Re: [PATCH 02/19] dax: Pass dax_dev to dax_writeback_mapping_range()

[Dan Williams]

[ add Jan ]

On Mon, Aug 26, 2019 at 1:58 PM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>
> On Mon, Aug 26, 2019 at 04:33:26PM -0400, Vivek Goyal wrote:
> > On Mon, Aug 26, 2019 at 04:53:16AM -0700, Christoph Hellwig wrote:
> > > On Wed, Aug 21, 2019 at 01:57:03PM -0400, Vivek Goyal wrote:
> > > > Right now dax_writeback_mapping_range() is passed a bdev and dax_dev
> > > > is searched from that bdev name.
> > > >
> > > > virtio-fs does not have a bdev. So pass in dax_dev also to
> > > > dax_writeback_mapping_range(). If dax_dev is passed in, bdev is not
> > > > used otherwise dax_dev is searched using bdev.
> > >
> > > Please just pass in only the dax_device and get rid of the block device.
> > > The callers should have one at hand easily, e.g. for XFS just call
> > > xfs_find_daxdev_for_inode instead of xfs_find_bdev_for_inode.
> >
> > Sure. Here is the updated patch.
> >
> > This patch can probably go upstream independently. If you are fine with
> > the patch, I can post it separately for inclusion.
>
> Forgot to update function declaration in case of !CONFIG_FS_DAX. Here is
> the updated patch.
>
> Subject: dax: Pass dax_dev instead of bdev to dax_writeback_mapping_range()
>
> As of now dax_writeback_mapping_range() takes "struct block_device" as a
> parameter and dax_dev is searched from bdev name. This also involves taking
> a fresh reference on dax_dev and putting that reference at the end of
> function.
>
> We are developing a new filesystem virtio-fs and using dax to access host
> page cache directly. But there is no block device. IOW, we want to make
> use of dax but want to get rid of this assumption that there is always
> a block device associated with dax_dev.
>
> So pass in "struct dax_device" as parameter instead of bdev.
>
> ext2/ext4/xfs are current users and they already have a reference on
> dax_device. So there is no need to take reference and drop reference to
> dax_device on each call of this function.
>
> Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> ---
>  fs/dax.c            |    8 +-------
>  fs/ext2/inode.c     |    5 +++--
>  fs/ext4/inode.c     |    2 +-
>  fs/xfs/xfs_aops.c   |    2 +-
>  include/linux/dax.h |    4 ++--

Looks good to me. Would be nice to get some ext4 and xfs acks then
I'll take it through the dax tree for v5.4.

>  5 files changed, 8 insertions(+), 13 deletions(-)
>
> Index: rhvgoyal-linux-fuse/fs/dax.c
> ===================================================================
> --- rhvgoyal-linux-fuse.orig/fs/dax.c   2019-08-26 16:45:26.093710196 -0400
> +++ rhvgoyal-linux-fuse/fs/dax.c        2019-08-26 16:45:29.462710196 -0400
> @@ -936,12 +936,11 @@ static int dax_writeback_one(struct xa_s
>   * on persistent storage prior to completion of the operation.
>   */
>  int dax_writeback_mapping_range(struct address_space *mapping,
> -               struct block_device *bdev, struct writeback_control *wbc)
> +               struct dax_device *dax_dev, struct writeback_control *wbc)
>  {
>         XA_STATE(xas, &mapping->i_pages, wbc->range_start >> PAGE_SHIFT);
>         struct inode *inode = mapping->host;
>         pgoff_t end_index = wbc->range_end >> PAGE_SHIFT;
> -       struct dax_device *dax_dev;
>         void *entry;
>         int ret = 0;
>         unsigned int scanned = 0;
> @@ -952,10 +951,6 @@ int dax_writeback_mapping_range(struct a
>         if (!mapping->nrexceptional || wbc->sync_mode != WB_SYNC_ALL)
>                 return 0;
>
> -       dax_dev = dax_get_by_host(bdev->bd_disk->disk_name);
> -       if (!dax_dev)
> -               return -EIO;
> -
>         trace_dax_writeback_range(inode, xas.xa_index, end_index);
>
>         tag_pages_for_writeback(mapping, xas.xa_index, end_index);
> @@ -976,7 +971,6 @@ int dax_writeback_mapping_range(struct a
>                 xas_lock_irq(&xas);
>         }
>         xas_unlock_irq(&xas);
> -       put_dax(dax_dev);
>         trace_dax_writeback_range_done(inode, xas.xa_index, end_index);
>         return ret;
>  }
> Index: rhvgoyal-linux-fuse/include/linux/dax.h
> ===================================================================
> --- rhvgoyal-linux-fuse.orig/include/linux/dax.h        2019-08-26 16:45:26.094710196 -0400
> +++ rhvgoyal-linux-fuse/include/linux/dax.h     2019-08-26 16:46:08.101710196 -0400
> @@ -141,7 +141,7 @@ static inline void fs_put_dax(struct dax
>
>  struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev);
>  int dax_writeback_mapping_range(struct address_space *mapping,
> -               struct block_device *bdev, struct writeback_control *wbc);
> +               struct dax_device *dax_dev, struct writeback_control *wbc);
>
>  struct page *dax_layout_busy_page(struct address_space *mapping);
>  dax_entry_t dax_lock_page(struct page *page);
> @@ -180,7 +180,7 @@ static inline struct page *dax_layout_bu
>  }
>
>  static inline int dax_writeback_mapping_range(struct address_space *mapping,
> -               struct block_device *bdev, struct writeback_control *wbc)
> +               struct dax_device *dax_dev, struct writeback_control *wbc)
>  {
>         return -EOPNOTSUPP;
>  }
> Index: rhvgoyal-linux-fuse/fs/xfs/xfs_aops.c
> ===================================================================
> --- rhvgoyal-linux-fuse.orig/fs/xfs/xfs_aops.c  2019-08-26 16:45:26.094710196 -0400
> +++ rhvgoyal-linux-fuse/fs/xfs/xfs_aops.c       2019-08-26 16:45:29.471710196 -0400
> @@ -1120,7 +1120,7 @@ xfs_dax_writepages(
>  {
>         xfs_iflags_clear(XFS_I(mapping->host), XFS_ITRUNCATED);
>         return dax_writeback_mapping_range(mapping,
> -                       xfs_find_bdev_for_inode(mapping->host), wbc);
> +                       xfs_find_daxdev_for_inode(mapping->host), wbc);
>  }
>
>  STATIC int
> Index: rhvgoyal-linux-fuse/fs/ext4/inode.c
> ===================================================================
> --- rhvgoyal-linux-fuse.orig/fs/ext4/inode.c    2019-08-26 16:45:26.093710196 -0400
> +++ rhvgoyal-linux-fuse/fs/ext4/inode.c 2019-08-26 16:45:29.475710196 -0400
> @@ -2992,7 +2992,7 @@ static int ext4_dax_writepages(struct ad
>         percpu_down_read(&sbi->s_journal_flag_rwsem);
>         trace_ext4_writepages(inode, wbc);
>
> -       ret = dax_writeback_mapping_range(mapping, inode->i_sb->s_bdev, wbc);
> +       ret = dax_writeback_mapping_range(mapping, sbi->s_daxdev, wbc);
>         trace_ext4_writepages_result(inode, wbc, ret,
>                                      nr_to_write - wbc->nr_to_write);
>         percpu_up_read(&sbi->s_journal_flag_rwsem);
> Index: rhvgoyal-linux-fuse/fs/ext2/inode.c
> ===================================================================
> --- rhvgoyal-linux-fuse.orig/fs/ext2/inode.c    2019-08-26 16:45:26.093710196 -0400
> +++ rhvgoyal-linux-fuse/fs/ext2/inode.c 2019-08-26 16:45:29.477710196 -0400
> @@ -957,8 +957,9 @@ ext2_writepages(struct address_space *ma
>  static int
>  ext2_dax_writepages(struct address_space *mapping, struct writeback_control *wbc)
>  {
> -       return dax_writeback_mapping_range(mapping,
> -                       mapping->host->i_sb->s_bdev, wbc);
> +       struct ext2_sb_info *sbi = EXT2_SB(mapping->host->i_sb);
> +
> +       return dax_writeback_mapping_range(mapping, sbi->s_daxdev, wbc);
>  }
>
>  const struct address_space_operations ext2_aops = {