[PATCH] dax: Fix last_page check in __bdev_dax_supported()

[Vaibhav Jain]

Presently __bdev_dax_supported() checks if first sector of last
page ( last_page ) on the block device is aligned to page
boundary. However the code to compute 'last_page' assumes that there
are 8 sectors/page assuming a 4K page-size.

This assumption breaks on architectures which use a different page
size specifically PPC64 where page-size == 64K. Hence a warning is
seen while trying to mount a xfs/ext4 file-system with dax enabled:

$ sudo mount -o dax /dev/pmem0 /mnt/pmem
XFS (pmem0): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
XFS (pmem0): DAX unsupported by block device. Turning off DAX.

The patch fixes this issue by updating calculation of 'last_var' to
take into account number-of-sectors/page instead of assuming it to be
'8'.

Fixes: ad428cdb525a ("dax: Check the end of the block-device capacity with dax_direct_access()")
Signed-off-by: Chandan Rajendra <chandan@xxxxxxxxxxxxx>
Signed-off-by: Vaibhav Jain <vaibhav@xxxxxxxxxxxxx>
---
 drivers/dax/super.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index bbd57ca0634a..6b50ed3673d3 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -116,7 +116,9 @@ bool __bdev_dax_supported(struct block_device *bdev, int blocksize)
 		return false;
 	}
 
-	last_page = PFN_DOWN(i_size_read(bdev->bd_inode) - 1) * 8;
+	/* Calculate the first sector of last page on the block device */
+	last_page = PFN_DOWN(i_size_read(bdev->bd_inode) - 1) *
+		(PAGE_SIZE >> SECTOR_SHIFT);
 	err = bdev_dax_pgoff(bdev, last_page, PAGE_SIZE, &pgoff_end);
 	if (err) {
 		pr_debug("%s: error: unaligned partition for dax\n",
-- 
2.21.0