lib: add version into /proc/allocinfo output

Add version string and a header at the beginning of /proc/allocinfo to
allow later format changes. Example output:

> head /proc/allocinfo
allocinfo - version: 1.0
# <size> <calls> <tag info>
0 0 init/main.c:1314 func:do_initcalls
0 0 init/do_mounts.c:353 func:mount_nodev_root
0 0 init/do_mounts.c:187 func:mount_root_generic
0 0 init/do_mounts.c:158 func:do_mount_root
0 0 init/initramfs.c:493 func:unpack_to_rootfs
0 0 init/initramfs.c:492 func:unpack_to_rootfs
0 0 init/initramfs.c:491 func:unpack_to_rootfs
512 1 arch/x86/events/rapl.c:681 func:init_rapl_pmus
128 1 arch/x86/events/rapl.c:571 func:rapl_cpu_online

[[e-mail]: remove stray newline from struct allocinfo_private]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Suren Baghdasaryan <[e-mail]>
Reviewed-by: Pasha Tatashin <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Cc: Kent Overstreet <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/filesystems/proc.rst

···
 base. Each allocation in the code is identified by its source file, line
 number, module (if originates from a loadable module) and the function calling
 the allocation. The number of bytes allocated and number of calls at each
-location are reported.
+location are reported. The first line indicates the version of the file, the
+second line is the header listing fields in the file.
 
 Example output.
 
 ::
 
-    > sort -rn /proc/allocinfo
+    > tail -n +3 /proc/allocinfo | sort -rn
    127664128    31168 mm/page_ext.c:270 func:alloc_page_ext
     56373248     4737 mm/slub.c:2259 func:alloc_slab_page
     14880768     3633 mm/readahead.c:247 func:page_cache_ra_unbounded

lib/alloc_tag.c

···
 DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
 			mem_alloc_profiling_key);
 
+struct allocinfo_private {
+	struct codetag_iterator iter;
+	bool print_header;
+};
+
 static void *allocinfo_start(struct seq_file *m, loff_t *pos)
 {
-	struct codetag_iterator *iter;
+	struct allocinfo_private *priv;
 	struct codetag *ct;
 	loff_t node = *pos;
 
-	iter = kzalloc(sizeof(*iter), GFP_KERNEL);
-	m->private = iter;
-	if (!iter)
+	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
+	m->private = priv;
+	if (!priv)
 		return NULL;
 
+	priv->print_header = (node == 0);
 	codetag_lock_module_list(alloc_tag_cttype, true);
-	*iter = codetag_get_ct_iter(alloc_tag_cttype);
-	while ((ct = codetag_next_ct(iter)) != NULL && node)
+	priv->iter = codetag_get_ct_iter(alloc_tag_cttype);
+	while ((ct = codetag_next_ct(&priv->iter)) != NULL && node)
 		node--;
 
-	return ct ? iter : NULL;
+	return ct ? priv : NULL;
 }
 
 static void *allocinfo_next(struct seq_file *m, void *arg, loff_t *pos)
 {
-	struct codetag_iterator *iter = (struct codetag_iterator *)arg;
-	struct codetag *ct = codetag_next_ct(iter);
+	struct allocinfo_private *priv = (struct allocinfo_private *)arg;
+	struct codetag *ct = codetag_next_ct(&priv->iter);
 
 	(*pos)++;
 	if (!ct)
 		return NULL;
 
-	return iter;
+	return priv;
 }
 
 static void allocinfo_stop(struct seq_file *m, void *arg)
 {
-	struct codetag_iterator *iter = (struct codetag_iterator *)m->private;
+	struct allocinfo_private *priv = (struct allocinfo_private *)m->private;
 
-	if (iter) {
+	if (priv) {
 		codetag_lock_module_list(alloc_tag_cttype, false);
-		kfree(iter);
+		kfree(priv);
 	}
+}
+
+static void print_allocinfo_header(struct seq_buf *buf)
+{
+	/* Output format version, so we can change it. */
+	seq_buf_printf(buf, "allocinfo - version: 1.0\n");
+	seq_buf_printf(buf, "#     <size>  <calls> <tag info>\n");
 }
 
 static void alloc_tag_to_text(struct seq_buf *out, struct codetag *ct)
···
 
 static int allocinfo_show(struct seq_file *m, void *arg)
 {
-	struct codetag_iterator *iter = (struct codetag_iterator *)arg;
+	struct allocinfo_private *priv = (struct allocinfo_private *)arg;
 	char *bufp;
 	size_t n = seq_get_buf(m, &bufp);
 	struct seq_buf buf;
 
 	seq_buf_init(&buf, bufp, n);
-	alloc_tag_to_text(&buf, iter->ct);
+	if (priv->print_header) {
+		print_allocinfo_header(&buf);
+		priv->print_header = false;
+	}
+	alloc_tag_to_text(&buf, priv->iter.ct);
 	seq_commit(m, seq_buf_used(&buf));
 	return 0;
 }