Re: [PATCH 4/5] dax: remove the copy_from_iter and copy_to_iter methods

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Dec 09, 2021 at 07:38:27AM +0100, Christoph Hellwig wrote:
> These methods indirect the actual DAX read/write path.  In the end pmem
> uses magic flush and mc safe variants and fuse and dcssblk use plain ones
> while device mapper picks redirects to the underlying device.
> 
> Add set_dax_virtual() and set_dax_nomcsafe() APIs for fuse to skip these
> special variants, then use them everywhere as they fall back to the plain
> ones on s390 anyway and remove an indirect call from the read/write path
> as well as a lot of boilerplate code.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  drivers/dax/super.c           | 36 ++++++++++++++--
>  drivers/md/dm-linear.c        | 20 ---------
>  drivers/md/dm-log-writes.c    | 80 -----------------------------------
>  drivers/md/dm-stripe.c        | 20 ---------
>  drivers/md/dm.c               | 50 ----------------------
>  drivers/nvdimm/pmem.c         | 20 ---------
>  drivers/s390/block/dcssblk.c  | 14 ------
>  fs/dax.c                      |  5 ---
>  fs/fuse/virtio_fs.c           | 19 +--------
>  include/linux/dax.h           |  9 ++--
>  include/linux/device-mapper.h |  4 --
>  11 files changed, 37 insertions(+), 240 deletions(-)
> 

[..]
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 5c03a0364a9bb..754319ce2a29b 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -753,20 +753,6 @@ static long virtio_fs_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
>  	return nr_pages > max_nr_pages ? max_nr_pages : nr_pages;
>  }
>  
> -static size_t virtio_fs_copy_from_iter(struct dax_device *dax_dev,
> -				       pgoff_t pgoff, void *addr,
> -				       size_t bytes, struct iov_iter *i)
> -{
> -	return copy_from_iter(addr, bytes, i);
> -}
> -
> -static size_t virtio_fs_copy_to_iter(struct dax_device *dax_dev,
> -				       pgoff_t pgoff, void *addr,
> -				       size_t bytes, struct iov_iter *i)
> -{
> -	return copy_to_iter(addr, bytes, i);
> -}
> -
>  static int virtio_fs_zero_page_range(struct dax_device *dax_dev,
>  				     pgoff_t pgoff, size_t nr_pages)
>  {
> @@ -783,8 +769,6 @@ static int virtio_fs_zero_page_range(struct dax_device *dax_dev,
>  
>  static const struct dax_operations virtio_fs_dax_ops = {
>  	.direct_access = virtio_fs_direct_access,
> -	.copy_from_iter = virtio_fs_copy_from_iter,
> -	.copy_to_iter = virtio_fs_copy_to_iter,
>  	.zero_page_range = virtio_fs_zero_page_range,
>  };
>  
> @@ -853,7 +837,8 @@ static int virtio_fs_setup_dax(struct virtio_device *vdev, struct virtio_fs *fs)
>  	fs->dax_dev = alloc_dax(fs, &virtio_fs_dax_ops);
>  	if (IS_ERR(fs->dax_dev))
>  		return PTR_ERR(fs->dax_dev);
> -
> +	set_dax_cached(fs->dax_dev);

Looks good to me from virtiofs point of view.

Reviewed-by: Vivek Goyal <vgoyal@redhat.com>

Going forward, I am wondering should virtiofs use flushcache version as
well. What if host filesystem is using DAX and mapping persistent memory
pfn directly into qemu address space. I have never tested that.

Right now we are relying on applications to do fsync/msync on virtiofs
for data persistence.

> +	set_dax_nomcsafe(fs->dax_dev);
>  	return devm_add_action_or_reset(&vdev->dev, virtio_fs_cleanup_dax,
>  					fs->dax_dev);
>  }

Thanks
Vivek