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>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index dbe6ea321df42cb2fe430c22793a1df9d77200fe..8ff834e19c2460d33844e43154b058974a6c9849 100644 (file)
--- 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;
                        }