On Fri, Mar 19, 2021 at 06:52:58PM -0700, Hugh Dickins wrote:

quoted

+	/*
+	 * 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) {

The put_page_testzero() has released our reference to the first
subpage of page: it's now under the control of the racing speculative
lookup.  So it seems to me unsafe to be checking PageHead(page) here:
if it was actually a compound page, PageHead might already be cleared
by now, and we doubly free its tail pages below?  I think we need to
use a "bool compound = PageHead(page)" on entry to __free_pages().

Or alternatively, it's wrong to call __free_pages() on a compound
page anyway, so we should not check PageHead at all, except in a
WARN_ON_ONCE(PageCompound(page)) at the start?

Alas ...

$ git grep '__free_pages\>.*compound'
drivers/dma-buf/heaps/system_heap.c:            __free_pages(page, compound_order(page));
drivers/dma-buf/heaps/system_heap.c:            __free_pages(p, compound_order(p));
drivers/dma-buf/heaps/system_heap.c:            __free_pages(page, compound_order(page));
mm/huge_memory.c:               __free_pages(zero_page, compound_order(zero_page));
mm/huge_memory.c:               __free_pages(zero_page, compound_order(zero_page));
mm/slub.c:                      __free_pages(page, compound_order(page));

Maybe we should disallow it!

There are a few other places to check:

$ grep -l __GFP_COMP $(git grep -lw __free_pages) | wc -l
24

(assuming the pages are allocated and freed in the same file, which is a
reasonable approximation, but not guaranteed to catch everything.  Many
of these 24 will be false positives, of course.)