get vmalloc start address from vmcoreinfo

Below error is noticed when running crash on vmcore collected from a linux-next
kernel crash (linux-next tag next-20240121):

  # crash /boot/vmlinuz-6.8.0-rc5-next-20240221 ./vmcore
  ...
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  crash: page excluded: kernel virtual address: c00000000219a2c0  type: "vmlist"

This occured since getting the vmalloc area base address doesn't work in
crash now, due to 'vmap_area_list' being removed in the linux kernel
6.9-rc1 with below commit:

    commit 55c49fee57af99f3c663e69dedc5b85e691bbe50
         mm/vmalloc: remove vmap_area_list

As an alternative, the commit introduced 'VMALLOC_START' in vmcoreinfo to
get base address of vmalloc area, use it to return vmallow start address
instead of depending on vmap_area_list and vmlist.

Reported-by: Sachin Sant <[email protected]>
Signed-off-by: Aditya Gupta <[email protected]>
Tested-by: Sachin Sant <[email protected]>
Acked-by: Hari Bathini <[email protected]>

memory.c

@@ -17220,11 +17220,20 @@ first_vmalloc_address(void)
 {
 	static ulong vmalloc_start = 0;
         ulong vm_struct, vmap_area;
+	char *vmalloc_start_string;
 
 	if (DUMPFILE() && vmalloc_start)
 		return vmalloc_start;
 
-	if (vt->flags & USE_VMAP_AREA) {
+	/*
+	 * 'vmap_area_list' and 'vmlist' in Linux 6.9 and later kernels might be
+	 * empty, prefer NUMBER(VMALLOC_START) if exported in vmcoreinfo.
+	 */
+	vmalloc_start_string = pc->read_vmcoreinfo("NUMBER(VMALLOC_START)");
+	if (vmalloc_start_string) {
+		vmalloc_start = htol(vmalloc_start_string, QUIET, NULL);
+		free(vmalloc_start_string);
+	} else if (vt->flags & USE_VMAP_AREA) {
 		get_symbol_data("vmap_area_list", sizeof(void *), &vmap_area);
 		if (!vmap_area)
 			return 0;