fs/dax: Fix pmd vs pte conflict detection

Users reported a v5.3 performance regression and inability to establish
huge page mappings. A revised version of the ndctl "dax.sh" huge page
unit test identifies commit 23c84eb78375 "dax: Fix missed wakeup with
PMD faults" as the source.

Update get_unlocked_entry() to 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@intel.com>
Reported-by: Doug Nelson <doug.nelson@intel.com>
Cc: <stable@vger.kernel.org>
Fixes: 23c84eb78375 ("dax: Fix missed wakeup with PMD faults")
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Link: https://lore.kernel.org/r/157167532455.3945484.11971474077040503994.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --git a/fs/dax.c b/fs/dax.c
index 6bf81f931de39e48bc2983a1c901b1a6643443f8..2cc43cd914eb8332e3c1ccbdcfafa2e6ce64baee 100644 (file)
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -220,10 +220,11 @@ 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)
                        return XA_RETRY_ENTRY;
-               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);