On Sat, May 14, 2016 at 05:05:59AM +0000, Elliott, Robert (Persistent Memory) wrote:

quoted

-----Original Message-----
From: linux-block-owner@vger.kernel.org [mailto:linux-block-
owner@vger.kernel.org] On Behalf Of Jon Derrick
Sent: Friday, May 13, 2016 7:03 PM

...

quoted

Subject: [RFCv2 2/3] block: add helper for setting and clearing S_DAX on
inode

inode->i_flags locking rules suggest using the i_mutex lock. This patch
adds a helper to do the locking and setting of S_DAX on the block device
inode.

...

quoted

+static void bd_set_dax(struct block_device *bdev, bool enabled)
+{
+	struct inode *inode = bdev->bd_inode;
+
+	inode_lock(inode);
+	if (enabled)
+		inode->i_flags = S_DAX;
+	else
+		inode->i_flags &= ~S_DAX;
+	inode_unlock(inode);
+}

This is not symmetric - setting wipes out any other bits, but
clearing only clears the S_DAX bit. That seems confusing for
a helper function.

Using |= would be symmetric, but wouldn't replace what 
__blkdev_get does (if what it does is appropriate).

I'm under the impression that S_DAX is the only inode flag appropriate for a block device inode with direct-access, even though it does break the 'flag' semantic.

It does bring up an interesting question about the fate of the block device inode when S_DAX is set and then cleared later due to the two other conditions that can clear S_DAX.

I think the appropriate fix is to save the inode flags when setting DAX and restore them when we need to clear S_DAX.

quoted

@@ -1206,9 +1218,9 @@ static int __blkdev_get(struct block_device *bdev,

fmode_t mode, int for_part)
 		bdev->bd_queue = disk->queue;
 		bdev->bd_contains = bdev;
 		if (IS_ENABLED(CONFIG_BLK_DEV_DAX) && disk->fops-

quoted

direct_access)

-			bdev->bd_inode->i_flags = S_DAX;
+			bd_set_dax(bdev, 1);
 		else
-			bdev->bd_inode->i_flags &= ~S_DAX;
+			bd_set_dax(bdev, 0);

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