Date:   Tue, 15 Feb 2022 14:06:26 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "Darrick J. Wong" <djwong@...nel.org>, david <david@...morbit.com>,
        Jane Chu <jane.chu@...cle.com>
Subject: Re: [PATCH v10.1 1/9] dax: Introduce holder for dax_device

On Sun, Feb 13, 2022 at 4:58 AM Shiyang Ruan <ruansy.fnst@...itsu.com> wrote:
>
> v10.1 update:
>  - Fix build error reported by kernel test robot
>  - Add return code to dax_register_holder()
>  - Add write lock to prevent concurrent registrations
>  - Rename dax_get_holder() to dax_holder()
>  - Add kernel doc for new added functions
>
> 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
>    instance 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 <ruansy.fnst@...itsu.com>
> ---
>  drivers/dax/super.c | 95 +++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/dax.h | 30 ++++++++++++++
>  2 files changed, 125 insertions(+)
>
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index e3029389d809..d7fb4c36bf16 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -21,6 +21,9 @@
>   * @cdev: optional character interface for "device dax"
>   * @private: dax driver private data
>   * @flags: state and boolean properties
> + * @ops: operations for dax_device
> + * @holder_data: holder of a dax_device: could be filesystem or mapped device
> + * @holder_ops: operations for the inner holder
>   */
>  struct dax_device {
>         struct inode inode;
> @@ -28,6 +31,9 @@ struct dax_device {
>         void *private;
>         unsigned long flags;
>         const struct dax_operations *ops;
> +       void *holder_data;
> +       struct percpu_rw_semaphore holder_rwsem;

This lock is not necessary, see below...

> +       const struct dax_holder_operations *holder_ops;
>  };
>
>  static dev_t dax_devt;
> @@ -193,6 +199,29 @@ 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, u64 off,
> +                             u64 len, int mf_flags)
> +{
> +       int rc, id;
> +
> +       id = dax_read_lock();
> +       if (!dax_alive(dax_dev)) {
> +               rc = -ENXIO;
> +               goto out;
> +       }
> +
> +       if (!dax_dev->holder_ops) {
> +               rc = -EOPNOTSUPP;
> +               goto out;
> +       }
> +
> +       rc = dax_dev->holder_ops->notify_failure(dax_dev, off, len, mf_flags);
> +out:
> +       dax_read_unlock(id);
> +       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)
> @@ -268,6 +297,13 @@ void kill_dax(struct dax_device *dax_dev)
>
>         clear_bit(DAXDEV_ALIVE, &dax_dev->flags);
>         synchronize_srcu(&dax_srcu);
> +
> +       /* Lock to prevent concurrent registrations. */
> +       percpu_down_write(&dax_dev->holder_rwsem);
> +       /* clear holder data */
> +       dax_dev->holder_ops = NULL;
> +       dax_dev->holder_data = NULL;
> +       percpu_up_write(&dax_dev->holder_rwsem);
>  }
>  EXPORT_SYMBOL_GPL(kill_dax);
>
> @@ -393,6 +429,7 @@ struct dax_device *alloc_dax(void *private, const struct dax_operations *ops)
>
>         dax_dev->ops = ops;
>         dax_dev->private = private;
> +       percpu_init_rwsem(&dax_dev->holder_rwsem);
>         return dax_dev;
>
>   err_dev:
> @@ -409,6 +446,64 @@ void put_dax(struct dax_device *dax_dev)
>  }
>  EXPORT_SYMBOL_GPL(put_dax);
>
> +/**
> + * dax_holder() - obtain the holder of a dax device
> + * @dax_dev: a dax_device instance
> +
> + * Return: the holder's data which represents the holder if registered,
> + * otherwize NULL.
> + */
> +void *dax_holder(struct dax_device *dax_dev)
> +{
> +       if (!dax_alive(dax_dev))
> +               return NULL;
> +
> +       return dax_dev->holder_data;
> +}
> +EXPORT_SYMBOL_GPL(dax_holder);
> +
> +/**
> + * dax_register_holder() - register a holder to a dax device
> + * @dax_dev: a dax_device instance
> + * @holder: a pointer to a holder's data which represents the holder
> + * @ops: operations of this holder
> +
> + * Return: negative errno if an error occurs, otherwise 0.
> + */
> +int dax_register_holder(struct dax_device *dax_dev, void *holder,
> +               const struct dax_holder_operations *ops)
> +{
> +       if (!dax_alive(dax_dev))
> +               return -ENXIO;
> +
> +       /* Already registered */
> +       if (dax_holder(dax_dev))
> +               return -EBUSY;

Delete this...

> +
> +       /* Lock to prevent concurrent registrations. */
> +       percpu_down_write(&dax_dev->holder_rwsem);

...and just use cmpxchg:

if (cmpxchg(&dax_dev->holder_data, NULL, holder))
    return -EBUSY;
dax_dev->holder_ops = ops;

...and then on the release side you can require that the caller
specify @holder before clearing to make the unregistration symmetric:

if (cmpxchg(&dax_dev->holder_data, holder, NULL) != holder))
    return -EBUSY;
dax_dev->holder_ops = NULL;


> +       dax_dev->holder_data = holder;
> +       dax_dev->holder_ops = ops;
> +       percpu_up_write(&dax_dev->holder_rwsem);
> +
> +       return 0;
> +}
> +EXPORT_SYMBOL_GPL(dax_register_holder);
> +
> +/**
> + * dax_unregister_holder() - unregister the holder for a dax device
> + * @dax_dev: a dax_device instance
> + */
> +void dax_unregister_holder(struct dax_device *dax_dev)

Per above, require the holder to pass in their holder_data again.

> +{
> +       if (!dax_alive(dax_dev))
> +               return;
> +
> +       dax_dev->holder_data = NULL;
> +       dax_dev->holder_ops = NULL;
> +}
> +EXPORT_SYMBOL_GPL(dax_unregister_holder);
> +
>  /**
>   * inode_dax: convert a public inode into its dax_dev
>   * @inode: An inode with i_cdev pointing to a dax_dev
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 9fc5f99a0ae2..9800d84e5b7d 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -32,8 +32,24 @@ struct dax_operations {
>         int (*zero_page_range)(struct dax_device *, pgoff_t, size_t);
>  };
>
> +struct dax_holder_operations {
> +       /*
> +        * notify_failure - notify memory failure into inner holder device
> +        * @dax_dev: the dax device which contains the holder
> +        * @offset: offset on this dax device where memory failure occurs
> +        * @len: length of this memory failure event
> +        * @flags: action flags for memory failure handler
> +        */
> +       int (*notify_failure)(struct dax_device *dax_dev, u64 offset,
> +                       u64 len, int mf_flags);
> +};
> +
>  #if IS_ENABLED(CONFIG_DAX)
>  struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
> +int dax_register_holder(struct dax_device *dax_dev, void *holder,
> +               const struct dax_holder_operations *ops);
> +void dax_unregister_holder(struct dax_device *dax_dev);
> +void *dax_holder(struct dax_device *dax_dev);
>  void put_dax(struct dax_device *dax_dev);
>  void kill_dax(struct dax_device *dax_dev);
>  void dax_write_cache(struct dax_device *dax_dev, bool wc);
> @@ -53,6 +69,18 @@ static inline bool daxdev_mapping_supported(struct vm_area_struct *vma,
>         return dax_synchronous(dax_dev);
>  }
>  #else
> +static inline int dax_register_holder(struct dax_device *dax_dev, void *holder,
> +               const struct dax_holder_operations *ops)
> +{
> +       return 0;
> +}
> +static inline void dax_unregister_holder(struct dax_device *dax_dev)
> +{
> +}
> +static inline void *dax_holder(struct dax_device *dax_dev)
> +{
> +       return NULL;
> +}
>  static inline struct dax_device *alloc_dax(void *private,
>                 const struct dax_operations *ops)
>  {
> @@ -185,6 +213,8 @@ 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, u64 off, u64 len,
> +               int mf_flags);
>  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,
> --
> 2.34.1
>
>
>

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