[RFC 0/3] Add buffered write-through support to iomap & xfs

[Ojaswin Mujoo <ojaswin@linux.ibm.com>]

Hi all,

This patchset implements an early design prototype of buffered I/O
write-through semantics in linux.
 
This idea mainly picked up traction to enable RWF_ATOMIC buffered IO [1],
however write-through path can have many use cases beyond atomic writes, 
- such as enabling truly async AIO buffered I/O when issued with O_DSYNC   
- better scalability for buffered I/O

The implementation of write-through combines the buffered IO frontend
with async dio backend, which leads to some interesting interactions.
I've added most of the design notes in respective patches. Please note
that this is an initial RFC to iron out any early design issues. This is
largely based on suggestions from Dave an Jan in [1] so thanks for the
pointers!


* Testing Notes *

- I've added support for RWF_WRITETHROUGH to fsx and fsstress in
  xfstests and these patches survive fsx with integrity verification as
  well as fsstress parallel stressing.
- -g quick with blocks size == page size and blocksize < pagesize shows
  no new regressions.


* Design TODOs *

- Evaluate if we need to tag page cache dirty bit in xarray, since 
  PG_Writeback is already set on the folio.
- As mentioned in patch 2, we call ->iomap_begin() twice which is not
  ideal but is kept this way to avoid churn and keep the PoC minimal.
  Look into a better way to refactor this.
- Fix support with filesystem freezing.


* Future work (once design is finalized) *

- Add aio O_DSYNC buffered write-through support
- Add RWF_ATOMIC support for buffered IO via write-through path
- Add support of other RWF_ flags for write-through buffered I/O path including
- Benchmarking numbers and more thorough testing needed.

As usual, thoughts and suggestions are welcome.

[1] https://lore.kernel.org/all/d0c4d95b-8064-4a7e-996d-7ad40eb4976b@linux.dev/

Regards,
ojaswin

Ojaswin Mujoo (3):
  iomap: Support buffered RWF_WRITETHROUGH via async dio backend
  iomap: Enable stable writes for RWF_WRITETHROUGH inodes
  xfs: Add RWF_WRITETHROUGH support to xfs

 fs/inode.c              |   1 +
 fs/iomap/buffered-io.c  | 414 ++++++++++++++++++++++++++++++++++++++++
 fs/iomap/direct-io.c    |  64 ++++---
 fs/xfs/xfs_file.c       |  68 ++++++-
 include/linux/fs.h      |   9 +
 include/linux/iomap.h   |  34 ++++
 include/uapi/linux/fs.h |   5 +-
 7 files changed, 568 insertions(+), 27 deletions(-)

-- 
2.52.0