dax: Reinstate RCU protection of inode

For the device-dax case, it is possible that the inode can go away
underneath us.  The rcu_read_lock() was there to prevent it from
being freed, and not (as I thought) to protect the tree.  Bring back
the rcu_read_lock() protection.  Also add a little kernel-doc; while
this function is not exported to modules, it is used from outside dax.c

Reported-by: Dan Williams <dan.j.williams@intel.com>
Fixes: 9f32d221301c ("dax: Convert dax_lock_mapping_entry to XArray")
Signed-off-by: Matthew Wilcox <willy@infradead.org>

diff --git a/fs/dax.c b/fs/dax.c
index 7944417..ce87d21 100644 (file)
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -353,16 +353,27 @@ static struct page *dax_busy_page(void *entry)
        return NULL;
 }
 
+/*
+ * dax_lock_mapping_entry - Lock the DAX entry corresponding to a page
+ * @page: The page whose entry we want to lock
+ *
+ * Context: Process context.
+ * Return: %true if the entry was locked or does not need to be locked.
+ */
 bool dax_lock_mapping_entry(struct page *page)
 {
        XA_STATE(xas, NULL, 0);
        void *entry;
+       bool locked;
 
+       /* Ensure page->mapping isn't freed while we look at it */
+       rcu_read_lock();
        for (;;) {
                struct address_space *mapping = READ_ONCE(page->mapping);
 
+               locked = false;
                if (!dax_mapping(mapping))
-                       return false;
+                       break;
 
                /*
                 * In the device-dax case there's no need to lock, a
@@ -371,8 +382,9 @@ bool dax_lock_mapping_entry(struct page *page)
                 * otherwise we would not have a valid pfn_to_page()
                 * translation.
                 */
+               locked = true;
                if (S_ISCHR(mapping->host->i_mode))
-                       return true;
+                       break;
 
                xas.xa = &mapping->i_pages;
                xas_lock_irq(&xas);
@@ -383,14 +395,18 @@ bool dax_lock_mapping_entry(struct page *page)
                xas_set(&xas, page->index);
                entry = xas_load(&xas);
                if (dax_is_locked(entry)) {
+                       rcu_read_unlock();
                        entry = get_unlocked_entry(&xas);
                        xas_unlock_irq(&xas);
+                       rcu_read_lock();
                        continue;
                }
                dax_lock_entry(&xas, entry);
                xas_unlock_irq(&xas);
-               return true;
+               break;
        }
+       rcu_read_unlock();
+       return locked;
 }
 
 void dax_unlock_mapping_entry(struct page *page)