libnvdimm fixes for 5.9-rc6

- Fix an original bug in device-mapper table reference counting when
   interrogating dax capability in the component device. This bug was
   hidden by the following bug.
 
 - Fix device-mapper to use the proper helper (dax_supported() instead of
   the leaf helper generic_fsdax_supported()) to determine dax operation
   of a stacked block device configuration. The original implementation
   is only valid for one level of dax-capable block device stacking. This
   bug was discovered while fixing the below regression.
 
 - Fix an infinite recursion regression introduced by broken attempts to
   quiet the generic_fsdax_supported() path and make it bail out before
   logging "dax capability not found" errors.
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Merge tag 'libnvdimm-fixes-5.9-rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/nvdimm/nvdimm

Pull libnvdimm fixes from Dan Williams:
 "A handful of fixes to address a string of mistakes in the mechanism
  for device-mapper to determine if its component devices are dax
  capable.

   - Fix an original bug in device-mapper table reference counting when
     interrogating dax capability in the component device. This bug was
     hidden by the following bug.

   - Fix device-mapper to use the proper helper (dax_supported() instead
     of the leaf helper generic_fsdax_supported()) to determine dax
     operation of a stacked block device configuration. The original
     implementation is only valid for one level of dax-capable block
     device stacking. This bug was discovered while fixing the below
     regression.

   - Fix an infinite recursion regression introduced by broken attempts
     to quiet the generic_fsdax_supported() path and make it bail out
     before logging "dax capability not found" errors"

* tag 'libnvdimm-fixes-5.9-rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/nvdimm/nvdimm:
  dax: Fix stack overflow when mounting fsdax pmem device
  dm: Call proper helper to determine dax support
  dm/dax: Fix table reference counts

drivers/dax/super.c

@@ -85,6 +85,12 @@ bool __generic_fsdax_supported(struct dax_device *dax_dev,
 		return false;
 	}
 
+	if (!dax_dev) {
+		pr_debug("%s: error: dax unsupported by block device\n",
+				bdevname(bdev, buf));
+		return false;
+	}
+
 	err = bdev_dax_pgoff(bdev, start, PAGE_SIZE, &pgoff);
 	if (err) {
 		pr_info("%s: error: unaligned partition for dax\n",
@@ -100,12 +106,6 @@ bool __generic_fsdax_supported(struct dax_device *dax_dev,
 		return false;
 	}
 
-	if (!dax_dev || !bdev_dax_supported(bdev, blocksize)) {
-		pr_debug("%s: error: dax unsupported by block device\n",
-				bdevname(bdev, buf));
-		return false;
-	}
-
 	id = dax_read_lock();
 	len = dax_direct_access(dax_dev, pgoff, 1, &kaddr, &pfn);
 	len2 = dax_direct_access(dax_dev, pgoff_end, 1, &end_kaddr, &end_pfn);
@@ -325,11 +325,15 @@ EXPORT_SYMBOL_GPL(dax_direct_access);
 bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
 		int blocksize, sector_t start, sector_t len)
 {
+	if (!dax_dev)
+		return false;
+
 	if (!dax_alive(dax_dev))
 		return false;
 
 	return dax_dev->ops->dax_supported(dax_dev, bdev, blocksize, start, len);
 }
+EXPORT_SYMBOL_GPL(dax_supported);
 
 size_t dax_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
 		size_t bytes, struct iov_iter *i)

drivers/md/dm-table.c

@@ -860,10 +860,14 @@ EXPORT_SYMBOL_GPL(dm_table_set_type);
 int device_supports_dax(struct dm_target *ti, struct dm_dev *dev,
 			sector_t start, sector_t len, void *data)
 {
-	int blocksize = *(int *) data;
+	int blocksize = *(int *) data, id;
+	bool rc;
 
-	return generic_fsdax_supported(dev->dax_dev, dev->bdev, blocksize,
+	id = dax_read_lock();
-				       start, len);
+	rc = dax_supported(dev->dax_dev, dev->bdev, blocksize, start, len);
+	dax_read_unlock(id);
+
+	return rc;
 }
 
 /* Check devices support synchronous DAX */

drivers/md/dm.c

@@ -1136,15 +1136,16 @@ static bool dm_dax_supported(struct dax_device *dax_dev, struct block_device *bd
 {
 	struct mapped_device *md = dax_get_private(dax_dev);
 	struct dm_table *map;
+	bool ret = false;
 	int srcu_idx;
-	bool ret;
 
 	map = dm_get_live_table(md, &srcu_idx);
 	if (!map)
-		return false;
+		goto out;
 
 	ret = dm_table_supports_dax(map, device_supports_dax, &blocksize);
 
+out:
 	dm_put_live_table(md, srcu_idx);
 
 	return ret;

include/linux/dax.h

@@ -130,6 +130,8 @@ static inline bool generic_fsdax_supported(struct dax_device *dax_dev,
 	return __generic_fsdax_supported(dax_dev, bdev, blocksize, start,
 			sectors);
 }
+bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
+		int blocksize, sector_t start, sector_t len);
 
 static inline void fs_put_dax(struct dax_device *dax_dev)
 {
@@ -157,6 +159,13 @@ static inline bool generic_fsdax_supported(struct dax_device *dax_dev,
 	return false;
 }
 
+static inline bool dax_supported(struct dax_device *dax_dev,
+		struct block_device *bdev, int blocksize, sector_t start,
+		sector_t len)
+{
+	return false;
+}
+
 static inline void fs_put_dax(struct dax_device *dax_dev)
 {
 }
@@ -189,14 +198,23 @@ static inline void dax_unlock_page(struct page *page, dax_entry_t cookie)
 }
 #endif
 
+#if IS_ENABLED(CONFIG_DAX)
 int dax_read_lock(void);
 void dax_read_unlock(int id);
+#else
+static inline int dax_read_lock(void)
+{
+	return 0;
+}
+
+static inline void dax_read_unlock(int id)
+{
+}
+#endif /* CONFIG_DAX */
 bool dax_alive(struct dax_device *dax_dev);
 void *dax_get_private(struct dax_device *dax_dev);
 long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
 		void **kaddr, pfn_t *pfn);
-bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
-		int blocksize, sector_t start, sector_t len);
 size_t dax_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
 		size_t bytes, struct iov_iter *i);
 size_t dax_copy_to_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,