Re: [PATCH v2 2/2] dax: move writeback calls into the filesystems

Ross Zwisler
2016-02-10T22:43:57Z

On Thu, Feb 11, 2016 at 09:03:12AM +1100, Dave Chinner wrote:
> On Wed, Feb 10, 2016 at 01:48:56PM -0700, Ross Zwisler wrote:
> > 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 <ross.zwisler@linux.intel.com>
> > 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.

- Ross