[PATCH v8 18/20] dcssblk: Mark DAX broken, remove FS_DAX_LIMITED support

[Alistair Popple]

From: Dan Williams <dan.j.williams@xxxxxxxxx>

The dcssblk driver has long needed special case supoprt to enable
limited dax operation, so called CONFIG_FS_DAX_LIMITED. This mode
works around the incomplete support for ZONE_DEVICE on s390 by forgoing
the ability of dax-mapped pages to support GUP.

Now, pending cleanups to fsdax that fix its reference counting [1] depend on
the ability of all dax drivers to supply ZONE_DEVICE pages.

To allow that work to move forward, dax support needs to be paused for
dcssblk until ZONE_DEVICE support arrives. That work has been known for
a few years [2], and the removal of "pte_devmap" requirements [3] makes the
conversion easier.

For now, place the support behind CONFIG_BROKEN, and remove PFN_SPECIAL
(dcssblk was the only user).

Link: http://lore.kernel.org/cover.9f0e45d52f5cff58807831b6b867084d0b14b61c.1725941415.git-series.apopple@xxxxxxxxxx [1]
Link: http://lore.kernel.org/20210820210318.187742e8@thinkpad/ [2]
Link: http://lore.kernel.org/4511465a4f8429f45e2ac70d2e65dc5e1df1eb47.1725941415.git-series.apopple@xxxxxxxxxx [3]
Reviewed-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
Tested-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Heiko Carstens <hca@xxxxxxxxxxxxx>
Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
Cc: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
Cc: Sven Schnelle <svens@xxxxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Alistair Popple <apopple@xxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 Documentation/filesystems/dax.rst |  1 -
 drivers/s390/block/Kconfig        | 12 ++++++++++--
 drivers/s390/block/dcssblk.c      | 27 +++++++++++++++++----------
 3 files changed, 27 insertions(+), 13 deletions(-)

diff --git a/Documentation/filesystems/dax.rst b/Documentation/filesystems/dax.rst
index 719e90f..08dd5e2 100644
--- a/Documentation/filesystems/dax.rst
+++ b/Documentation/filesystems/dax.rst
@@ -207,7 +207,6 @@ implement direct_access.
 
 These block devices may be used for inspiration:
 - brd: RAM backed block device driver
-- dcssblk: s390 dcss block device driver
 - pmem: NVDIMM persistent memory driver
 
 
diff --git a/drivers/s390/block/Kconfig b/drivers/s390/block/Kconfig
index e3710a7..4bfe469 100644
--- a/drivers/s390/block/Kconfig
+++ b/drivers/s390/block/Kconfig
@@ -4,13 +4,21 @@ comment "S/390 block device drivers"
 
 config DCSSBLK
 	def_tristate m
-	select FS_DAX_LIMITED
-	select DAX
 	prompt "DCSSBLK support"
 	depends on S390 && BLOCK
 	help
 	  Support for dcss block device
 
+config DCSSBLK_DAX
+	def_bool y
+	depends on DCSSBLK
+	# requires S390 ZONE_DEVICE support
+	depends on BROKEN
+	select DAX
+	prompt "DCSSBLK DAX support"
+	help
+	  Enable DAX operation for the dcss block device
+
 config DASD
 	def_tristate y
 	prompt "Support for DASD devices"
diff --git a/drivers/s390/block/dcssblk.c b/drivers/s390/block/dcssblk.c
index 0f14d27..7248e54 100644
--- a/drivers/s390/block/dcssblk.c
+++ b/drivers/s390/block/dcssblk.c
@@ -534,6 +534,21 @@ static const struct attribute_group *dcssblk_dev_attr_groups[] = {
 	NULL,
 };
 
+static int dcssblk_setup_dax(struct dcssblk_dev_info *dev_info)
+{
+	struct dax_device *dax_dev;
+
+	if (!IS_ENABLED(CONFIG_DCSSBLK_DAX))
+		return 0;
+
+	dax_dev = alloc_dax(dev_info, &dcssblk_dax_ops);
+	if (IS_ERR(dax_dev))
+		return PTR_ERR(dax_dev);
+	set_dax_synchronous(dax_dev);
+	dev_info->dax_dev = dax_dev;
+	return dax_add_host(dev_info->dax_dev, dev_info->gd);
+}
+
 /*
  * device attribute for adding devices
  */
@@ -547,7 +562,6 @@ dcssblk_add_store(struct device *dev, struct device_attribute *attr, const char 
 	int rc, i, j, num_of_segments;
 	struct dcssblk_dev_info *dev_info;
 	struct segment_info *seg_info, *temp;
-	struct dax_device *dax_dev;
 	char *local_buf;
 	unsigned long seg_byte_size;
 
@@ -674,14 +688,7 @@ dcssblk_add_store(struct device *dev, struct device_attribute *attr, const char 
 	if (rc)
 		goto put_dev;
 
-	dax_dev = alloc_dax(dev_info, &dcssblk_dax_ops);
-	if (IS_ERR(dax_dev)) {
-		rc = PTR_ERR(dax_dev);
-		goto put_dev;
-	}
-	set_dax_synchronous(dax_dev);
-	dev_info->dax_dev = dax_dev;
-	rc = dax_add_host(dev_info->dax_dev, dev_info->gd);
+	rc = dcssblk_setup_dax(dev_info);
 	if (rc)
 		goto out_dax;
 
@@ -917,7 +924,7 @@ __dcssblk_direct_access(struct dcssblk_dev_info *dev_info, pgoff_t pgoff,
 		*kaddr = __va(dev_info->start + offset);
 	if (pfn)
 		*pfn = __pfn_to_pfn_t(PFN_DOWN(dev_info->start + offset),
-				PFN_DEV|PFN_SPECIAL);
+				      PFN_DEV);
 
 	return (dev_sz - offset) / PAGE_SIZE;
 }
-- 
git-series 0.9.1