Patch "dax: fix deadlock with DAX 4k holes" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

dax: fix deadlock with DAX 4k holes

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
dax-fix-deadlock-with-dax-4k-holes.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ross Zwisler <[email protected]>
Date: Tue, 10 Jan 2017 16:57:15 -0800
Subject: dax: fix deadlock with DAX 4k holes

From: Ross Zwisler <[email protected]>

commit 965d004af54088d138f806d04d803fb60d441986 upstream.

Currently in DAX if we have three read faults on the same hole address we can end up with the following:

Thread 0 Thread 1 Thread 2
-------- -------- --------
dax_iomap_fault
grab_mapping_entry
lock_slot
<locks empty DAX entry>

dax_iomap_fault
grab_mapping_entry
get_unlocked_mapping_entry
<sleeps on empty DAX entry>

dax_iomap_fault
grab_mapping_entry
get_unlocked_mapping_entry
<sleeps on empty DAX entry>
dax_load_hole
find_or_create_page
...
page_cache_tree_insert
dax_wake_mapping_entry_waiter
<wakes one sleeper>
__radix_tree_replace
<swaps empty DAX entry with 4k zero page>

<wakes>
get_page
lock_page
...
put_locked_mapping_entry
unlock_page
put_page

<sleeps forever on the DAX
wait queue>

The crux of the problem is that once we insert a 4k zero page, all locking from then on is done in terms of that 4k zero page and any additional threads sleeping on the empty DAX entry will never be woken.

Fix this by waking all sleepers when we replace the DAX radix tree entry with a 4k zero page. This will allow all sleeping threads to successfully transition from locking based on the DAX empty entry to locking on the 4k zero page.

With the test case reported by Xiong this happens very regularly in my test setup, with some runs resulting in 9+ threads in this deadlocked state. With this fix I've been able to run that same test dozens of times in a loop without issue.

---
 mm/filemap.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -144,7 +144,7 @@ static int page_cache_tree_insert(struct
 				workingset_node_pages_dec(node);
 			/* Wakeup waiters for exceptional entry lock */
 			dax_wake_mapping_entry_waiter(mapping, page->index,
-						      false);
+						      true);
 		}
 	}
 	radix_tree_replace_slot(slot, page);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/dax-fix-deadlock-with-dax-4k-holes.patch