perf/core: Restore mmap record type correctly [Linux 5.1]

perf/core: Restore mmap record type correctly [Linux 5.1]

This Linux kernel change "perf/core: Restore mmap record type correctly" is included in the Linux 5.1 release. This change is authored by Stephane Eranian <eranian [at] google.com> on Thu Mar 7 10:52:33 2019 -0800. The commit for this change in Linux stable tree is d9c1bb2 (patch).

perf/core: Restore mmap record type correctly

On mmap(), perf_events generates a RECORD_MMAP record and then checks
which events are interested in this record. There are currently 2
versions of mmap records: RECORD_MMAP and RECORD_MMAP2. MMAP2 is larger.
The event configuration controls which version the user level tool
accepts.

If the event->attr.mmap2=1 field then MMAP2 record is returned.  The
perf_event_mmap_output() takes care of this. It checks attr->mmap2 and
corrects the record fields before putting it in the sampling buffer of
the event.  At the end the function restores the modified MMAP record
fields.

The problem is that the function restores the size but not the type.
Thus, if a subsequent event only accepts MMAP type, then it would
instead receive an MMAP2 record with a size of MMAP record.

This patch fixes the problem by restoring the record type on exit.

Signed-off-by: Stephane Eranian <eranian@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Fixes: 13d7a2410fa6 ("perf: Add attr->mmap2 attribute to an event")
Link: http://lkml.kernel.org/r/20190307185233.225521-1-eranian@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

There are 2 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 kernel/events/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6fb27b5..514b8e0 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7189,6 +7189,7 @@ static void perf_event_mmap_output(struct perf_event *event,
    struct perf_output_handle handle;
    struct perf_sample_data sample;
    int size = mmap_event->event_id.header.size;
+   u32 type = mmap_event->event_id.header.type;
    int ret;

    if (!perf_event_mmap_match(event, data))
@@ -7232,6 +7233,7 @@ static void perf_event_mmap_output(struct perf_event *event,
    perf_output_end(&handle);
 out:
    mmap_event->event_id.header.size = size;
+   mmap_event->event_id.header.type = type;
 }

 static void perf_event_mmap_event(struct perf_mmap_event *mmap_event)

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