Fix "kmem -v" option displaying no regions on Linux 6.3 and later

Kernel commit 869176a09606 ("mm/vmalloc.c: add flags to mark vm_map_ram
area"), which is contained in Linux 6.3 and later, added "flags" member
to struct vmap_area.  This was the revival of the "flags" member as
kernel commit 688fcbfc06e4 had eliminated it before.

As a result, crash started to use the old procedure using the member and
displays no vmalloc'd regions, because it does not have the same flag
value as the old one.

  crash> kmem -v
     VMAP_AREA         VM_STRUCT                 ADDRESS RANGE                SIZE
  crash>

To fix this, also check if vmap_area.purge_list exists, which was
introduced with the flags and removed later, to determine that the flags
member is the old one.

Related vmap_area history:
 v2.6.28 db64fe02258f introduced vmap_area with flags and purge_list
 v5.4    688fcbfc06e4 removed flags
 v5.11   96e2db456135 removed purge_list
 v6.3    869176a09606 added flags again

Signed-off-by: Kazuhito Hagio <k-hagio-ab@nec.com>

defs.h

@@ -2216,6 +2216,7 @@ struct offset_table {                    /* stash of commonly-used offsets */
 	long in6_addr_in6_u;
 	long kset_kobj;
 	long subsys_private_subsys;
+	long vmap_area_purge_list;
 };
 
 struct size_table {         /* stash of commonly-used sizes */

memory.c

@@ -429,6 +429,7 @@ vm_init(void)
 	MEMBER_OFFSET_INIT(vmap_area_vm, "vmap_area", "vm");
 	if (INVALID_MEMBER(vmap_area_vm))
 		MEMBER_OFFSET_INIT(vmap_area_vm, "vmap_area", "private");
+	MEMBER_OFFSET_INIT(vmap_area_purge_list, "vmap_area", "purge_list");
 	STRUCT_SIZE_INIT(vmap_area, "vmap_area");
 	if (VALID_MEMBER(vmap_area_va_start) &&
 	    VALID_MEMBER(vmap_area_va_end) &&
@@ -9063,7 +9064,8 @@ dump_vmap_area(struct meminfo *vi)
 		readmem(ld->list_ptr[i], KVADDR, vmap_area_buf,
                         SIZE(vmap_area), "vmap_area struct", FAULT_ON_ERROR); 
 
-		if (VALID_MEMBER(vmap_area_flags)) {
+		if (VALID_MEMBER(vmap_area_flags) &&
+		    VALID_MEMBER(vmap_area_purge_list)) {
 			flags = ULONG(vmap_area_buf + OFFSET(vmap_area_flags));
 			if (flags != VM_VM_AREA)
 				continue;

symbols.c

@@ -9169,6 +9169,7 @@ dump_offset_table(char *spec, ulong makestruct)
 		OFFSET(vmap_area_vm));
 	fprintf(fp, "               vmap_area_flags: %ld\n", 
 		OFFSET(vmap_area_flags));
+	fprintf(fp, "          vmap_area_purge_list: %ld\n", OFFSET(vmap_area_purge_list));
 
 	fprintf(fp, "         module_size_of_struct: %ld\n", 
 		OFFSET(module_size_of_struct));