linux-kernel - [PATCH 1/2] xen/grant-table: Use put_page instead of free

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Date:   Thu, 11 Jan 2018 09:36:37 +0000
From:   Ross Lagerwall <ross.lagerwall@...rix.com>
To:     <xen-devel@...ts.xenproject.org>
CC:     Ross Lagerwall <ross.lagerwall@...rix.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>, <linux-kernel@...r.kernel.org>
Subject: [PATCH 1/2] 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@...rix.com>
---
 drivers/xen/grant-table.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index f45114f..27be107 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -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);
-- 
2.9.5