -2309,22 +2326,47 @@ static void __init reserved_mem_add(phys_addr_t start, phys_addr_t size,
 int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *size)
 {
 	struct reserve_mem_table *map;
-	int i;
 
-	for (i = 0; i < reserved_mem_count; i++) {
-		map = &reserved_mem_table[i];
-		if (!map->size)
-			continue;
-		if (strcmp(name, map->name) == 0) {
-			*start = map->start;
-			*size = map->size;
-			return 1;
-		}
-	}
-	return 0;
+	guard(mutex)(&reserve_mem_lock);
+	map = reserve_mem_find_by_name_nolock(name);
+	if (!map)
+		return 0;
+
+	*start = map->start;
+	*size = map->size;
+	return 1;
 }
 EXPORT_SYMBOL_GPL(reserve_mem_find_by_name);
 
+/**
+ * reserve_mem_release_by_name - Release reserved memory region with a given name
+ * @name: The name that is attatched to a reserved memory region
+ *
+ * Forcibly release the pages in the reserved memory region so that those memory
+ * can be used as free memory. After released the reserved region size becomes 0.
+ *
+ * Returns: 1 if released or 0 if not found.
+ */
+int reserve_mem_release_by_name(const char *name)
+{
+	char buf[RESERVE_MEM_NAME_SIZE + 12];
+	struct reserve_mem_table *map;
+	void *start, *end;
+
+	guard(mutex)(&reserve_mem_lock);
+	map = reserve_mem_find_by_name_nolock(name);
+	if (!map)
+		return 0;
+
+	start = phys_to_virt(map->start);
+	end = start + map->size - 1;
+	snprintf(buf, sizeof(buf), "reserve_mem:%s", name);
+	free_reserved_area(start, end, 0, buf);
+	map->size = 0;
+
+	return 1;
+}
+
 /*
  * Parse reserve_mem=nn:align:name
  */