[PATCH] mm: page_alloc: fix memcg accounting leak in speculative cache lookup

[Johannes Weiner]

When the freeing of a higher-order page block (non-compound) races
with a speculative page cache lookup, __free_pages() needs to leave
the first order-0 page in the chunk to the lookup but free the buddy
pages that the lookup doesn't know about separately.

However, if such a higher-order page is charged to a memcg (e.g. !vmap
kernel stack)), only the first page of the block has page->memcg
set. That means we'll uncharge only one order-0 page from the entire
block, and leak the remainder.

Add a split_page_memcg() to __free_pages() right before it starts
taking the higher-order page apart and freeing its individual
constituent pages. This ensures all of them will have the memcg
linkage set up for correct uncharging. Also update the comments a bit
to clarify what exactly is happening to the page during that race.

This bug is old and has its roots in the speculative page cache patch
and adding cgroup accounting of kernel pages. There are no known user
reports. A backport to stable is therefor not warranted.

Reported-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/page_alloc.c | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c53fe4fa10bf..f4bd56656402 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5112,10 +5112,9 @@ static inline void free_the_page(struct page *page, unsigned int order)
  * the allocation, so it is easy to leak memory.  Freeing more memory
  * than was allocated will probably emit a warning.
  *
- * If the last reference to this page is speculative, it will be released
- * by put_page() which only frees the first page of a non-compound
- * allocation.  To prevent the remaining pages from being leaked, we free
- * the subsequent pages here.  If you want to use the page's reference
+ * This function isn't a put_page(). Don't let the put_page_testzero()
+ * fool you, it's only to deal with speculative cache references. It
+ * WILL free pages directly. If you want to use the page's reference
  * count to decide when to free the allocation, you should allocate a
  * compound page, and use put_page() instead of __free_pages().
  *
@@ -5124,11 +5123,33 @@ static inline void free_the_page(struct page *page, unsigned int order)
  */
 void __free_pages(struct page *page, unsigned int order)
 {
-	if (put_page_testzero(page))
+	/*
+	 * Drop the base reference from __alloc_pages and free. In
+	 * case there is an outstanding speculative reference, from
+	 * e.g. the page cache, it will put and free the page later.
+	 */
+	if (likely(put_page_testzero(page))) {
 		free_the_page(page, order);
-	else if (!PageHead(page))
+		return;
+	}
+
+	/*
+	 * The speculative reference will put and free the page.
+	 *
+	 * However, if the speculation was into a higher-order page
+	 * chunk that isn't marked compound, the other side will know
+	 * nothing about our buddy pages and only free the order-0
+	 * page at the start of our chunk! We must split off and free
+	 * the buddy pages here.
+	 *
+	 * The buddy pages aren't individually refcounted, so they
+	 * can't have any pending speculative references themselves.
+	 */
+	if (!PageHead(page) && order > 0) {
+		split_page_memcg(page, 1 << order);
 		while (order-- > 0)
 			free_the_page(page + (1 << order), order);
+	}
 }
 EXPORT_SYMBOL(__free_pages);
 
-- 
2.30.1