On Thu, Feb 11, 2016 at 4:50 AM, Jan Kara [off-list ref] wrote:

On Wed 10-02-16 15:43:40, Ross Zwisler wrote:

quoted

On Thu, Feb 11, 2016 at 09:03:12AM +1100, Dave Chinner wrote:

quoted

On Wed, Feb 10, 2016 at 01:48:56PM -0700, Ross Zwisler wrote:

quoted

Previously calls to dax_writeback_mapping_range() for all DAX filesystems
(ext2, ext4 & xfs) were centralized in filemap_write_and_wait_range().
dax_writeback_mapping_range() needs a struct block_device, and it used to
get that from inode->i_sb->s_bdev.  This is correct for normal inodes
mounted on ext2, ext4 and XFS filesystems, but is incorrect for DAX raw
block devices and for XFS real-time files.

Instead, call dax_writeback_mapping_range() directly from the filesystem
->writepages function so that it can supply us with a valid block
device. This also fixes DAX code to properly flush caches in response to
sync(2).

Signed-off-by: Ross Zwisler <redacted>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/block_dev.c      | 16 +++++++++++++++-
 fs/dax.c            | 13 ++++++++-----
 fs/ext2/inode.c     | 11 +++++++++++
 fs/ext4/inode.c     |  7 +++++++
 fs/xfs/xfs_aops.c   |  9 +++++++++
 include/linux/dax.h |  6 ++++--
 mm/filemap.c        | 12 ++++--------
 7 files changed, 58 insertions(+), 16 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 39b3a17..fc01e43 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c

@@ -1693,13 +1693,27 @@ static int blkdev_releasepage(struct page *page, gfp_t wait)
  return try_to_free_buffers(page);
 }

+static int blkdev_writepages(struct address_space *mapping,
+                      struct writeback_control *wbc)
+{
+ if (dax_mapping(mapping)) {
+         struct block_device *bdev = I_BDEV(mapping->host);
+         int error;
+
+         error = dax_writeback_mapping_range(mapping, bdev, wbc);
+         if (error)
+                 return error;
+ }
+ return generic_writepages(mapping, wbc);
+}

Can you remind of the reason for calling generic_writepages() on DAX
enabled address spaces?

Sure.  The initial version of this patch didn't do this, and during testing I
hit a bunch of xfstests failures.  In ext2 at least I believe these were
happening because we were skipping the call into generic_writepages() for DAX
inodes. Without a lot of data to back this up, my guess is that this is due
to metadata inodes or something being marked as DAX (so dax_mapping(mapping)
returns true), but having dirty page cache pages that need to be written back
as part of the writeback.

Changing this so we always call generic_writepages() even in the DAX case
solved the xfstest failures.

If this sounds incorrect, please let me know and I'll go and gather more data.

So I think a more correct fix it to not set S_DAX for inodes that will have
any pagecache pages - e.g. don't set S_DAX for block device inodes when
filesystem is mounted on it (probably the easiest is to just refuse to
mount filesystem on block device which has S_DAX set).

I think we have a wider problem here.  See __blkdev_get, we set S_DAX
on all block devices that have ->direct_access() and have a
page-aligned starting address.  It seems to me we need to modify the
metadata i/o paths to bypass the page cache, or teach the fsync code
how to flush populated data pages out of the radix.

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