tracing: fix splice return too large

I got these from strace:

splice(0x3, 0, 0x5, 0, 0x1000, 0x1) = 12288
splice(0x3, 0, 0x5, 0, 0x1000, 0x1) = 12288
splice(0x3, 0, 0x5, 0, 0x1000, 0x1) = 12288
splice(0x3, 0, 0x5, 0, 0x1000, 0x1) = 16384
splice(0x3, 0, 0x5, 0, 0x1000, 0x1) = 8192
splice(0x3, 0, 0x5, 0, 0x1000, 0x1) = 8192
splice(0x3, 0, 0x5, 0, 0x1000, 0x1) = 8192

I wanted to splice_read 4096 bytes, but it returns 8192 or larger.

It is because the return value of tracing_buffers_splice_read()
does not include "zero out any left over data" bytes.

But tracing_buffers_read() includes these bytes, we make them
consistent.

Signed-off-by: Lai Jiangshan <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
LKML-Reference: <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/trace/trace.c

···
 	int size, i;
 	size_t ret;
 
-	for (i = 0; i < PIPE_BUFFERS && len; i++, len -= size) {
+	if (*ppos & (PAGE_SIZE - 1)) {
+		WARN_ONCE(1, "Ftrace: previous read must page-align\n");
+		return -EINVAL;
+	}
+
+	if (len & (PAGE_SIZE - 1)) {
+		WARN_ONCE(1, "Ftrace: splice_read should page-align\n");
+		if (len < PAGE_SIZE)
+			return -EINVAL;
+		len &= PAGE_MASK;
+	}
+
+	for (i = 0; i < PIPE_BUFFERS && len; i++, len -= PAGE_SIZE) {
 		struct page *page;
 		int r;
 
···
 		spd.partial[i].offset = 0;
 		spd.partial[i].private = (unsigned long)ref;
 		spd.nr_pages++;
-		*ppos += size;
+		*ppos += PAGE_SIZE;
 	}
 
 	spd.nr_pages = i;