[PATCH] page_alloc: Fix freeing non-compound pages

[Matthew Wilcox (Oracle)]

Here is a very rare race which leaks memory:

Page P0 is allocated to the page cache.
Page P1 is free.

Thread A		Thread B		Thread C
find_get_entry():
xas_load() returns P0
						Removes P0 from page cache
						Frees P0
						P0 merged with its buddy P1
			alloc_pages(GFP_KERNEL, 1) returns P0
			P0 has refcount 1
page_cache_get_speculative(P0)
P0 has refcount 2
			__free_pages(P0)
			P0 has refcount 1
put_page(P0)
P1 is not freed

Fix this by freeing all the pages in __free_pages() that won't be freed
by the call to put_page().  It's usually not a good idea to split a page,
but this is a very unlikely scenario.

Fixes: e286781d5f2e ("mm: speculative page references")
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/page_alloc.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fab5e97dc9ca..5db74797db39 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4943,10 +4943,19 @@ static inline void free_the_page(struct page *page, unsigned int order)
 		__free_pages_ok(page, order);
 }
 
+/*
+ * If we free a non-compound allocation, another thread may have a
+ * speculative reference to the first page.  It has no way of knowing
+ * about the rest of the allocation, so we have to free all but the
+ * first page here.
+ */
 void __free_pages(struct page *page, unsigned int order)
 {
 	if (put_page_testzero(page))
 		free_the_page(page, order);
+	else
+		while (order-- > 0)
+			free_the_page(page + (1 << order), order);
 }
 EXPORT_SYMBOL(__free_pages);
 
-- 
2.28.0