Re: [PATCH v7 2/8] dax: Introduce holder for dax_device

[Shiyang Ruan]

在 2021/10/15 2:00, Darrick J. Wong 写道:
> On Fri, Sep 24, 2021 at 09:09:53PM +0800, Shiyang Ruan wrote:
> > 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 <ruansy.fnst@xxxxxxxxxxx>
> > ---
> >   drivers/dax/super.c | 59 +++++++++++++++++++++++++++++++++++++++++++++
> >   include/linux/dax.h | 29 ++++++++++++++++++++++
> >   2 files changed, 88 insertions(+)
> >
> > diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> > index 48ce86501d93..7d4a11dcba90 100644
> > --- a/drivers/dax/super.c
> > +++ b/drivers/dax/super.c
> > @@ -23,7 +23,10 @@
> >    * @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_data: holder of a dax_device: could be filesystem or mapped device
> >    * @flags: state and boolean properties
> > + * @ops: operations for dax_device
> > + * @holder_ops: operations for the inner holder
> >    */
> >   struct dax_device {
> >   	struct hlist_node list;
> > @@ -31,8 +34,10 @@ struct dax_device {
> >   	struct cdev cdev;
> >   	const char *host;
> >   	void *private;
> > +	void *holder_data;
> >   	unsigned long flags;
> >   	const struct dax_operations *ops;
> > +	const struct dax_holder_operations *holder_ops;
> >   };
> >   
> >   static dev_t dax_devt;
> > @@ -374,6 +379,29 @@ 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, loff_t offset,
> > +			      size_t size, int flags)
> > +{
> > +	int rc;
> > +
> > +	dax_read_lock();
> > +	if (!dax_alive(dax_dev)) {
> > +		rc = -ENXIO;
> > +		goto out;
> > +	}
> > +
> > +	if (!dax_dev->holder_data) {
> > +		rc = -EOPNOTSUPP;
> > +		goto out;
> > +	}
> > +
> > +	rc = dax_dev->holder_ops->notify_failure(dax_dev, offset, size, flags);
>
> Shouldn't this check if dax_dev->holder_ops != NULL before dereferencing
> it for the function call?  Imagine an implementation that wants to
> attach a ->notify_failure function to a dax_device, maintains its own
> lookup table, and decides that it doesn't need to set holder_data.
>
> (Or, imagine someone who writes a garbage into holder_data and *boom*)

My mistake. I should check @holder_ops instead of @holder_data.

> How does the locking work here?  If there's a media failure, we'll take
> dax_rwsem and call ->notify_failure.  If the ->notify_failure function
> wants to access the pmem to handle the error by calling back into the
> dax code, will that cause nested locking on dax_rwsem?

Won't for now.  I have tested it with my simple testcases.
> Jumping ahead a bit, I think the rmap btree accesses that the xfs
> implementation performs can cause xfs_buf(fer) cache IO, which would
> trigger that if the buffers aren't already in memory, if I'm reading
> this correctly?

I didn't think of this case.  But I think this uses read lock too.  It 
won't be blocked.  Only dax_set_holder() takes write lock.

> > +out:
> > +	dax_read_unlock();
> > +	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)
> > @@ -618,6 +646,37 @@ 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)
> > +{
> > +	dax_write_lock();
> > +	if (!dax_alive(dax_dev)) {
> > +		dax_write_unlock();
> > +		return;
> > +	}
> > +
> > +	dax_dev->holder_data = holder;
> > +	dax_dev->holder_ops = ops;
> > +	dax_write_unlock();
>
> I guess this means that the holder has to detach itself before anyone
> calls kill_dax, or else a dead dax device ends up with a dangling
> reference to the holder?

Yes.

> > +}
> > +EXPORT_SYMBOL_GPL(dax_set_holder);
> > +
> > +void *dax_get_holder(struct dax_device *dax_dev)
> > +{
> > +	void *holder;
> > +
> > +	dax_read_lock();
> > +	if (!dax_alive(dax_dev)) {
> > +		dax_read_unlock();
> > +		return NULL;
> > +	}
> > +
> > +	holder = dax_dev->holder_data;
> > +	dax_read_unlock();
> > +	return holder;
> > +}
> > +EXPORT_SYMBOL_GPL(dax_get_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 097b3304f9b9..d273d59723cd 100644
> > --- a/include/linux/dax.h
> > +++ b/include/linux/dax.h
> > @@ -38,9 +38,24 @@ 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
> > +	 * @size: length of this memory failure event
> > +	 * @flags: action flags for memory failure handler
> > +	 */
> > +	int (*notify_failure)(struct dax_device *dax_dev, loff_t offset,
> > +			size_t size, int flags);
>
> Shouldn't size be u64 or something?  Let's say that 8GB of your pmem go
> bad, wouldn't you want a single call?  Though I guess the current
> implementation only goes a single page at a time, doesn't it?

Right.

> > +};
> > +
> >   extern struct attribute_group dax_attribute_group;
> >   
> >   #if IS_ENABLED(CONFIG_DAX)
> > +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);
> > @@ -70,6 +85,18 @@ static inline bool daxdev_mapping_supported(struct vm_area_struct *vma,
> >   	return dax_synchronous(dax_dev);
> >   }
> >   #else
> > +static inline struct dax_device *dax_get_by_host(const char *host)
>
> Not sure why this is being added here?  AFAICT none of the patches call
> this function...?

It's mistake when I rebase my code to the latest.  These lines were 
deleted but I didn't notice.  Will fix it.


--
Thanks,
Ruan.

> --D
>
> > +{
> > +	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)
> >   {
> > @@ -198,6 +225,8 @@ 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, int 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,
> > --
> > 2.33.0