tracing: Have saved_cmdlines use the seq_read infrastructure

Current tracing_saved_cmdlines_read() implementation is naive; It allocates
a large buffer, constructs output data to that buffer for each read
operation, and then copies a portion of the buffer to the user space
buffer. This has several issues such as slow memory allocation, high
CPU usage, and even corruption of the output data.

The seq_read infrastructure is made to handle this type of work.
By converting it to use seq_read() the code becomes smaller, simplified,
as well as correct.

Link: http://lkml.kernel.org/p/20140220084431.3839.51793.stgit@yunodevel

Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 54 insertions, 35 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index eb228b9de170..1855dae73f34 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3699,55 +3699,74 @@ static const struct file_operations tracing_readme_fops = {
         .llseek         = generic_file_llseek,
 };
 
-static ssize_t
-tracing_saved_cmdlines_read(struct file *file, char __user *ubuf,
-                                size_t cnt, loff_t *ppos)
+static void *saved_cmdlines_next(struct seq_file *m, void *v, loff_t *pos)
 {
-        char *buf_comm;
-        char *file_buf;
-        char *buf;
-        int len = 0;
-        int pid;
-        int i;
+        unsigned int *ptr = v;
 
-        file_buf = kmalloc(SAVED_CMDLINES*(16+TASK_COMM_LEN), GFP_KERNEL);
-        if (!file_buf)
-                return -ENOMEM;
+        if (*pos || m->count)
+                ptr++;
 
-        buf_comm = kmalloc(TASK_COMM_LEN, GFP_KERNEL);
-        if (!buf_comm) {
-                kfree(file_buf);
-                return -ENOMEM;
-        }
+        (*pos)++;
+
+        for (; ptr < &map_cmdline_to_pid[SAVED_CMDLINES]; ptr++) {
+                if (*ptr == -1 || *ptr == NO_CMDLINE_MAP)
+                        continue;
 
-        buf = file_buf;
+                return ptr;
+        }
 
-        for (i = 0; i < SAVED_CMDLINES; i++) {
-                int r;
+        return NULL;
+}
 
-                pid = map_cmdline_to_pid[i];
-                if (pid == -1 || pid == NO_CMDLINE_MAP)
-                        continue;
+static void *saved_cmdlines_start(struct seq_file *m, loff_t *pos)
+{
+        void *v;
+        loff_t l = 0;
 
-                trace_find_cmdline(pid, buf_comm);
-                r = sprintf(buf, "%d %s\n", pid, buf_comm);
-                buf += r;
-                len += r;
+        v = &map_cmdline_to_pid[0];
+        while (l <= *pos) {
+                v = saved_cmdlines_next(m, v, &l);
+                if (!v)
+                        return NULL;
         }
 
-        len = simple_read_from_buffer(ubuf, cnt, ppos,
-                                      file_buf, len);
+        return v;
+}
 
-        kfree(file_buf);
-        kfree(buf_comm);
+static void saved_cmdlines_stop(struct seq_file *m, void *v)
+{
+}
 
-        return len;
+static int saved_cmdlines_show(struct seq_file *m, void *v)
+{
+        char buf[TASK_COMM_LEN];
+        unsigned int *pid = v;
+
+        trace_find_cmdline(*pid, buf);
+        seq_printf(m, "%d %s\n", *pid, buf);
+        return 0;
+}
+
+static const struct seq_operations tracing_saved_cmdlines_seq_ops = {
+        .start          = saved_cmdlines_start,
+        .next           = saved_cmdlines_next,
+        .stop           = saved_cmdlines_stop,
+        .show           = saved_cmdlines_show,
+};
+
+static int tracing_saved_cmdlines_open(struct inode *inode, struct file *filp)
+{
+        if (tracing_disabled)
+                return -ENODEV;
+
+        return seq_open(filp, &tracing_saved_cmdlines_seq_ops);
 }
 
 static const struct file_operations tracing_saved_cmdlines_fops = {
-    .open       = tracing_open_generic,
-    .read       = tracing_saved_cmdlines_read,
-    .llseek     = generic_file_llseek,
+        .open           = tracing_saved_cmdlines_open,
+        .read           = seq_read,
+        .llseek         = seq_lseek,
+        .release        = seq_release,
 };
 
 static ssize_t