[PATCH] fs/dax: Fix pmd vs pte conflict detection

From: Dan Williams
Date: Sat Oct 19 2019 - 12:40:45 EST

Check for NULL entries before checking the entry order, otherwise NULL
is misinterpreted as a present pte conflict. The 'order' check needs to
happen before the locked check as an unlocked entry at the wrong order
must fallback to lookup the correct order.

Reported-by: Jeff Smits <jeff.smits@xxxxxxxxx>
Reported-by: Doug Nelson <doug.nelson@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: 23c84eb78375 ("dax: Fix missed wakeup with PMD faults")
Cc: Jan Kara <jack@xxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
fs/dax.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index a71881e77204..08160011d94c 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -221,10 +221,11 @@ static void *get_unlocked_entry(struct xa_state *xas, unsigned int order)

for (;;) {
entry = xas_find_conflict(xas);
+ if (!entry || WARN_ON_ONCE(!xa_is_value(entry)))
+ return entry;
if (dax_entry_order(entry) < order)
- if (!entry || WARN_ON_ONCE(!xa_is_value(entry)) ||
- !dax_is_locked(entry))
+ if (!dax_is_locked(entry))
return entry;

wq = dax_entry_waitqueue(xas, entry, &ewait.key);