Re: [PATCH RESEND v6 2/9] dax: Introduce holder for dax_device

To easily track filesystem from a pmem device, we introduce a holder for
dax_device structure, and also its operation.  This holder is used to
remember who is using this dax_device:
 - When it is the backend of a filesystem, the holder will be the
   superblock of this filesystem.
 - When this pmem device is one of the targets in a mapped device, the
   holder will be this mapped device.  In this case, the mapped device
   has its own dax_device and it will follow the first rule.  So that we
   can finally track to the filesystem we needed.

The holder and holder_ops will be set when filesystem is being mounted,
or an target device is being activated.

Signed-off-by: Shiyang Ruan <redacted>
---
 drivers/dax/super.c | 46 +++++++++++++++++++++++++++++++++++++++++++++
 include/linux/dax.h | 17 +++++++++++++++++
 2 files changed, 63 insertions(+)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 5fa6ae9dbc8b..00c32dfa5665 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c

@@ -214,6 +214,8 @@ enum dax_device_flags {
  * @cdev: optional character interface for "device dax"
  * @host: optional name for lookups where the device path is not available
  * @private: dax driver private data
+ * @holder_rwsem: prevent unregistration while holder_ops is in progress
+ * @holder_data: holder of a dax_device: could be filesystem or mapped device
  * @flags: state and boolean properties
  */
 struct dax_device {

@@ -222,8 +224,11 @@ struct dax_device {
        struct cdev cdev;
        const char *host;
        void *private;
+       struct rw_semaphore holder_rwsem;

+       void *holder_data;
        unsigned long flags;
        const struct dax_operations *ops;
+       const struct dax_holder_operations *holder_ops;
 };

 static ssize_t write_cache_show(struct device *dev,

@@ -373,6 +378,25 @@ int dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
 }
 EXPORT_SYMBOL_GPL(dax_zero_page_range);

+int dax_holder_notify_failure(struct dax_device *dax_dev, loff_t offset,
+                             size_t size, void *data)
+{
+       int rc;
+
+       if (!dax_dev)
+               return -ENXIO;

+       return rc;
+}
+EXPORT_SYMBOL_GPL(dax_holder_notify_failure);
+
 #ifdef CONFIG_ARCH_HAS_PMEM_API
 void arch_wb_cache_pmem(void *addr, size_t size);
 void dax_flush(struct dax_device *dax_dev, void *addr, size_t size)

@@ -603,6 +627,7 @@ struct dax_device *alloc_dax(void *private, const char *__host,
        dax_add_host(dax_dev, host);
        dax_dev->ops = ops;
        dax_dev->private = private;
+       init_rwsem(&dax_dev->holder_rwsem);
        if (flags & DAXDEV_F_SYNC)
                set_dax_synchronous(dax_dev);

@@ -624,6 +649,27 @@ void put_dax(struct dax_device *dax_dev)
 }
 EXPORT_SYMBOL_GPL(put_dax);

+void dax_set_holder(struct dax_device *dax_dev, void *holder,
+               const struct dax_holder_operations *ops)
+{
+       if (!dax_dev)

+
+       return dax_dev->holder_data;
+}
+EXPORT_SYMBOL_GPL(dax_get_holder);
+
 /**
  * dax_get_by_host() - temporary lookup mechanism for filesystem-dax
  * @host: alternate name for the device registered by a dax driver

diff --git a/include/linux/dax.h b/include/linux/dax.h
index b52f084aa643..6f4b5c97ceb0 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h

@@ -38,10 +38,17 @@ struct dax_operations {
        int (*zero_page_range)(struct dax_device *, pgoff_t, size_t);
 };

+struct dax_holder_operations {
+       int (*notify_failure)(struct dax_device *, loff_t, size_t, void *);
+};
+
 extern struct attribute_group dax_attribute_group;

 #if IS_ENABLED(CONFIG_DAX)
 struct dax_device *dax_get_by_host(const char *host);
+void dax_set_holder(struct dax_device *dax_dev, void *holder,
+               const struct dax_holder_operations *ops);
+void *dax_get_holder(struct dax_device *dax_dev);
 struct dax_device *alloc_dax(void *private, const char *host,
                const struct dax_operations *ops, unsigned long flags);
 void put_dax(struct dax_device *dax_dev);

@@ -77,6 +84,14 @@ static inline struct dax_device *dax_get_by_host(const char *host)
 {
        return NULL;
 }
+static inline void dax_set_holder(struct dax_device *dax_dev, void *holder,
+               const struct dax_holder_operations *ops)
+{
+}
+static inline void *dax_get_holder(struct dax_device *dax_dev)
+{
+       return NULL;
+}
 static inline struct dax_device *alloc_dax(void *private, const char *host,
                const struct dax_operations *ops, unsigned long flags)
 {

@@ -226,6 +241,8 @@ size_t dax_copy_to_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
                size_t bytes, struct iov_iter *i);
 int dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
                        size_t nr_pages);
+int dax_holder_notify_failure(struct dax_device *dax_dev, loff_t offset,
+               size_t size, void *data);
 void dax_flush(struct dax_device *dax_dev, void *addr, size_t size);

 ssize_t dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
--

2.32.0