Subject: [PATCH v5 08/10] libnvdimm/dax: Add a dax flag to control synchronous fault support

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Wed, 10 Jun 2020 11:53:41 +0530

With POWER10, architecture is adding new pmem flush and sync instructions.
The kernel should prevent the usage of MAP_SYNC if applications are not using
the new instructions on newer hardware

This patch adds a dax attribute (/sys/bus/nd/devices/region0/pfn0.1/block/pmem0/dax/sync_fault)
which can be used to control this flag. If the device supports synchronous flush
then userspace can update this attribute to enable/disable the synchronous
fault. The attribute is only visible if there is write cache enabled on the device.

In a followup patch on ppc64 device with compat string "ibm,pmemory-v2"
will disable the sync fault feature.

Signed-off-by: Aneesh Kumar K.V <[email-masked]>
---
 drivers/dax/bus.c            |  2 +-
 drivers/dax/super.c          | 73 ++++++++++++++++++++++++++++++++++++
 drivers/nvdimm/pmem.c        |  4 ++
 drivers/nvdimm/region_devs.c | 16 ++++++++
 include/linux/dax.h          | 16 ++++++++
 include/linux/libnvdimm.h    |  4 ++
 mm/Kconfig                   |  3 ++
 7 files changed, 117 insertions(+), 1 deletion(-)

diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index df238c8b6ef2..8a825ecff49b 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -420,7 +420,7 @@ struct dev_dax *__devm_create_dev_dax(struct dax_region *dax_region, int id,
 	 * No 'host' or dax_operations since there is no access to this
 	 * device outside of mmap of the resulting character device.
 	 */
-	dax_dev = alloc_dax(dev_dax, NULL, NULL, DAXDEV_F_SYNC);
+	dax_dev = alloc_dax(dev_dax, NULL, NULL, DAXDEV_F_SYNC | DAXDEV_F_SYNC_ENABLED);
 	if (IS_ERR(dax_dev)) {
 		rc = PTR_ERR(dax_dev);
 		goto err;
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 8e32345be0f7..f93e6649d452 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -198,6 +198,12 @@ enum dax_device_flags {
 	DAXDEV_WRITE_CACHE,
 	/* flag to check if device supports synchronous flush */
 	DAXDEV_SYNC,
+	/*
+	 * flag to indicate whether synchronous flush is enabled.
+	 * Some platform may want to disable synchronous flush support
+	 * even though device supports the same.
+	 */
+	DAXDEV_SYNC_ENABLED,
 };
 
 /**
@@ -254,6 +260,63 @@ static ssize_t write_cache_store(struct device *dev,
 }
 static DEVICE_ATTR_RW(write_cache);
 
+bool __dax_synchronous_enabled(struct dax_device *dax_dev)
+{
+	return test_bit(DAXDEV_SYNC_ENABLED, &dax_dev->flags);
+}
+EXPORT_SYMBOL_GPL(__dax_synchronous_enabled);
+
+static void set_dax_synchronous_enable(struct dax_device *dax_dev, bool enable)
+{
+	if (!test_bit(DAXDEV_SYNC, &dax_dev->flags))
+		return;
+
+	if (enable)
+		set_bit(DAXDEV_SYNC_ENABLED, &dax_dev->flags);
+	else
+		clear_bit(DAXDEV_SYNC_ENABLED, &dax_dev->flags);
+}
+
+
+static ssize_t sync_fault_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	int enabled;
+	struct dax_device *dax_dev = dax_get_by_host(dev_name(dev));
+	ssize_t rc;
+
+	WARN_ON_ONCE(!dax_dev);
+	if (!dax_dev)
+		return -ENXIO;
+
+	enabled = (dax_synchronous(dax_dev) && dax_synchronous_enabled(dax_dev));
+	rc = sprintf(buf, "%d\n", enabled);
+	put_dax(dax_dev);
+	return rc;
+}
+
+static ssize_t sync_fault_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t len)
+{
+	bool enable_sync;
+	int rc = strtobool(buf, &enable_sync);
+	struct dax_device *dax_dev = dax_get_by_host(dev_name(dev));
+
+	WARN_ON_ONCE(!dax_dev);
+	if (!dax_dev)
+		return -ENXIO;
+
+	if (rc)
+		len = rc;
+	else
+		set_dax_synchronous_enable(dax_dev, enable_sync);
+
+	put_dax(dax_dev);
+	return len;
+}
+
+static DEVICE_ATTR_RW(sync_fault);
+
 static umode_t dax_visible(struct kobject *kobj, struct attribute *a, int n)
 {
 	struct device *dev = container_of(kobj, typeof(*dev), kobj);
@@ -267,11 +330,18 @@ static umode_t dax_visible(struct kobject *kobj, struct attribute *a, int n)
 	if (a == &dev_attr_write_cache.attr)
 		return 0;
 #endif
+	if (a == &dev_attr_sync_fault.attr) {
+		if (dax_write_cache_enabled(dax_dev))
+			return a->mode;
+		return 0;
+	}
+
 	return a->mode;
 }
 
 static struct attribute *dax_attributes[] = {
 	&dev_attr_write_cache.attr,
+	&dev_attr_sync_fault.attr,
 	NULL,
 };
 
@@ -594,6 +664,9 @@ struct dax_device *alloc_dax(void *private, const char *__host,
 	if (flags & DAXDEV_F_SYNC)
 		set_dax_synchronous(dax_dev);
 
+	if (flags & DAXDEV_F_SYNC_ENABLED)
+		set_dax_synchronous_enable(dax_dev, true);
+
 	return dax_dev;
 
  err_dev:
diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
index 97f948f8f4e6..a738b237a3ff 100644
--- a/drivers/nvdimm/pmem.c
+++ b/drivers/nvdimm/pmem.c
@@ -487,6 +487,10 @@ static int pmem_attach_disk(struct device *dev,
 
 	if (is_nvdimm_sync(nd_region))
 		flags = DAXDEV_F_SYNC;
+
+	if (is_nvdimm_sync_enabled(nd_region))
+		flags |= DAXDEV_F_SYNC_ENABLED;
+
 	dax_dev = alloc_dax(pmem, disk->disk_name, &pmem_dax_ops, flags);
 	if (IS_ERR(dax_dev)) {
 		put_disk(disk);
diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
index 88ea34a9c7fd..d60d159b812b 100644
--- a/drivers/nvdimm/region_devs.c
+++ b/drivers/nvdimm/region_devs.c
@@ -1283,6 +1283,22 @@ bool is_nvdimm_sync(struct nd_region *nd_region)
 }
 EXPORT_SYMBOL_GPL(is_nvdimm_sync);
 
+bool is_nvdimm_sync_enabled(struct nd_region *nd_region)
+{
+#ifdef CONFIG_ARCH_MAP_SYNC_DISABLE
+	if (is_nd_volatile(&nd_region->dev))
+		return true;
+
+	return is_nd_pmem(&nd_region->dev) &&
+		test_bit(ND_REGION_SYNC_ENABLED, &nd_region->flags);
+#else
+	return true;
+#endif
+
+}
+EXPORT_SYMBOL_GPL(is_nvdimm_sync_enabled);
+
+
 struct conflict_context {
 	struct nd_region *nd_region;
 	resource_size_t start, size;
diff --git a/include/linux/dax.h b/include/linux/dax.h
index d7af5d243f24..c4a3551557de 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -10,6 +10,9 @@
 /* Flag for synchronous flush */
 #define DAXDEV_F_SYNC (1UL << 0)
 
+/* flag for platform forcing synchronous flush disable */
+#define DAXDEV_F_SYNC_ENABLED (1UL << 1)
+
 typedef unsigned long dax_entry_t;
 
 struct iomap_ops;
@@ -59,6 +62,13 @@ static inline void set_dax_synchronous(struct dax_device *dax_dev)
 {
 	__set_dax_synchronous(dax_dev);
 }
+
+bool __dax_synchronous_enabled(struct dax_device *dax_dev);
+static inline bool dax_synchronous_enabled(struct dax_device *dax_dev)
+{
+	return  __dax_synchronous_enabled(dax_dev);
+}
+
 /*
  * Check if given mapping is supported by the file / underlying device.
  */
@@ -69,6 +79,12 @@ static inline bool daxdev_mapping_supported(struct vm_area_struct *vma,
 		return true;
 	if (!IS_DAX(file_inode(vma->vm_file)))
 		return false;
+	/*
+	 * check MAP_SYNC is disabled by platform for this device.
+	 */
+	if (!dax_synchronous_enabled(dax_dev))
+		return false;
+
 	return dax_synchronous(dax_dev);
 }
 #else
diff --git a/include/linux/libnvdimm.h b/include/linux/libnvdimm.h
index 66f6c65bd789..53d4d4b8b6b9 100644
--- a/include/linux/libnvdimm.h
+++ b/include/linux/libnvdimm.h
@@ -63,6 +63,9 @@ enum {
 	/* Platform provides asynchronous flush mechanism */
 	ND_REGION_ASYNC = 3,
 
+	/* Platform wants to disable synchronous flush mechanism */
+	ND_REGION_SYNC_ENABLED= 4,
+
 	/* mark newly adjusted resources as requiring a label update */
 	DPA_RESOURCE_ADJUSTED = 1 << 0,
 };
@@ -262,6 +265,7 @@ int nvdimm_has_flush(struct nd_region *nd_region);
 int nvdimm_has_cache(struct nd_region *nd_region);
 int nvdimm_in_overwrite(struct nvdimm *nvdimm);
 bool is_nvdimm_sync(struct nd_region *nd_region);
+bool is_nvdimm_sync_enabled(struct nd_region *nd_region);
 
 static inline int nvdimm_ctl(struct nvdimm *nvdimm, unsigned int cmd, void *buf,
 		unsigned int buf_len, int *cmd_rc)
diff --git a/mm/Kconfig b/mm/Kconfig
index cffc276fa19c..aebe7e9c079d 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -868,4 +868,7 @@ config ARCH_HAS_HUGEPD
 config MAPPING_DIRTY_HELPERS
         bool
 
+config ARCH_MAP_SYNC_DISABLE
+	bool
+
 endmenu
-- 
2.26.2