seq_file: add seq_set_overflow(), seq_overflow()

It is undocumented but a seq_file's overflow state is indicated by
m->count == m->size.  Add seq_set_overflow() and seq_overflow() to
set/check overflow status explicitly.

Based on an idea from Eric Dumazet.

[akpm@linux-foundation.org: tweak code comment]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/seq_file.c

@@ -13,6 +13,22 @@
 #include <asm/uaccess.h>
 #include <asm/page.h>
 
+
+/*
+ * seq_files have a buffer which can may overflow. When this happens a larger
+ * buffer is reallocated and all the data will be printed again.
+ * The overflow state is true when m->count == m->size.
+ */
+static bool seq_overflow(struct seq_file *m)
+{
+	return m->count == m->size;
+}
+
+static void seq_set_overflow(struct seq_file *m)
+{
+	m->count = m->size;
+}
+
 /**
  *	seq_open -	initialize sequential file
  *	@file: file we initialize
@@ -92,7 +108,7 @@ static int traverse(struct seq_file *m, loff_t offset)
 			error = 0;
 			m->count = 0;
 		}
-		if (m->count == m->size)
+		if (seq_overflow(m))
 			goto Eoverflow;
 		if (pos + m->count > offset) {
 			m->from = offset - pos;
@@ -234,7 +250,7 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
 			break;
 		}
 		err = m->op->show(m, p);
-		if (m->count == m->size || err) {
+		if (seq_overflow(m) || err) {
 			m->count = offs;
 			if (likely(err <= 0))
 				break;
@@ -361,7 +377,7 @@ int seq_escape(struct seq_file *m, const char *s, const char *esc)
 			*p++ = '0' + (c & 07);
 			continue;
 		}
-		m->count = m->size;
+		seq_set_overflow(m);
 		return -1;
         }
 	m->count = p - m->buf;
@@ -383,7 +399,7 @@ int seq_printf(struct seq_file *m, const char *f, ...)
 			return 0;
 		}
 	}
-	m->count = m->size;
+	seq_set_overflow(m);
 	return -1;
 }
 EXPORT_SYMBOL(seq_printf);
@@ -512,7 +528,7 @@ int seq_bitmap(struct seq_file *m, const unsigned long *bits,
 			return 0;
 		}
 	}
-	m->count = m->size;
+	seq_set_overflow(m);
 	return -1;
 }
 EXPORT_SYMBOL(seq_bitmap);
@@ -528,7 +544,7 @@ int seq_bitmap_list(struct seq_file *m, const unsigned long *bits,
 			return 0;
 		}
 	}
-	m->count = m->size;
+	seq_set_overflow(m);
 	return -1;
 }
 EXPORT_SYMBOL(seq_bitmap_list);
@@ -639,7 +655,7 @@ int seq_puts(struct seq_file *m, const char *s)
 		m->count += len;
 		return 0;
 	}
-	m->count = m->size;
+	seq_set_overflow(m);
 	return -1;
 }
 EXPORT_SYMBOL(seq_puts);
@@ -673,7 +689,7 @@ int seq_put_decimal_ull(struct seq_file *m, char delimiter,
 	m->count += len;
 	return 0;
 overflow:
-	m->count = m->size;
+	seq_set_overflow(m);
 	return -1;
 }
 EXPORT_SYMBOL(seq_put_decimal_ull);
@@ -683,7 +699,7 @@ int seq_put_decimal_ll(struct seq_file *m, char delimiter,
 {
 	if (num < 0) {
 		if (m->count + 3 >= m->size) {
-			m->count = m->size;
+			seq_set_overflow(m);
 			return -1;
 		}
 		if (delimiter)
@@ -711,7 +727,7 @@ int seq_write(struct seq_file *seq, const void *data, size_t len)
 		seq->count += len;
 		return 0;
 	}
-	seq->count = seq->size;
+	seq_set_overflow(seq);
 	return -1;
 }
 EXPORT_SYMBOL(seq_write);