perf tools: Start the perf.data mapping at data offset in perf trace

Currently, we are mapping perf.data in the beginning of the file
and use the data offset as a buffer offset.

This may exceed the mapping area if the data offset is upper than
page_size * mmap_window and result in a page fault (thing that
happen if we merge trace.info in perf.data).

Instead, let's start the mapping in the page that matches our data
offset.

v2: Drop a junk from another patch (trace_report() removal)

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <1254856886-10348-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

tools/perf/builtin-trace.c

@@ -143,6 +143,7 @@ static int __cmd_trace(void)
 	int ret, rc = EXIT_FAILURE;
 	unsigned long offset = 0;
 	unsigned long head = 0;
+	unsigned long shift;
 	struct stat perf_stat;
 	event_t *event;
 	uint32_t size;
@@ -180,6 +181,10 @@ static int __cmd_trace(void)
 		return EXIT_FAILURE;
 	}
 
+	shift = page_size * (head / page_size);
+	offset += shift;
+	head -= shift;
+
 remap:
 	buf = (char *)mmap(NULL, page_size * mmap_window, PROT_READ,
 			   MAP_SHARED, input, offset);
@@ -192,9 +197,9 @@ more:
 	event = (event_t *)(buf + head);
 
 	if (head + event->header.size >= page_size * mmap_window) {
-		unsigned long shift = page_size * (head / page_size);
 		int res;
 
+		shift = page_size * (head / page_size);
 		res = munmap(buf, page_size * mmap_window);
 		assert(res == 0);