dax: Fix missed wakeup with PMD faults

RocksDB can hang indefinitely when using a DAX file.  This is due to
a bug in the XArray conversion when handling a PMD fault and finding a
PTE entry.  We use the wrong index in the hash and end up waiting on
the wrong waitqueue.

There's actually no need to wait; if we find a PTE entry while looking
for a PMD entry, we can return immediately as we know we should fall
back to a PTE fault (which may not conflict with the lock held).

We reuse the XA_RETRY_ENTRY to signal a conflicting entry was found.
This value can never be found in an XArray while holding its lock, so
it does not create an ambiguity.

Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/CAPcyv4hwHpX-MkUEqxwdTj7wCCZCN4RV-L4jsnuwLGyL_UEG4A@mail.gmail.com
Fixes: b15cd800682f ("dax: Convert page fault handlers to XArray")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Dan Williams <dan.j.williams@intel.com>
Reported-by: Robert Barror <robert.barror@intel.com>
Reported-by: Seema Pandit <seema.pandit@intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --git a/fs/dax.c b/fs/dax.c
index 2e48c7e..7a75031 100644 (file)
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -124,6 +124,15 @@ static int dax_is_empty_entry(void *entry)
 }
 
 /*
+ * true if the entry that was found is of a smaller order than the entry
+ * we were looking for
+ */
+static bool dax_is_conflict(void *entry)
+{
+       return entry == XA_RETRY_ENTRY;
+}
+
+/*
  * DAX page cache entry locking
  */
 struct exceptional_entry_key {
@@ -195,11 +204,13 @@ static void dax_wake_entry(struct xa_state *xas, void *entry, bool wake_all)
  * Look up entry in page cache, wait for it to become unlocked if it
  * is a DAX entry and return it.  The caller must subsequently call
  * put_unlocked_entry() if it did not lock the entry or dax_unlock_entry()
- * if it did.
+ * if it did.  The entry returned may have a larger order than @order.
+ * If @order is larger than the order of the entry found in i_pages, this
+ * function returns a dax_is_conflict entry.
  *
  * Must be called with the i_pages lock held.
  */
-static void *get_unlocked_entry(struct xa_state *xas)
+static void *get_unlocked_entry(struct xa_state *xas, unsigned int order)
 {
        void *entry;
        struct wait_exceptional_entry_queue ewait;
@@ -210,6 +221,8 @@ static void *get_unlocked_entry(struct xa_state *xas)
 
        for (;;) {
                entry = xas_find_conflict(xas);
+               if (dax_entry_order(entry) < order)
+                       return XA_RETRY_ENTRY;
                if (!entry || WARN_ON_ONCE(!xa_is_value(entry)) ||
                                !dax_is_locked(entry))
                        return entry;
@@ -254,7 +267,7 @@ static void wait_entry_unlocked(struct xa_state *xas, void *entry)
 static void put_unlocked_entry(struct xa_state *xas, void *entry)
 {
        /* If we were the only waiter woken, wake the next one */
-       if (entry)
+       if (entry && dax_is_conflict(entry))
                dax_wake_entry(xas, entry, false);
 }
 
@@ -461,7 +474,7 @@ void dax_unlock_page(struct page *page, dax_entry_t cookie)
  * overlap with xarray value entries.
  */
 static void *grab_mapping_entry(struct xa_state *xas,
-               struct address_space *mapping, unsigned long size_flag)
+               struct address_space *mapping, unsigned int order)
 {
        unsigned long index = xas->xa_index;
        bool pmd_downgrade = false; /* splitting PMD entry into PTE entries? */
@@ -469,20 +482,17 @@ static void *grab_mapping_entry(struct xa_state *xas,
 
 retry:
        xas_lock_irq(xas);
-       entry = get_unlocked_entry(xas);
+       entry = get_unlocked_entry(xas, order);
 
        if (entry) {
+               if (dax_is_conflict(entry))
+                       goto fallback;
                if (!xa_is_value(entry)) {
                        xas_set_err(xas, EIO);
                        goto out_unlock;
                }
 
-               if (size_flag & DAX_PMD) {
-                       if (dax_is_pte_entry(entry)) {
-                               put_unlocked_entry(xas, entry);
-                               goto fallback;
-                       }
-               } else { /* trying to grab a PTE entry */
+               if (order == 0) {
                        if (dax_is_pmd_entry(entry) &&
                            (dax_is_zero_entry(entry) ||
                             dax_is_empty_entry(entry))) {
@@ -523,7 +533,11 @@ retry:
        if (entry) {
                dax_lock_entry(xas, entry);
        } else {
-               entry = dax_make_entry(pfn_to_pfn_t(0), size_flag | DAX_EMPTY);
+               unsigned long flags = DAX_EMPTY;
+
+               if (order > 0)
+                       flags |= DAX_PMD;
+               entry = dax_make_entry(pfn_to_pfn_t(0), flags);
                dax_lock_entry(xas, entry);
                if (xas_error(xas))
                        goto out_unlock;
@@ -594,7 +608,7 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
                if (WARN_ON_ONCE(!xa_is_value(entry)))
                        continue;
                if (unlikely(dax_is_locked(entry)))
-                       entry = get_unlocked_entry(&xas);
+                       entry = get_unlocked_entry(&xas, 0);
                if (entry)
                        page = dax_busy_page(entry);
                put_unlocked_entry(&xas, entry);
@@ -621,7 +635,7 @@ static int __dax_invalidate_entry(struct address_space *mapping,
        void *entry;
 
        xas_lock_irq(&xas);
-       entry = get_unlocked_entry(&xas);
+       entry = get_unlocked_entry(&xas, 0);
        if (!entry || WARN_ON_ONCE(!xa_is_value(entry)))
                goto out;
        if (!trunc &&
@@ -849,7 +863,7 @@ static int dax_writeback_one(struct xa_state *xas, struct dax_device *dax_dev,
        if (unlikely(dax_is_locked(entry))) {
                void *old_entry = entry;
 
-               entry = get_unlocked_entry(xas);
+               entry = get_unlocked_entry(xas, 0);
 
                /* Entry got punched out / reallocated? */
                if (!entry || WARN_ON_ONCE(!xa_is_value(entry)))
@@ -1510,7 +1524,7 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
         * entry is already in the array, for instance), it will return
         * VM_FAULT_FALLBACK.
         */
-       entry = grab_mapping_entry(&xas, mapping, DAX_PMD);
+       entry = grab_mapping_entry(&xas, mapping, PMD_ORDER);
        if (xa_is_internal(entry)) {
                result = xa_to_internal(entry);
                goto fallback;
@@ -1659,11 +1673,10 @@ dax_insert_pfn_mkwrite(struct vm_fault *vmf, pfn_t pfn, unsigned int order)
        vm_fault_t ret;
 
        xas_lock_irq(&xas);
-       entry = get_unlocked_entry(&xas);
+       entry = get_unlocked_entry(&xas, order);
        /* Did we race with someone splitting entry or so? */
-       if (!entry ||
-           (order == 0 && !dax_is_pte_entry(entry)) ||
-           (order == PMD_ORDER && !dax_is_pmd_entry(entry))) {
+       if (!entry || dax_is_conflict(entry) ||
+           (order == 0 && !dax_is_pte_entry(entry))) {
                put_unlocked_entry(&xas, entry);
                xas_unlock_irq(&xas);
                trace_dax_insert_pfn_mkwrite_no_entry(mapping->host, vmf,