[PATCH v3 0/3] minimal DAX support for XFS realtime device

[PATCH v3 0/3] minimal DAX support for XFS realtime device

[Dave Jiang]

Darrick,
After reading the comments from you, Dave Chinner, and Dan, it looks like
the dyanmic S_DAX flag support won't be coming or not any time soon at the
least. Here are the the collection of patches so far to address yours and
Dave C's comments for minimal support. Please let me know what else I am
missing. Thanks!

---

Darrick J. Wong (1):
      fs: allow per-device dax status checking for filesystems

Dave Jiang (2):
      dax: change bdev_dax_supported() to support boolean returns
      xfs: reject removal of realtime flag when datadev doesn't support DAX


 drivers/dax/super.c |   25 +++++++++++++------------
 fs/ext2/super.c     |    5 +++--
 fs/ext4/super.c     |    5 +++--
 fs/xfs/xfs_ioctl.c  |   18 +++++++++++++++++-
 fs/xfs/xfs_iops.c   |   30 +++++++++++++++++++++++++-----
 fs/xfs/xfs_super.c  |   11 +++++++++--
 include/linux/dax.h |   18 +++++++++++++-----
 7 files changed, 83 insertions(+), 29 deletions(-)

--
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[PATCH v3 1/3] fs: allow per-device dax status checking for filesystems

[Dave Jiang]

From: Darrick J. Wong <darrick.wong@oracle.com>

Refactor __bdev_dax_supported into a sb_dax_supported helper for
single-bdev filesystems and a regular bdev_dax_supported that takes a
bdev parameter.  This enables multi-device filesystems like xfs to check
that a dax device can work for the particular filesystem.  Once that's
in place, actually fix all the parts of XFS where we need to be able to
distinguish between datadev and rtdev.

This patch fixes the problem where we screw up the dax support checking
in xfs if the datadev and rtdev have different dax capabilities.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 drivers/dax/super.c |    9 +++++----
 fs/ext2/super.c     |    2 +-
 fs/ext4/super.c     |    2 +-
 fs/xfs/xfs_ioctl.c  |    3 ++-
 fs/xfs/xfs_iops.c   |   30 +++++++++++++++++++++++++-----
 fs/xfs/xfs_super.c  |   11 +++++++++--
 include/linux/dax.h |   16 ++++++++++++----
 7 files changed, 55 insertions(+), 18 deletions(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 3ec804672601..c4db84f26c0f 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -72,8 +72,9 @@ EXPORT_SYMBOL_GPL(fs_dax_get_by_bdev);
 #endif
 
 /**
- * __bdev_dax_supported() - Check if the device supports dax for filesystem
+ * bdev_dax_supported() - Check if the device supports dax for filesystem
  * @sb: The superblock of the device
+ * @bdev: block device to check
  * @blocksize: The block size of the device
  *
  * This is a library function for filesystems to check if the block device
@@ -81,9 +82,9 @@ EXPORT_SYMBOL_GPL(fs_dax_get_by_bdev);
  *
  * Return: negative errno if unsupported, 0 if supported.
  */
-int __bdev_dax_supported(struct super_block *sb, int blocksize)
+int bdev_dax_supported(struct super_block *sb, struct block_device *bdev,
+		       int blocksize)
 {
-	struct block_device *bdev = sb->s_bdev;
 	struct dax_device *dax_dev;
 	pgoff_t pgoff;
 	int err, id;
@@ -125,7 +126,7 @@ int __bdev_dax_supported(struct super_block *sb, int blocksize)
 
 	return 0;
 }
-EXPORT_SYMBOL_GPL(__bdev_dax_supported);
+EXPORT_SYMBOL_GPL(bdev_dax_supported);
 #endif
 
 enum dax_device_flags {
diff --git a/fs/ext2/super.c b/fs/ext2/super.c
index 7646818ab266..655699321c45 100644
--- a/fs/ext2/super.c
+++ b/fs/ext2/super.c
@@ -958,7 +958,7 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
 	blocksize = BLOCK_SIZE << le32_to_cpu(sbi->s_es->s_log_block_size);
 
 	if (sbi->s_mount_opt & EXT2_MOUNT_DAX) {
-		err = bdev_dax_supported(sb, blocksize);
+		err = sb_dax_supported(sb, blocksize);
 		if (err)
 			goto failed_mount;
 	}
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 7c46693a14d7..804a2d6729af 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3712,7 +3712,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 					" that may contain inline data");
 			goto failed_mount;
 		}
-		err = bdev_dax_supported(sb, blocksize);
+		err = sb_dax_supported(sb, blocksize);
 		if (err)
 			goto failed_mount;
 	}
diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index 20dc65fef6a4..f9d3f4314f7c 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1102,7 +1102,8 @@ xfs_ioctl_setattr_dax_invalidate(
 	if (fa->fsx_xflags & FS_XFLAG_DAX) {
 		if (!(S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode)))
 			return -EINVAL;
-		if (bdev_dax_supported(sb, sb->s_blocksize) < 0)
+		if (bdev_dax_supported(sb, xfs_find_bdev_for_inode(VFS_I(ip)),
+				sb->s_blocksize) < 0)
 			return -EINVAL;
 	}
 
diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 56475fcd76f2..66cd61c172af 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -1182,6 +1182,30 @@ static const struct inode_operations xfs_inline_symlink_inode_operations = {
 	.update_time		= xfs_vn_update_time,
 };
 
+/* Figure out if this file actually supports DAX. */
+static bool
+xfs_inode_supports_dax(
+	struct xfs_inode	*ip)
+{
+	struct xfs_mount	*mp = ip->i_mount;
+
+	/* Only supported on non-reflinked files. */
+	if (!S_ISREG(VFS_I(ip)->i_mode) || xfs_is_reflink_inode(ip))
+		return false;
+
+	/* DAX mount option or DAX iflag must be set. */
+	if (!(mp->m_flags & XFS_MOUNT_DAX) &&
+	    !(ip->i_d.di_flags2 & XFS_DIFLAG2_DAX))
+		return false;
+
+	/* Block size must match page size */
+	if (mp->m_sb.sb_blocksize != PAGE_SIZE)
+		return false;
+
+	/* Device has to support DAX too. */
+	return xfs_find_daxdev_for_inode(VFS_I(ip)) != NULL;
+}
+
 STATIC void
 xfs_diflags_to_iflags(
 	struct inode		*inode,
@@ -1200,11 +1224,7 @@ xfs_diflags_to_iflags(
 		inode->i_flags |= S_SYNC;
 	if (flags & XFS_DIFLAG_NOATIME)
 		inode->i_flags |= S_NOATIME;
-	if (S_ISREG(inode->i_mode) &&
-	    ip->i_mount->m_sb.sb_blocksize == PAGE_SIZE &&
-	    !xfs_is_reflink_inode(ip) &&
-	    (ip->i_mount->m_flags & XFS_MOUNT_DAX ||
-	     ip->i_d.di_flags2 & XFS_DIFLAG2_DAX))
+	if (xfs_inode_supports_dax(ip))
 		inode->i_flags |= S_DAX;
 }
 
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 1dacccc367f8..5fd60c1fa9db 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1649,11 +1649,18 @@ xfs_fs_fill_super(
 		sb->s_flags |= SB_I_VERSION;
 
 	if (mp->m_flags & XFS_MOUNT_DAX) {
+		int	error2 = 0;
+
 		xfs_warn(mp,
 		"DAX enabled. Warning: EXPERIMENTAL, use at your own risk");
 
-		error = bdev_dax_supported(sb, sb->s_blocksize);
-		if (error) {
+		error = bdev_dax_supported(sb, mp->m_ddev_targp->bt_bdev,
+				sb->s_blocksize);
+		if (mp->m_rtdev_targp)
+			error2 = bdev_dax_supported(sb,
+					mp->m_rtdev_targp->bt_bdev,
+					sb->s_blocksize);
+		if (error && error2) {
 			xfs_alert(mp,
 			"DAX unsupported by block device. Turning off DAX.");
 			mp->m_flags &= ~XFS_MOUNT_DAX;
diff --git a/include/linux/dax.h b/include/linux/dax.h
index 5258346c558c..1107a980f5c3 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -40,10 +40,11 @@ static inline void put_dax(struct dax_device *dax_dev)
 
 int bdev_dax_pgoff(struct block_device *, sector_t, size_t, pgoff_t *pgoff);
 #if IS_ENABLED(CONFIG_FS_DAX)
-int __bdev_dax_supported(struct super_block *sb, int blocksize);
-static inline int bdev_dax_supported(struct super_block *sb, int blocksize)
+int bdev_dax_supported(struct super_block *sb, struct block_device *bdev,
+		       int blocksize);
+static inline int sb_dax_supported(struct super_block *sb, int blocksize)
 {
-	return __bdev_dax_supported(sb, blocksize);
+	return bdev_dax_supported(sb, sb->s_bdev, blocksize);
 }
 
 static inline struct dax_device *fs_dax_get_by_host(const char *host)
@@ -58,7 +59,14 @@ static inline void fs_put_dax(struct dax_device *dax_dev)
 
 struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev);
 #else
-static inline int bdev_dax_supported(struct super_block *sb, int blocksize)
+static inline int bdev_dax_supported(struct super_block *sb,
+				     struct block_device *bdev,
+				     int blocksize)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline int sb_dax_supported(struct super_block *sb, int blocksize)
 {
 	return -EOPNOTSUPP;
 }

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[PATCH v3 2/3] dax: change bdev_dax_supported() to support boolean returns

[Dave Jiang]

The function return values are confusing with the way the function is
named. We expect a true or false return value but it actually returns
0/-errno.  This makes the code very confusing. Changing the return values
to return a bool where if DAX is supported then return true and no DAX
support returns false.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
---
 drivers/dax/super.c |   18 +++++++++---------
 fs/ext2/super.c     |    5 +++--
 fs/ext4/super.c     |    5 +++--
 fs/xfs/xfs_ioctl.c  |    4 ++--
 fs/xfs/xfs_super.c  |   10 +++++-----
 include/linux/dax.h |   12 ++++++------
 6 files changed, 28 insertions(+), 26 deletions(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index c4db84f26c0f..ef008e384042 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -80,9 +80,9 @@ EXPORT_SYMBOL_GPL(fs_dax_get_by_bdev);
  * This is a library function for filesystems to check if the block device
  * can be mounted with dax option.
  *
- * Return: negative errno if unsupported, 0 if supported.
+ * Return: true if supported, false if unsupported
  */
-int bdev_dax_supported(struct super_block *sb, struct block_device *bdev,
+bool bdev_dax_supported(struct super_block *sb, struct block_device *bdev,
 		       int blocksize)
 {
 	struct dax_device *dax_dev;
@@ -95,21 +95,21 @@ int bdev_dax_supported(struct super_block *sb, struct block_device *bdev,
 	if (blocksize != PAGE_SIZE) {
 		pr_debug("VFS (%s): error: unsupported blocksize for dax\n",
 				sb->s_id);
-		return -EINVAL;
+		return false;
 	}
 
 	err = bdev_dax_pgoff(bdev, 0, PAGE_SIZE, &pgoff);
 	if (err) {
-		pr_debug("VFS (%s): error: unaligned partition for dax\n",
-				sb->s_id);
-		return err;
+		pr_debug("VFS (%s): error: unaligned partition for dax: %d\n",
+				sb->s_id, err);
+		return false;
 	}
 
 	dax_dev = dax_get_by_host(bdev->bd_disk->disk_name);
 	if (!dax_dev) {
 		pr_debug("VFS (%s): error: device does not support dax\n",
 				sb->s_id);
-		return -EOPNOTSUPP;
+		return false;
 	}
 
 	id = dax_read_lock();
@@ -121,10 +121,10 @@ int bdev_dax_supported(struct super_block *sb, struct block_device *bdev,
 	if (len < 1) {
 		pr_debug("VFS (%s): error: dax access failed (%ld)\n",
 				sb->s_id, len);
-		return len < 0 ? len : -EIO;
+		return false;
 	}
 
-	return 0;
+	return true;
 }
 EXPORT_SYMBOL_GPL(bdev_dax_supported);
 #endif
diff --git a/fs/ext2/super.c b/fs/ext2/super.c
index 655699321c45..636b9c5e1bff 100644
--- a/fs/ext2/super.c
+++ b/fs/ext2/super.c
@@ -958,9 +958,10 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
 	blocksize = BLOCK_SIZE << le32_to_cpu(sbi->s_es->s_log_block_size);
 
 	if (sbi->s_mount_opt & EXT2_MOUNT_DAX) {
-		err = sb_dax_supported(sb, blocksize);
-		if (err)
+		if(!sb_dax_supported(sb, blocksize)) {
+			err = -EIO;
 			goto failed_mount;
+		}
 	}
 
 	/* If the blocksize doesn't match, re-read the thing.. */
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 804a2d6729af..7b7650ac9c53 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3712,9 +3712,10 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 					" that may contain inline data");
 			goto failed_mount;
 		}
-		err = sb_dax_supported(sb, blocksize);
-		if (err)
+		if (!sb_dax_supported(sb, blocksize)) {
+			err = -EIO;
 			goto failed_mount;
+		}
 	}
 
 	if (ext4_has_feature_encrypt(sb) && es->s_encryption_level) {
diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index f9d3f4314f7c..e440d789ed1b 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1102,8 +1102,8 @@ xfs_ioctl_setattr_dax_invalidate(
 	if (fa->fsx_xflags & FS_XFLAG_DAX) {
 		if (!(S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode)))
 			return -EINVAL;
-		if (bdev_dax_supported(sb, xfs_find_bdev_for_inode(VFS_I(ip)),
-				sb->s_blocksize) < 0)
+		if (!bdev_dax_supported(sb, xfs_find_bdev_for_inode(VFS_I(ip)),
+				sb->s_blocksize))
 			return -EINVAL;
 	}
 
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 5fd60c1fa9db..39b2295bf646 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1649,18 +1649,18 @@ xfs_fs_fill_super(
 		sb->s_flags |= SB_I_VERSION;
 
 	if (mp->m_flags & XFS_MOUNT_DAX) {
-		int	error2 = 0;
+		bool rtdev_is_dax, datadev_is_dax;
 
 		xfs_warn(mp,
 		"DAX enabled. Warning: EXPERIMENTAL, use at your own risk");
 
-		error = bdev_dax_supported(sb, mp->m_ddev_targp->bt_bdev,
-				sb->s_blocksize);
+		datadev_is_dax = bdev_dax_supported(sb,
+				mp->m_ddev_targp->bt_bdev, sb->s_blocksize);
 		if (mp->m_rtdev_targp)
-			error2 = bdev_dax_supported(sb,
+			rtdev_is_dax = bdev_dax_supported(sb,
 					mp->m_rtdev_targp->bt_bdev,
 					sb->s_blocksize);
-		if (error && error2) {
+		if (!rtdev_is_dax && !datadev_is_dax) {
 			xfs_alert(mp,
 			"DAX unsupported by block device. Turning off DAX.");
 			mp->m_flags &= ~XFS_MOUNT_DAX;
diff --git a/include/linux/dax.h b/include/linux/dax.h
index 1107a980f5c3..ee962705fffb 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -40,9 +40,9 @@ static inline void put_dax(struct dax_device *dax_dev)
 
 int bdev_dax_pgoff(struct block_device *, sector_t, size_t, pgoff_t *pgoff);
 #if IS_ENABLED(CONFIG_FS_DAX)
-int bdev_dax_supported(struct super_block *sb, struct block_device *bdev,
+bool bdev_dax_supported(struct super_block *sb, struct block_device *bdev,
 		       int blocksize);
-static inline int sb_dax_supported(struct super_block *sb, int blocksize)
+static inline bool sb_dax_supported(struct super_block *sb, int blocksize)
 {
 	return bdev_dax_supported(sb, sb->s_bdev, blocksize);
 }
@@ -59,16 +59,16 @@ static inline void fs_put_dax(struct dax_device *dax_dev)
 
 struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev);
 #else
-static inline int bdev_dax_supported(struct super_block *sb,
+static inline bool bdev_dax_supported(struct super_block *sb,
 				     struct block_device *bdev,
 				     int blocksize)
 {
-	return -EOPNOTSUPP;
+	return false;
 }
 
-static inline int sb_dax_supported(struct super_block *sb, int blocksize)
+static inline bool sb_dax_supported(struct super_block *sb, int blocksize)
 {
-	return -EOPNOTSUPP;
+	return false;
 }
 
 static inline struct dax_device *fs_dax_get_by_host(const char *host)

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Re: [PATCH v3 2/3] dax: change bdev_dax_supported() to support boolean returns

[Ross Zwisler]

On Thu, Feb 08, 2018 at 05:38:19PM -0700, Dave Jiang wrote:
> The function return values are confusing with the way the function is
> named. We expect a true or false return value but it actually returns
> 0/-errno.  This makes the code very confusing. Changing the return values
> to return a bool where if DAX is supported then return true and no DAX
> support returns false.
> 
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> ---

> diff --git a/fs/ext2/super.c b/fs/ext2/super.c
> index 655699321c45..636b9c5e1bff 100644
> --- a/fs/ext2/super.c
> +++ b/fs/ext2/super.c
> @@ -958,9 +958,10 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
>  	blocksize = BLOCK_SIZE << le32_to_cpu(sbi->s_es->s_log_block_size);
>  
>  	if (sbi->s_mount_opt & EXT2_MOUNT_DAX) {
> -		err = sb_dax_supported(sb, blocksize);
> -		if (err)
> +		if(!sb_dax_supported(sb, blocksize)) {
> +			err = -EIO;

No need to set 'err' here.  This is just a temporary variable used for some
local checks later in the function.  'ret' is the value that will be returned,
and that is already initialized to -EINVAL which should be fine.

>  			goto failed_mount;
> +		}
>  	}
>  
>  	/* If the blocksize doesn't match, re-read the thing.. */
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 804a2d6729af..7b7650ac9c53 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3712,9 +3712,10 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>  					" that may contain inline data");
>  			goto failed_mount;
>  		}
> -		err = sb_dax_supported(sb, blocksize);
> -		if (err)
> +		if (!sb_dax_supported(sb, blocksize)) {
> +			err = -EIO;

Same comment as with the ext2 case - no need to set 'err', 'ret' is -EINVAL
and has you covered.
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Re: [PATCH v3 2/3] dax: change bdev_dax_supported() to support boolean returns

[Dave Jiang]

On 02/13/2018 01:31 PM, Ross Zwisler wrote:
> On Thu, Feb 08, 2018 at 05:38:19PM -0700, Dave Jiang wrote:
>> The function return values are confusing with the way the function is
>> named. We expect a true or false return value but it actually returns
>> 0/-errno.  This makes the code very confusing. Changing the return values
>> to return a bool where if DAX is supported then return true and no DAX
>> support returns false.
>>
>> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
>> ---
> 
>> diff --git a/fs/ext2/super.c b/fs/ext2/super.c
>> index 655699321c45..636b9c5e1bff 100644
>> --- a/fs/ext2/super.c
>> +++ b/fs/ext2/super.c
>> @@ -958,9 +958,10 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
>>  	blocksize = BLOCK_SIZE << le32_to_cpu(sbi->s_es->s_log_block_size);
>>  
>>  	if (sbi->s_mount_opt & EXT2_MOUNT_DAX) {
>> -		err = sb_dax_supported(sb, blocksize);
>> -		if (err)
>> +		if(!sb_dax_supported(sb, blocksize)) {
>> +			err = -EIO;
> 
> No need to set 'err' here.  This is just a temporary variable used for some
> local checks later in the function.  'ret' is the value that will be returned,
> and that is already initialized to -EINVAL which should be fine.

Change ret to -EIO instead to set the correct error return code?

> 
>>  			goto failed_mount;
>> +		}
>>  	}
>>  
>>  	/* If the blocksize doesn't match, re-read the thing.. */
>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>> index 804a2d6729af..7b7650ac9c53 100644
>> --- a/fs/ext4/super.c
>> +++ b/fs/ext4/super.c
>> @@ -3712,9 +3712,10 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>>  					" that may contain inline data");
>>  			goto failed_mount;
>>  		}
>> -		err = sb_dax_supported(sb, blocksize);
>> -		if (err)
>> +		if (!sb_dax_supported(sb, blocksize)) {
>> +			err = -EIO;
> 
> Same comment as with the ext2 case - no need to set 'err', 'ret' is -EINVAL
> and has you covered.
> 
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Re: [PATCH v3 2/3] dax: change bdev_dax_supported() to support boolean returns

[Ross Zwisler]

On Tue, Feb 13, 2018 at 01:35:19PM -0700, Dave Jiang wrote:
> 
> 
> On 02/13/2018 01:31 PM, Ross Zwisler wrote:
> > On Thu, Feb 08, 2018 at 05:38:19PM -0700, Dave Jiang wrote:
> >> The function return values are confusing with the way the function is
> >> named. We expect a true or false return value but it actually returns
> >> 0/-errno.  This makes the code very confusing. Changing the return values
> >> to return a bool where if DAX is supported then return true and no DAX
> >> support returns false.
> >>
> >> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> >> ---
> > 
> >> diff --git a/fs/ext2/super.c b/fs/ext2/super.c
> >> index 655699321c45..636b9c5e1bff 100644
> >> --- a/fs/ext2/super.c
> >> +++ b/fs/ext2/super.c
> >> @@ -958,9 +958,10 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
> >>  	blocksize = BLOCK_SIZE << le32_to_cpu(sbi->s_es->s_log_block_size);
> >>  
> >>  	if (sbi->s_mount_opt & EXT2_MOUNT_DAX) {
> >> -		err = sb_dax_supported(sb, blocksize);
> >> -		if (err)
> >> +		if(!sb_dax_supported(sb, blocksize)) {
> >> +			err = -EIO;
> > 
> > No need to set 'err' here.  This is just a temporary variable used for some
> > local checks later in the function.  'ret' is the value that will be returned,
> > and that is already initialized to -EINVAL which should be fine.
> 
> Change ret to -EIO instead to set the correct error return code?

I'm not sure that -EIO is the 'correct' return code.  The old
sb_dax_supported() code could have returned -EINVAL, -EOPNOTSUPP or -EIO,
based on what went wrong.  All the other error cases in this function just
goto failed_mount without messing with 'ret', and we should probably do the
same.
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[PATCH v3 3/3] xfs: reject removal of realtime flag when datadev doesn't support DAX

[Dave Jiang]

In a situation where the rt_dev is DAX and data_dev is not DAX, if the user
requests to remove the realtime flag via ioctl we can no longer support DAX
for that file. Dynamic changing of S_DAX on the inode is not supported due
to various complications in the existing implementation. Therefore until we
address the dynamic S_DAX change issues, we must disallow realtime flag
being removed.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
---
 fs/xfs/xfs_ioctl.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index e440d789ed1b..cd5a5c61da39 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1029,6 +1029,21 @@ xfs_ioctl_setattr_xflags(
 {
 	struct xfs_mount	*mp = ip->i_mount;
 	uint64_t		di_flags2;
+	struct inode		*inode = VFS_I(ip);
+	struct super_block	*sb = inode->i_sb;
+
+	/*
+	 * In the case that the inode is realtime, and we are trying to remove
+	 * the realtime flag, and the rtdev supports DAX but the datadev does
+	 * not support DAX, we can't allow the realtime flag to be removed
+	 * since we do not support dynamic S_DAX flag removal yet.
+	 */
+	if ((XFS_IS_REALTIME_INODE(ip)) &&
+	    !(fa->fsx_xflags & FS_XFLAG_REALTIME) &&
+	    bdev_dax_supported(sb, mp->m_rtdev_targp->bt_bdev,
+		    sb->s_blocksize) &&
+	    !bdev_dax_supported(sb, mp->m_ddev_targp->bt_bdev, sb->s_blocksize))
+		return -ENOTSUPP;
 
 	/* Can't change realtime flag if any extents are allocated. */
 	if ((ip->i_d.di_nextents || ip->i_delayed_blks) &&

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