kexec: add helper __crash_shrink_memory()

No functional change, in preparation for the next patch so that it is
easier to review.

[akpm@linux-foundation.org: make  __crash_shrink_memory() static]
  Link: https://lore.kernel.org/oe-kbuild-all/202305280717.Pw06aLkz-lkp@intel.com/
Link: https://lkml.kernel.org/r/20230527123439.772-6-thunder.leizhen@huawei.comSigned-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Cong Wang <amwang@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/kexec_core.c

@@ -1105,11 +1105,38 @@ ssize_t crash_get_memory_size(void)
 	return size;
 }
 
+static int __crash_shrink_memory(struct resource *old_res,
+				 unsigned long new_size)
+{
+	struct resource *ram_res;
+
+	ram_res = kzalloc(sizeof(*ram_res), GFP_KERNEL);
+	if (!ram_res)
+		return -ENOMEM;
+
+	ram_res->start = old_res->start + new_size;
+	ram_res->end   = old_res->end;
+	ram_res->flags = IORESOURCE_BUSY | IORESOURCE_SYSTEM_RAM;
+	ram_res->name  = "System RAM";
+
+	if (!new_size) {
+		release_resource(old_res);
+		old_res->start = 0;
+		old_res->end   = 0;
+	} else {
+		crashk_res.end = ram_res->start - 1;
+	}
+
+	crash_free_reserved_phys_range(ram_res->start, ram_res->end);
+	insert_resource(&iomem_resource, ram_res);
+
+	return 0;
+}
+
 int crash_shrink_memory(unsigned long new_size)
 {
 	int ret = 0;
 	unsigned long old_size;
-	struct resource *ram_res;
 
 	if (!kexec_trylock())
 		return -EBUSY;
@@ -1125,27 +1152,7 @@ int crash_shrink_memory(unsigned long new_size)
 		goto unlock;
 	}
 
-	ram_res = kzalloc(sizeof(*ram_res), GFP_KERNEL);
-	if (!ram_res) {
-		ret = -ENOMEM;
-		goto unlock;
-	}
-
-	ram_res->start = crashk_res.start + new_size;
-	ram_res->end = crashk_res.end;
-	ram_res->flags = IORESOURCE_BUSY | IORESOURCE_SYSTEM_RAM;
-	ram_res->name = "System RAM";
-
-	if (!new_size) {
-		release_resource(&crashk_res);
-		crashk_res.start = 0;
-		crashk_res.end = 0;
-	} else {
-		crashk_res.end = ram_res->start - 1;
-	}
-
-	crash_free_reserved_phys_range(ram_res->start, ram_res->end);
-	insert_resource(&iomem_resource, ram_res);
+	ret = __crash_shrink_memory(&crashk_res, new_size);
 
 unlock:
 	kexec_unlock();