[PATCH] mm: move call to compound_head() in release_pages()

[Ralph Campbell]

The function is_huge_zero_page() doesn't call compound_head() to make sure
the page pointer is a head page. The call to is_huge_zero_page() in
release_pages() is made before compound_head() is called so the test would
fail if release_pages() was called with a tail page of the huge_zero_page
and put_page_testzero() would be called releasing the page.
This is unlikely to be happening in normal use or we would be seeing all
sorts of process data corruption when accessing a THP zero page.

Looking at other places where is_huge_zero_page() is called, all seem to
only pass a head page so I think the right solution is to move the call
to compound_head() in release_pages() to a point before calling
is_huge_zero_page().

Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
---

I found this by code inspection while working on my patch
("mm: remove extra ZONE_DEVICE struct page refcount").

This applies cleanly on the latest linux-mm and is for Andrew Morton's
tree.

 mm/swap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/swap.c b/mm/swap.c
index eca95afe7ad4..7e79829a2e73 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -889,6 +889,7 @@ void release_pages(struct page **pages, int nr)
 			locked_pgdat = NULL;
 		}
 
+		page = compound_head(page);
 		if (is_huge_zero_page(page))
 			continue;
 
@@ -910,7 +911,6 @@ void release_pages(struct page **pages, int nr)
 			}
 		}
 
-		page = compound_head(page);
 		if (!put_page_testzero(page))
 			continue;
 
-- 
2.20.1