linux-kernel - [PATCH] Fix perf mmap limitations on 32-bit.

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Date:	Sat, 10 Nov 2012 14:12:19 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	acme@...stprotocols.net
CC:	a.p.zijlstra@...llo.nl, paulus@...ba.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org
Subject: [PATCH] Fix perf mmap limitations on 32-bit.

This is a suggested patch to fix the bug I reported at:

	http://marc.info/?l=linux-kernel&m=135033028924652&w=2

Essentially, there is a hard requirement that when perf analyzes a
trace, it must have the entire thing mmap()'d.

Therefore the scheme used on 32-bit where we have a fixed (8) number
of 32MB mmaps, and cycle through them, simply does not work.

One of the reasons this requirement exists is because the iterators
maintain references to perf entry objects and those references don't
just simply go away when this mmap code decides to cycle an old mmap
area out and reuse it.  At this point, those entry pointers now point
to garbage resulting in unpredictable behavior and crashes.

It is better to try to mmap() as much as we can and if we do actually
run into address space limitations, the failure of the mmap() call
will indicate that and stop processing.

I noticed that perf_session->mmap_window is set to a constant in one
location, and only used in one other location.  So I got rid of it
altogether.

So we adjust the size of the mmaps[] array to the maximum we could
need.  On 64-bit we only need one slot.  On 32-bit we could need
up to 128 (128 * 32MB == 4GB).

I've verified that this allows a large (~600MB) perf.data file to
be analyzed properly with a 32-bit perf binary, which previously
was not possible.

Signed-off-by: David S. Miller <davem@...emloft.net>

diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index 8cdd232..2cd3cc3 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -128,15 +128,6 @@ struct perf_session *perf_session__new(const char *filename, int mode,
 		goto out;

 	memcpy(self->filename, filename, len);
-	/*
-	 * On 64bit we can mmap the data file in one go. No need for tiny mmap
-	 * slices. On 32bit we use 32MB.
-	 */
-#if BITS_PER_LONG == 64
-	self->mmap_window = ULLONG_MAX;
-#else
-	self->mmap_window = 32 * 1024 * 1024ULL;
-#endif
 	self->machines = RB_ROOT;
 	self->repipe = repipe;
 	INIT_LIST_HEAD(&self->ordered_samples.samples);
@@ -1369,6 +1360,18 @@ fetch_mmaped_event(struct perf_session *session,
 	return event;
 }

+/*
+ * On 64bit we can mmap the data file in one go. No need for tiny mmap
+ * slices. On 32bit we use 32MB.
+ */
+#if BITS_PER_LONG == 64
+#define MMAP_SIZE ULLONG_MAX
+#define NUM_MMAPS 1
+#else
+#define MMAP_SIZE (32 * 1024 * 1024ULL)
+#define NUM_MMAPS 128
+#endif
+
 int __perf_session__process_events(struct perf_session *session,
 				   u64 data_offset, u64 data_size,
 				   u64 file_size, struct perf_tool *tool)
@@ -1376,7 +1379,7 @@ int __perf_session__process_events(struct perf_session *session,
 	u64 head, page_offset, file_offset, file_pos, progress_next;
 	int err, mmap_prot, mmap_flags, map_idx = 0;
 	size_t	page_size, mmap_size;
-	char *buf, *mmaps[8];
+	char *buf, *mmaps[NUM_MMAPS];
 	union perf_event *event;
 	uint32_t size;

@@ -1393,7 +1396,7 @@ int __perf_session__process_events(struct perf_session *session,

 	progress_next = file_size / 16;

-	mmap_size = session->mmap_window;
+	mmap_size = MMAP_SIZE;
 	if (mmap_size > file_size)
 		mmap_size = file_size;

diff --git a/tools/perf/util/session.h b/tools/perf/util/session.h
index dd64261..903966b 100644
--- a/tools/perf/util/session.h
+++ b/tools/perf/util/session.h
@@ -29,7 +29,6 @@ struct ordered_samples {
 struct perf_session {
 	struct perf_header	header;
 	unsigned long		size;
-	unsigned long		mmap_window;
 	struct machine		host_machine;
 	struct rb_root		machines;
 	struct perf_evlist	*evlist;
--
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