[PATCH v2 0/5] fs, xfs: block map immutable files for dax, dma-to-storage, and swap

Dan Williams
2017-08-04T02:32:17Z

Changes since v1 [1]:
* Add IS_IOMAP_IMMUTABLE() checks to xfs ioctl paths that perform block
map changes (xfs_alloc_file_space and xfs_free_file_space) (Darrick)

* Rather than complete a partial write, fail all writes that would
attempt to extend the file size (Darrick)

* Introduce FALLOC_FL_UNSEAL_BLOCK_MAP as an explicit operation type for
clearing S_IOMAP_IMMUTABLE (Dave)

* Rework xfs_seal_file_space() to first complete hole-fill and unshare
operations and then check the file for suitability under
XFS_ILOCK_EXCL. (Darrick)

* Add an FS_XFLAG_IOMAP_IMMUTABLE flag so the immutable state can be
seen by xfs_io. (Dave)

* Move the setting of S_IOMAP_IMMUTABLE to be atomic with respect to the
successful transaction that records XFS_DIFLAG2_IOMAP_IMMUTABLE.
(Darrick, Dave)

* Switch to a 'goto out_unlock' style in xfs_seal_file_space() to
cleanup 'if / else' tree, and use the mapping_mapped() helper. (Dave)

* Rely on XFS_MMAPLOCK_EXCL for reading a stable state of
mapping->i_mmap. (Dave)

[1]: http://marc.info/?l=linux-fsdevel&m=150135785712967&w=2

---

The daxfile proposal a few weeks back [2] sought to piggy back on the
swapfile implementation to approximate a block map immutable file. This
is an idea Dave originated last year to solve the dax "flush from
userspace" problem [3].

The discussion yielded several results. First, Christoph pointed out
that swapfiles are subtly broken [4]. Second, Darrick [5] and Dave [6]
proposed how to properly implement a block map immutable file. Finally,
Dave identified some improvements to swapfiles that can be built on the
block-map-immutable mechanism. These patches seek to implement the first
part of the proposal and save the swapfile work to build on top once the
base mechanism is complete.

While the initial motivation for this feature is support for
byte-addressable updates of persistent memory and managing cache
maintenance from userspace, the applications of the feature are broader.
In addition to being the start of a better swapfile mechanism it can
also support a DMA-to-storage use case. This use case enables
data-acquisition hardware to DMA directly to a storage device address
while being safe in the knowledge that storage mappings will not change.

[2]: https://lkml.org/lkml/2017/6/16/790
[3]: https://lkml.org/lkml/2016/9/11/159
[4]: https://lkml.org/lkml/2017/6/18/31
[5]: https://lkml.org/lkml/2017/6/20/49
[6]: https://www.spinics.net/lists/linux-xfs/msg07871.html

---

Dan Williams (5):
fs, xfs: introduce S_IOMAP_IMMUTABLE
fs, xfs: introduce FALLOC_FL_SEAL_BLOCK_MAP
fs, xfs: introduce FALLOC_FL_UNSEAL_BLOCK_MAP
xfs: introduce XFS_DIFLAG2_IOMAP_IMMUTABLE
xfs: toggle XFS_DIFLAG2_IOMAP_IMMUTABLE in response to fallocate

fs/attr.c | 10 ++
fs/open.c | 22 +++++
fs/read_write.c | 3 +
fs/xfs/libxfs/xfs_format.h | 5 +
fs/xfs/xfs_bmap_util.c | 181 +++++++++++++++++++++++++++++++++++++++++++
fs/xfs/xfs_bmap_util.h | 5 +
fs/xfs/xfs_file.c | 16 +++-
fs/xfs/xfs_inode.c | 2
fs/xfs/xfs_ioctl.c | 7 ++
fs/xfs/xfs_iops.c | 8 +-
include/linux/falloc.h | 4 +
include/linux/fs.h | 2
include/uapi/linux/falloc.h | 20 +++++
include/uapi/linux/fs.h | 1
mm/filemap.c | 5 +
15 files changed, 282 insertions(+), 9 deletions(-)