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

Ross Zwisler
2016-02-10T20:49:17Z

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(-)

--
2.5.0