[PATCH v2 0/2] DAX bdev fixes - move flushing calls to FS

[Ross Zwisler]

During testing of raw block devices + DAX I noticed that the struct
block_device that we were using for DAX operations was incorrect.  For the
fault handlers, etc. we can just get the correct bdev via get_block(),
which is passed in as a function pointer, but for the *sync code and for
sector zeroing we don't have access to get_block().  This is also an issue
for XFS real-time devices, whenever we get those working.

Patch one of this series fixes the DAX sector zeroing code by explicitly
passing in a valid struct block_device.

Patch two of this series fixes DAX *sync support by moving calls to
dax_writeback_mapping_range() out of filemap_write_and_wait_range() and
into the filesystem/block device ->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).

Thanks to Jan Kara for his initial draft of patch 2:
https://lkml.org/lkml/2016/2/9/485

Here are the changes that I've made to that patch:

1) For DAX mappings, only return after calling
dax_writeback_mapping_range() if we encountered an error.  In the non-error
case we still need to write back normal pages, else we lose metadata
updates. 

2) In dax_writeback_mapping_range(), move the new check for 
        if (!mapping->nrexceptional || wbc->sync_mode != WB_SYNC_ALL)
above the i_blkbits check.  In my testing I found cases where
dax_writeback_mapping_range() was called for inodes with i_blkbits !=
PAGE_SHIFT - I'm assuming these are internal metadata inodes?  They have no
exceptional DAX entries to flush, so we have no work to do, but if we
return error from the i_blkbits check we will fail the overall writeback
operation.  Please let me know if it seems wrong for us to be seeing inodes
set to use DAX but with i_blkbits != PAGE_SHIFT and I'll get more info.

3) In filemap_write_and_wait() and filemap_write_and_wait_range(), continue
the writeback in the case that DAX is enabled but we only have a nonzero
mapping->nrpages.  As with 1) and 2), I believe this is necessary to
properly writeback metadata changes.  If this sounds wrong, please let me
know and I'll get more info.

A working tree can be found here:
https://git.kernel.org/cgit/linux/kernel/git/zwisler/linux.git/log/?h=fsync_bdev_v2

Ross Zwisler (2):
  dax: supply DAX clearing code with correct bdev
  dax: move writeback calls into the filesystems

 fs/block_dev.c         | 16 +++++++++++++++-
 fs/dax.c               | 22 ++++++++++++----------
 fs/ext2/inode.c        | 17 +++++++++++++++--
 fs/ext4/inode.c        |  7 +++++++
 fs/xfs/xfs_aops.c      | 11 ++++++++++-
 fs/xfs/xfs_aops.h      |  1 +
 fs/xfs/xfs_bmap_util.c |  3 ++-
 include/linux/dax.h    |  8 +++++---
 mm/filemap.c           | 12 ++++--------
 9 files changed, 71 insertions(+), 26 deletions(-)

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2.5.0