@@ -5072,10 +5072,9 @@ static inline void free_the_page(struct
* 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().
*
@@ -5084,11 +5083,33 @@ static inline void free_the_page(struct
*/
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);
The patch titled Subject: mm: page_alloc: fix allocation imbalances from speculative cache lookup has been removed from the -mm tree. Its filename was mm-page_alloc-fix-memcg-accounting-leak-in-speculative-cache-lookup.patch This patch was dropped because an updated version will be merged ------------------------------------------------------ From: Johannes Weiner <hannes@cmpxchg.org> Subject: mm: page_alloc: fix allocation imbalances from speculative cache lookup 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. There are currently two problems with it: 1. It checks PageHead() to see whether we're dealing with a compound page after put_page_testzero(). But the speculative lookup could have freed the page after our put and cleared PageHead, in which case we would double free the tail pages. To fix this, test PageHead before the put and cache the result for afterwards. 2. 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. To fix this, add a split_page_memcg() before it starts freeing tail pages, to ensure they all have page->memcg set up. While at it, also update the comments a bit to clarify what exactly is happening to the page during that race. Link: https://lkml.kernel.org/r/20210319071547.60973-1-hannes@cmpxchg.org Fixes: e320d3012d25 ("mm/page_alloc.c: fix freeing non-compound pages") Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Reported-by: Hugh Dickins <hughd@google.com> Reported-by: Matthew Wilcox <willy@infradead.org> Acked-by: Hugh Dickins <hughd@google.com> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org> Acked-by: Hugh Dickins <hughd@google.com> Acked-by: Michal Hocko <mhocko@suse.com> Cc: Zhou Guanghui <zhouguanghui1@huawei.com> Cc: Zi Yan <ziy@nvidia.com> Cc: Shakeel Butt <shakeelb@google.com> Cc: Roman Gushchin <guro@fb.com> Cc: <stable@vger.kernel.org> # 5.10+ Signed-off-by: Andrew Morton <akpm@linux-foundation.org> --- mm/page_alloc.c | 33 +++++++++++++++++++++++++++------ 1 file changed, 27 insertions(+), 6 deletions(-)