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

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