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Subject[PATCH] page_alloc: Fix freeing non-compound pages
Date
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@infradead.org>
---
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
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