[PATCH 2/2] perf/x86/intel/bts: Fix the use of page_private()

[Alexander Shishkin <alexander.shishkin@linux.intel.com>]

Commit

  8062382c8dbe2 ("perf/x86/intel/bts: Add BTS PMU driver")

uses page_private(page) without checking the PagePrivate(page) first,
which seems like a potential bug, considering that page->private aliases
with other stuff in struct page.

Fix this by checking PagePrivate() first.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: 8062382c8dbe2 ("perf/x86/intel/bts: Add BTS PMU driver")
---
 arch/x86/events/intel/bts.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/events/intel/bts.c b/arch/x86/events/intel/bts.c
index d53b4fb86d87..9e4da1c5a129 100644
--- a/arch/x86/events/intel/bts.c
+++ b/arch/x86/events/intel/bts.c
@@ -63,9 +63,17 @@ struct bts_buffer {
 
 static struct pmu bts_pmu;
 
+static int buf_nr_pages(struct page *page)
+{
+	if (!PagePrivate(page))
+		return 1;
+
+	return 1 << page_private(page);
+}
+
 static size_t buf_size(struct page *page)
 {
-	return 1 << (PAGE_SHIFT + page_private(page));
+	return 1 << (PAGE_SHIFT + buf_nr_pages(page));
 }
 
 static void *
@@ -83,7 +91,7 @@ bts_buffer_setup_aux(struct perf_event *event, void **pages,
 	/* count all the high order buffers */
 	for (pg = 0, nbuf = 0; pg < nr_pages;) {
 		page = virt_to_page(pages[pg]);
-		pg += 1 << page_private(page);
+		pg += buf_nr_pages(page);
 		nbuf++;
 	}
 
@@ -107,7 +115,7 @@ bts_buffer_setup_aux(struct perf_event *event, void **pages,
 		unsigned int __nr_pages;
 
 		page = virt_to_page(pages[pg]);
-		__nr_pages = PagePrivate(page) ? 1 << page_private(page) : 1;
+		__nr_pages = buf_nr_pages(page);
 		buf->buf[nbuf].page = page;
 		buf->buf[nbuf].offset = offset;
 		buf->buf[nbuf].displacement = (pad ? BTS_RECORD_SIZE - pad : 0);
-- 
2.24.0