perf: Add data_{offset,size} to user_page

Currently, the actual perf ring buffer is one page into the mmap area,
following the user page and the userspace follows this convention. This
patch adds data_{offset,size} fields to user_page that can be used by
userspace instead for locating perf data in the mmap area. This is also
helpful when mapping existing or shared buffers if their size is not
known in advance.

Right now, it is made to follow the existing convention that

	data_offset == PAGE_SIZE and
	data_offset + data_size == mmap_size.

Signed-off-by: Alexander Shishkin <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Kaixu Xia <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Robert Richter <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/1421237903-181015-2-git-send-email-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <[email protected]>

2 files changed, 7 insertions, 0 deletions

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 91803e54ee73..86c44ae66d43 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -522,9 +522,14 @@ struct perf_event_mmap_page {
 	 * In this case the kernel will not over-write unread data.
 	 *
 	 * See perf_output_put_handle() for the data ordering.
+	 *
+	 * data_{offset,size} indicate the location and size of the perf record
+	 * buffer within the mmapped area.
 	 */
 	__u64   data_head;		/* head in the data section */
 	__u64	data_tail;		/* user-space written tail */
+	__u64	data_offset;		/* where the buffer starts */
+	__u64	data_size;		/* data buffer size */
 };
 
 #define PERF_RECORD_MISC_CPUMODE_MASK		(7 << 0)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 5c13862d3e85..6efa516f1ab8 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4105,6 +4105,8 @@ static void perf_event_init_userpage(struct perf_event *event)
 	/* Allow new userspace to detect that bit 0 is deprecated */
 	userpg->cap_bit0_is_deprecated = 1;
 	userpg->size = offsetof(struct perf_event_mmap_page, __reserved);
+	userpg->data_offset = PAGE_SIZE;
+	userpg->data_size = perf_data_size(rb);
 
 unlock:
 	rcu_read_unlock();