mm: vmscan: do not use page_count without a page pin

It is unsafe to run page_count during the physical pfn scan because
compound_head could trip on a dangling pointer when reading
page->first_page if the compound page is being freed by another CPU.

[mgorman@suse.de: split out patch]
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 2 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index dbe6ea321df4..8ff834e19c24 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1124,8 +1124,20 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
                                         nr_lumpy_dirty++;
                                 scan++;
                         } else {
-                                /* the page is freed already. */
-                                if (!page_count(cursor_page))
+                                /*
+                                 * Check if the page is freed already.
+                                 *
+                                 * We can't use page_count() as that
+                                 * requires compound_head and we don't
+                                 * have a pin on the page here. If a
+                                 * page is tail, we may or may not
+                                 * have isolated the head, so assume
+                                 * it's not free, it'd be tricky to
+                                 * track the head status without a
+                                 * page pin.
+                                 */
+                                if (!PageTail(cursor_page) &&
+                                    !atomic_read(&cursor_page->_count))
                                         continue;
                                 break;
                         }