From: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Subject: dax: fix deadlock with DAX 4k holes
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.
Fixes: ac401cc78242 ("dax: New fault locking")
Link: http://lkml.kernel.org/r/1483479365-13607-1-git-send-email-ross.zwisler@xxxxxxxxxxxxxxx
Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Reported-by: Xiong Zhou <xzhou@xxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [4.7+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff -puN mm/filemap.c~dax-fix-deadlock-with-dax-4k-holes mm/filemap.c
--- a/mm/filemap.c~dax-fix-deadlock-with-dax-4k-holes
+++ a/mm/filemap.c
@@ -138,7 +138,7 @@ static int page_cache_tree_insert(struct
dax_radix_locked_entry(0, RADIX_DAX_EMPTY));
/* Wakeup waiters for exceptional entry lock */
dax_wake_mapping_entry_waiter(mapping, page->index, p,
- false);
+ true);
}
}
__radix_tree_replace(&mapping->page_tree, node, slot, page,
_
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