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

Dan Williams
2017-08-04T02:36:00Z

[ adding linux-api to the cover letter for notification, will send the
full set to linux-api for v3 ]

On Thu, Aug 3, 2017 at 7:28 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> 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(-)