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

Alistair Popple
2025-02-18T03:57:34Z

From: Dan Williams <dan.j.williams@intel.com>

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@nvidia.com [1]
Link: http://lore.kernel.org/20210820210318.187742e8@thinkpad/ [2]
Link: http://lore.kernel.org/4511465a4f8429f45e2ac70d2e65dc5e1df1eb47.1725941415.git-series.apopple@nvidia.com [3]
Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Tested-by: Alexander Gordeev <agordeev@linux.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
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