coredump: kill call_count, add core_name_size

Imho, "atomic_t call_count" is ugly and should die.  It buys nothing and
in fact it can grow more than necessary, expand doesn't check if it was
already incremented by another task.

Kill it, and introduce "static int core_name_size" updated by
expand_corename().  This is obviously racy too but harmless, and
core_name_size never grows for no reason.

We do not bother to to calculate the "right" new size, we simply do
kmalloc(size_we_need) and use ksize() to rely on kmalloc_index's decision.

Finally change format_corename() to use expand_corename(), krealloc(NULL)
is fine.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Colin Walters <walters@verbum.org>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Lennart Poettering <mzxreary@0pointer.de>
Cc: Lucas De Marchi <lucas.de.marchi@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/coredump.c

@@ -45,26 +45,28 @@
 #include <trace/events/sched.h>
 
 int core_uses_pid;
-char core_pattern[CORENAME_MAX_SIZE] = "core";
 unsigned int core_pipe_limit;
+char core_pattern[CORENAME_MAX_SIZE] = "core";
+static int core_name_size = CORENAME_MAX_SIZE;
 
 struct core_name {
 	char *corename;
 	int used, size;
 };
-static atomic_t call_count = ATOMIC_INIT(1);
 
 /* The maximal length of core_pattern is also specified in sysctl.c */
 
-static int expand_corename(struct core_name *cn)
+static int expand_corename(struct core_name *cn, int size)
 {
-	int size = CORENAME_MAX_SIZE * atomic_inc_return(&call_count);
 	char *corename = krealloc(cn->corename, size, GFP_KERNEL);
 
 	if (!corename)
 		return -ENOMEM;
 
-	cn->size = size;
+	if (size > core_name_size) /* racy but harmless */
+		core_name_size = size;
+
+	cn->size = ksize(corename);
 	cn->corename = corename;
 	return 0;
 }
@@ -81,7 +83,7 @@ again:
 		return 0;
 	}
 
-	if (!expand_corename(cn))
+	if (!expand_corename(cn, cn->size + need - free + 1))
 		goto again;
 
 	return -ENOMEM;
@@ -160,9 +162,8 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm)
 	int err = 0;
 
 	cn->used = 0;
-	cn->size = CORENAME_MAX_SIZE * atomic_read(&call_count);
-	cn->corename = kmalloc(cn->size, GFP_KERNEL);
-	if (!cn->corename)
+	cn->corename = NULL;
+	if (expand_corename(cn, core_name_size))
 		return -ENOMEM;
 
 	/* Repeat as long as we have more pattern to process and more output