block: enable dax for raw block devices

If an application wants exclusive access to all of the persistent memory
provided by an NVDIMM namespace it can use this raw-block-dax facility
to forgo establishing a filesystem.  This capability is targeted
primarily to hypervisors wanting to provision persistent memory for
guests.  It can be disabled / enabled dynamically via the new BLKDAXSET
ioctl.

Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reviewed-by: Jan Kara <jack@suse.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

block/ioctl.c

@@ -406,6 +406,62 @@ static inline int is_unrecognized_ioctl(int ret)
 		ret == -ENOIOCTLCMD;
 }
 
+#ifdef CONFIG_FS_DAX
+bool blkdev_dax_capable(struct block_device *bdev)
+{
+	struct gendisk *disk = bdev->bd_disk;
+
+	if (!disk->fops->direct_access)
+		return false;
+
+	/*
+	 * If the partition is not aligned on a page boundary, we can't
+	 * do dax I/O to it.
+	 */
+	if ((bdev->bd_part->start_sect % (PAGE_SIZE / 512))
+			|| (bdev->bd_part->nr_sects % (PAGE_SIZE / 512)))
+		return false;
+
+	return true;
+}
+
+static int blkdev_daxset(struct block_device *bdev, unsigned long argp)
+{
+	unsigned long arg;
+	int rc = 0;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EACCES;
+
+	if (get_user(arg, (int __user *)(argp)))
+		return -EFAULT;
+	arg = !!arg;
+	if (arg == !!(bdev->bd_inode->i_flags & S_DAX))
+		return 0;
+
+	if (arg)
+		arg = S_DAX;
+
+	if (arg && !blkdev_dax_capable(bdev))
+		return -ENOTTY;
+
+	mutex_lock(&bdev->bd_inode->i_mutex);
+	if (bdev->bd_map_count == 0)
+		inode_set_flags(bdev->bd_inode, arg, S_DAX);
+	else
+		rc = -EBUSY;
+	mutex_unlock(&bdev->bd_inode->i_mutex);
+	return rc;
+}
+#else
+static int blkdev_daxset(struct block_device *bdev, int arg)
+{
+	if (arg)
+		return -ENOTTY;
+	return 0;
+}
+#endif
+
 static int blkdev_flushbuf(struct block_device *bdev, fmode_t mode,
 		unsigned cmd, unsigned long arg)
 {
@@ -568,6 +624,11 @@ int blkdev_ioctl(struct block_device *bdev, fmode_t mode, unsigned cmd,
 	case BLKTRACESETUP:
 	case BLKTRACETEARDOWN:
 		return blk_trace_ioctl(bdev, cmd, argp);
+	case BLKDAXSET:
+		return blkdev_daxset(bdev, arg);
+	case BLKDAXGET:
+		return put_int(arg, !!(bdev->bd_inode->i_flags & S_DAX));
+		break;
 	case IOC_PR_REGISTER:
 		return blkdev_pr_register(bdev, argp);
 	case IOC_PR_RESERVE:

fs/block_dev.c

@@ -1235,8 +1235,11 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
 				}
 			}
 
-			if (!ret)
+			if (!ret) {
 				bd_set_size(bdev,(loff_t)get_capacity(disk)<<9);
+				if (!blkdev_dax_capable(bdev))
+					bdev->bd_inode->i_flags &= ~S_DAX;
+			}
 
 			/*
 			 * If the device is invalidated, rescan partition
@@ -1250,6 +1253,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
 				else if (ret == -ENOMEDIUM)
 					invalidate_partitions(disk, bdev);
 			}
+
 			if (ret)
 				goto out_clear;
 		} else {
@@ -1270,12 +1274,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
 				goto out_clear;
 			}
 			bd_set_size(bdev, (loff_t)bdev->bd_part->nr_sects << 9);
-			/*
-			 * If the partition is not aligned on a page
-			 * boundary, we can't do dax I/O to it.
-			 */
-			if ((bdev->bd_part->start_sect % (PAGE_SIZE / 512)) ||
-			    (bdev->bd_part->nr_sects % (PAGE_SIZE / 512)))
+			if (!blkdev_dax_capable(bdev))
 				bdev->bd_inode->i_flags &= ~S_DAX;
 		}
 	} else {
@@ -1713,13 +1712,101 @@ static const struct address_space_operations def_blk_aops = {
 	.is_dirty_writeback = buffer_check_dirty_writeback,
 };
 
+#ifdef CONFIG_FS_DAX
+/*
+ * In the raw block case we do not need to contend with truncation nor
+ * unwritten file extents.  Without those concerns there is no need for
+ * additional locking beyond the mmap_sem context that these routines
+ * are already executing under.
+ *
+ * Note, there is no protection if the block device is dynamically
+ * resized (partition grow/shrink) during a fault. A stable block device
+ * size is already not enforced in the blkdev_direct_IO path.
+ *
+ * For DAX, it is the responsibility of the block device driver to
+ * ensure the whole-disk device size is stable while requests are in
+ * flight.
+ *
+ * Finally, unlike the filemap_page_mkwrite() case there is no
+ * filesystem superblock to sync against freezing.  We still include a
+ * pfn_mkwrite callback for dax drivers to receive write fault
+ * notifications.
+ */
+static int blkdev_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+	return __dax_fault(vma, vmf, blkdev_get_block, NULL);
+}
+
+static int blkdev_dax_pmd_fault(struct vm_area_struct *vma, unsigned long addr,
+		pmd_t *pmd, unsigned int flags)
+{
+	return __dax_pmd_fault(vma, addr, pmd, flags, blkdev_get_block, NULL);
+}
+
+static void blkdev_vm_open(struct vm_area_struct *vma)
+{
+	struct inode *bd_inode = bdev_file_inode(vma->vm_file);
+	struct block_device *bdev = I_BDEV(bd_inode);
+
+	mutex_lock(&bd_inode->i_mutex);
+	bdev->bd_map_count++;
+	mutex_unlock(&bd_inode->i_mutex);
+}
+
+static void blkdev_vm_close(struct vm_area_struct *vma)
+{
+	struct inode *bd_inode = bdev_file_inode(vma->vm_file);
+	struct block_device *bdev = I_BDEV(bd_inode);
+
+	mutex_lock(&bd_inode->i_mutex);
+	bdev->bd_map_count--;
+	mutex_unlock(&bd_inode->i_mutex);
+}
+
+static const struct vm_operations_struct blkdev_dax_vm_ops = {
+	.open		= blkdev_vm_open,
+	.close		= blkdev_vm_close,
+	.fault		= blkdev_dax_fault,
+	.pmd_fault	= blkdev_dax_pmd_fault,
+	.pfn_mkwrite	= blkdev_dax_fault,
+};
+
+static const struct vm_operations_struct blkdev_default_vm_ops = {
+	.open		= blkdev_vm_open,
+	.close		= blkdev_vm_close,
+	.fault		= filemap_fault,
+	.map_pages	= filemap_map_pages,
+};
+
+static int blkdev_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct inode *bd_inode = bdev_file_inode(file);
+	struct block_device *bdev = I_BDEV(bd_inode);
+
+	file_accessed(file);
+	mutex_lock(&bd_inode->i_mutex);
+	bdev->bd_map_count++;
+	if (IS_DAX(bd_inode)) {
+		vma->vm_ops = &blkdev_dax_vm_ops;
+		vma->vm_flags |= VM_MIXEDMAP | VM_HUGEPAGE;
+	} else {
+		vma->vm_ops = &blkdev_default_vm_ops;
+	}
+	mutex_unlock(&bd_inode->i_mutex);
+
+	return 0;
+}
+#else
+#define blkdev_mmap generic_file_mmap
+#endif
+
 const struct file_operations def_blk_fops = {
 	.open		= blkdev_open,
 	.release	= blkdev_close,
 	.llseek		= block_llseek,
 	.read_iter	= blkdev_read_iter,
 	.write_iter	= blkdev_write_iter,
-	.mmap		= generic_file_mmap,
+	.mmap		= blkdev_mmap,
 	.fsync		= blkdev_fsync,
 	.unlocked_ioctl	= block_ioctl,
 #ifdef CONFIG_COMPAT

include/linux/fs.h

@@ -482,6 +482,9 @@ struct block_device {
 	int			bd_fsfreeze_count;
 	/* Mutex for freeze */
 	struct mutex		bd_fsfreeze_mutex;
+#ifdef CONFIG_FS_DAX
+	int			bd_map_count;
+#endif
 };
 
 /*
@@ -2264,6 +2267,14 @@ extern struct super_block *freeze_bdev(struct block_device *);
 extern void emergency_thaw_all(void);
 extern int thaw_bdev(struct block_device *bdev, struct super_block *sb);
 extern int fsync_bdev(struct block_device *);
+#ifdef CONFIG_FS_DAX
+extern bool blkdev_dax_capable(struct block_device *bdev);
+#else
+static inline bool blkdev_dax_capable(struct block_device *bdev)
+{
+	return false;
+}
+#endif
 
 extern struct super_block *blockdev_superblock;
 

include/uapi/linux/fs.h

@@ -152,6 +152,8 @@ struct inodes_stat_t {
 #define BLKSECDISCARD _IO(0x12,125)
 #define BLKROTATIONAL _IO(0x12,126)
 #define BLKZEROOUT _IO(0x12,127)
+#define BLKDAXSET _IO(0x12,128)
+#define BLKDAXGET _IO(0x12,129)
 
 #define BMAP_IOCTL 1		/* obsolete - kept for compatibility */
 #define FIBMAP	   _IO(0x00,1)	/* bmap access */