[PATCH] perf tools: fix: Force backward ring buffer mapped readonly

[Wang Nan]

perf record's --overwrite option doesn't work as we expect.
For example:

    $ ~/linux/tools/perf$ sudo rm ./perf.data*
    rm: cannot remove './perf.data*': No such file or directory
    : ~/linux/tools/perf$ sudo ./perf record -m 4 -e raw_syscalls:* -g --overwrite \
	    				--switch-output=1s --tail-synthesize   dd if=/dev/zero of=/dev/null
    [ perf record: dump data: Woken up 1 times ]
    [ perf record: Dump perf.data.2017101221460002 ]
    [ perf record: dump data: Woken up 1 times ]
    [ perf record: Dump perf.data.2017101221460102 ]
    [ perf record: dump data: Woken up 1 times ]
    [ perf record: Dump perf.data.2017101221460202 ]
    [ perf record: dump data: Woken up 1 times ]
    [ perf record: Dump perf.data.2017101221460302 ]
    ^C[ perf record: Woken up 1 times to write data ]
    2066346+0 records in
    2066346+0 records out
    1057969152 bytes (1.1 GB, 1009 MiB) copied, 4.25983 s, 248 MB/s
    [ perf record: Dump perf.data.2017101221460332 ]
    [ perf record: Captured and wrote 0.034 MB perf.data.<timestamp> ]

    $ ~/linux/tools/perf$ sudo ./perf script -i ./perf.data.2017101221460002 | head -n 1
    dd  3918 [006] 182589.668954:  raw_syscalls:sys_exit: NR 59 = 0
    $ ~/linux/tools/perf$ sudo ./perf script -i ./perf.data.2017101221460102 | head -n 1
    dd  3918 [006] 182589.668954:  raw_syscalls:sys_exit: NR 59 = 0
    $ ~/linux/tools/perf$ sudo ./perf script -i ./perf.data.2017101221460202 | head -n 1
    dd  3918 [006] 182589.668954:  raw_syscalls:sys_exit: NR 59 = 0
    $ ~/linux/tools/perf$ sudo ./perf script -i ./perf.data.2017101221460302 | head -n 1
    dd  3918 [006] 182589.668954:  raw_syscalls:sys_exit: NR 59 = 0

In the above example we get same records from the backward ring buffer
all the time. Overwriting is not triggered.

This commit maps backward ring buffers readonly, make it overwritable.
It is safe because we assume backward ring buffer always overwritable
in other part of code.

After applying this patch:

    $ ~/linux/tools/perf$ sudo ./perf record -m 4 -e raw_syscalls:* -g --overwrite \
	    			   --switch-output=1s --tail-synthesize   dd if=/dev/zero of=/dev/null
    [ perf record: dump data: Woken up 1 times ]
    [ perf record: Dump perf.data.2017101221540350 ]
    [ perf record: dump data: Woken up 1 times ]
    [ perf record: Dump perf.data.2017101221540451 ]
    [ perf record: dump data: Woken up 1 times ]
    [ perf record: Dump perf.data.2017101221540551 ]
    [ perf record: dump data: Woken up 1 times ]
    [ perf record: Dump perf.data.2017101221540651 ]
    ^C[ perf record: Woken up 1 times to write data ]
    1604606+0 records in
    1604605+0 records out
    821557760 bytes (822 MB, 783 MiB) copied, 4.42736 s, 186 MB/s
    [ perf record: Dump perf.data.2017101221540700 ]
    [ perf record: Captured and wrote 0.034 MB perf.data.<timestamp> ]

    $ ~/linux/tools/perf$ sudo ./perf script -i ./perf.data.2017101221540350 | head -n 1
    dd  5126 [003] 183074.104581:  raw_syscalls:sys_exit: NR 0 = 512
    $ ~/linux/tools/perf$ sudo ./perf script -i ./perf.data.2017101221540451 | head -n 1
    dd  5126 [003] 183075.106496:  raw_syscalls:sys_exit: NR 1 = 512
    $ ~/linux/tools/perf$ sudo ./perf script -i ./perf.data.2017101221540551 | head -n 1
    dd  5126 [003] 183076.108093: raw_syscalls:sys_enter: NR 1 (1, af8000, 200, 871, 0, af8060)
    $ ~/linux/tools/perf$ sudo ./perf script -i ./perf.data.2017101221540651 | head -n 1
    dd  5126 [003] 183077.109676:  raw_syscalls:sys_exit: NR 1 = 512


Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
Cc: Liang Kan <kan.liang@xxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Li Zefan <lizefan@xxxxxxxxxx>
---
 tools/perf/util/evlist.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index c6c891e..a86b0d2 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -799,12 +799,14 @@ perf_evlist__should_poll(struct perf_evlist *evlist __maybe_unused,
 }
 
 static int perf_evlist__mmap_per_evsel(struct perf_evlist *evlist, int idx,
-				       struct mmap_params *mp, int cpu_idx,
+				       struct mmap_params *_mp, int cpu_idx,
 				       int thread, int *_output, int *_output_backward)
 {
 	struct perf_evsel *evsel;
 	int revent;
 	int evlist_cpu = cpu_map__cpu(evlist->cpus, cpu_idx);
+	struct mmap_params *mp = _mp;
+	struct mmap_params backward_mp;
 
 	evlist__for_each_entry(evlist, evsel) {
 		struct perf_mmap *maps = evlist->mmap;
@@ -815,6 +817,9 @@ static int perf_evlist__mmap_per_evsel(struct perf_evlist *evlist, int idx,
 		if (evsel->attr.write_backward) {
 			output = _output_backward;
 			maps = evlist->backward_mmap;
+			backward_mp = *mp;
+			backward_mp.prot &= ~PROT_WRITE;
+			mp = &backward_mp;
 
 			if (!maps) {
 				maps = perf_evlist__alloc_mmap(evlist);
-- 
2.9.3