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

[Vivek Goyal <vgoyal@redhat.com>]

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@infradead.org>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 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 ++--
 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 = {