filesystem-dax: Introduce dax_lock_mapping_entry()

In preparation for implementing support for memory poison (media error)
handling via dax mappings, implement a lock_page() equivalent. Poison
error handling requires rmap and needs guarantees that the page->mapping
association is maintained / valid (inode not freed) for the duration of
the lookup.

In the device-dax case it is sufficient to simply hold a dev_pagemap
reference. In the filesystem-dax case we need to use the entry lock.

Export the entry lock via dax_lock_mapping_entry() that uses
rcu_read_lock() to protect against the inode being freed, and
revalidates the page->mapping association under xa_lock().

Cc: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

diff --git a/fs/dax.c b/fs/dax.c
index 4de11ed..57ec272 100644 (file)
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -226,8 +226,8 @@ static inline void *unlock_slot(struct address_space *mapping, void **slot)
  *
  * Must be called with the i_pages lock held.
  */
-static void *get_unlocked_mapping_entry(struct address_space *mapping,
-                                       pgoff_t index, void ***slotp)
+static void *__get_unlocked_mapping_entry(struct address_space *mapping,
+               pgoff_t index, void ***slotp, bool (*wait_fn)(void))
 {
        void *entry, **slot;
        struct wait_exceptional_entry_queue ewait;
@@ -237,6 +237,8 @@ static void *get_unlocked_mapping_entry(struct address_space *mapping,
        ewait.wait.func = wake_exceptional_entry_func;
 
        for (;;) {
+               bool revalidate;
+
                entry = __radix_tree_lookup(&mapping->i_pages, index, NULL,
                                          &slot);
                if (!entry ||
@@ -251,14 +253,31 @@ static void *get_unlocked_mapping_entry(struct address_space *mapping,
                prepare_to_wait_exclusive(wq, &ewait.wait,
                                          TASK_UNINTERRUPTIBLE);
                xa_unlock_irq(&mapping->i_pages);
-               schedule();
+               revalidate = wait_fn();
                finish_wait(wq, &ewait.wait);
                xa_lock_irq(&mapping->i_pages);
+               if (revalidate)
+                       return ERR_PTR(-EAGAIN);
        }
 }
 
-static void dax_unlock_mapping_entry(struct address_space *mapping,
-                                    pgoff_t index)
+static bool entry_wait(void)
+{
+       schedule();
+       /*
+        * Never return an ERR_PTR() from
+        * __get_unlocked_mapping_entry(), just keep looping.
+        */
+       return false;
+}
+
+static void *get_unlocked_mapping_entry(struct address_space *mapping,
+               pgoff_t index, void ***slotp)
+{
+       return __get_unlocked_mapping_entry(mapping, index, slotp, entry_wait);
+}
+
+static void unlock_mapping_entry(struct address_space *mapping, pgoff_t index)
 {
        void *entry, **slot;
 
@@ -277,7 +296,7 @@ static void dax_unlock_mapping_entry(struct address_space *mapping,
 static void put_locked_mapping_entry(struct address_space *mapping,
                pgoff_t index)
 {
-       dax_unlock_mapping_entry(mapping, index);
+       unlock_mapping_entry(mapping, index);
 }
 
 /*
@@ -374,6 +393,84 @@ static struct page *dax_busy_page(void *entry)
        return NULL;
 }
 
+static bool entry_wait_revalidate(void)
+{
+       rcu_read_unlock();
+       schedule();
+       rcu_read_lock();
+
+       /*
+        * Tell __get_unlocked_mapping_entry() to take a break, we need
+        * to revalidate page->mapping after dropping locks
+        */
+       return true;
+}
+
+bool dax_lock_mapping_entry(struct page *page)
+{
+       pgoff_t index;
+       struct inode *inode;
+       bool did_lock = false;
+       void *entry = NULL, **slot;
+       struct address_space *mapping;
+
+       rcu_read_lock();
+       for (;;) {
+               mapping = READ_ONCE(page->mapping);
+
+               if (!dax_mapping(mapping))
+                       break;
+
+               /*
+                * In the device-dax case there's no need to lock, a
+                * struct dev_pagemap pin is sufficient to keep the
+                * inode alive, and we assume we have dev_pagemap pin
+                * otherwise we would not have a valid pfn_to_page()
+                * translation.
+                */
+               inode = mapping->host;
+               if (S_ISCHR(inode->i_mode)) {
+                       did_lock = true;
+                       break;
+               }
+
+               xa_lock_irq(&mapping->i_pages);
+               if (mapping != page->mapping) {
+                       xa_unlock_irq(&mapping->i_pages);
+                       continue;
+               }
+               index = page->index;
+
+               entry = __get_unlocked_mapping_entry(mapping, index, &slot,
+                               entry_wait_revalidate);
+               if (!entry) {
+                       xa_unlock_irq(&mapping->i_pages);
+                       break;
+               } else if (IS_ERR(entry)) {
+                       WARN_ON_ONCE(PTR_ERR(entry) != -EAGAIN);
+                       continue;
+               }
+               lock_slot(mapping, slot);
+               did_lock = true;
+               xa_unlock_irq(&mapping->i_pages);
+               break;
+       }
+       rcu_read_unlock();
+
+       return did_lock;
+}
+
+void dax_unlock_mapping_entry(struct page *page)
+{
+       struct address_space *mapping = page->mapping;
+       struct inode *inode = mapping->host;
+
+       if (S_ISCHR(inode->i_mode))
+               return;
+
+       unlock_mapping_entry(mapping, page->index);
+}
+
 /*
  * Find radix tree entry at given index. If it points to an exceptional entry,
  * return it with the radix tree entry locked. If the radix tree doesn't

diff --git a/include/linux/dax.h b/include/linux/dax.h
index deb0f66..450b28d 100644 (file)
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -88,6 +88,8 @@ int dax_writeback_mapping_range(struct address_space *mapping,
                struct block_device *bdev, struct writeback_control *wbc);
 
 struct page *dax_layout_busy_page(struct address_space *mapping);
+bool dax_lock_mapping_entry(struct page *page);
+void dax_unlock_mapping_entry(struct page *page);
 #else
 static inline bool bdev_dax_supported(struct block_device *bdev,
                int blocksize)
@@ -119,6 +121,17 @@ static inline int dax_writeback_mapping_range(struct address_space *mapping,
 {
        return -EOPNOTSUPP;
 }
+
+static inline bool dax_lock_mapping_entry(struct page *page)
+{
+       if (IS_DAX(page->mapping->host))
+               return true;
+       return false;
+}
+
+static inline void dax_unlock_mapping_entry(struct page *page)
+{
+}
 #endif
 
 int dax_read_lock(void);