Commit 3ac7292a authored Jan 11, 2018 by Ross Lagerwall Committed by Juergen Gross Feb 06, 2018

xen/grant-table: Use put_page instead of free_page

The page given to gnttab_end_foreign_access() to free could be a
compound page so use put_page() instead of free_page() since it can
handle both compound and single pages correctly.

This bug was discovered when migrating a Xen VM with several VIFs and
CONFIG_DEBUG_VM enabled. It hits a BUG usually after fewer than 10
iterations. All netfront devices disconnect from the backend during a
suspend/resume and this will call gnttab_end_foreign_access() if a
netfront queue has an outstanding skb. The mismatch between calling
get_page() and free_page() on a compound page causes a reference
counting error which is detected when DEBUG_VM is enabled.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

parent 4f277295

drivers/xen/grant-table.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -382,7 +382,7 @@ static void gnttab_handle_deferred(struct timer_list *unused)
		if (entry->page) {
		pr_debug("freeing g.e. %#x (pfn %#lx)\n",
		entry->ref, page_to_pfn(entry->page));
		__free_page(entry->page);
		put_page(entry->page);
		} else
		pr_info("freeing g.e. %#x\n", entry->ref);
		kfree(entry);
		@@ -438,7 +438,7 @@ void gnttab_end_foreign_access(grant_ref_t ref, int readonly,
		if (gnttab_end_foreign_access_ref(ref, readonly)) {
		put_free_entry(ref);
		if (page != 0)
		free_page(page);
		put_page(virt_to_page(page));
		} else
		gnttab_add_deferred(ref, readonly,
		page ? virt_to_page(page) : NULL);