RFC xfs: single block atomic writes for buffered IO

This patch adds support to perform single block RWF_ATOMIC writes for iomap xfs buffered IO. This builds upon the inital RFC shared by John Garry last year [1]. Most of the details are present in the respective commit messages but I'd mention some of the design points below:

1. The first 4 patches introduce the statx and iomap plubming and page flags to add basic atomic writes support to buffered IO. However, there are still 2 key restrictions that apply:

FIRST: If the user buffer of atomic write crosses page boundary, there's a possibility of short write, example if 1 user page could not be faulted or got reclaimed before the copy operation. For now don't allow such a scenario by ensuring user buffer is page aligned. This way either the full write goes through or nothing does. This is also discussed in Mathew Wilcox's comment here
[2]

This is lifted in patch 5. The approach we took was to:
1. pin the user pages
2. Create a BVEC out of the struct page to pass to copy_folio_from_iter_atomic() rather than the USER backed iter. We don't use the user iter directly because the pinned user page could still get unmapped from the process, leading to short writes.

This approach allows us to only proceed if we are sure we will not have a short copy.

SECOND: We only support block size == page size buf-io atomic writes. This is to avoid the following scenario:
1. 4kb block atomic write marks the complete 64kb folio as atomic.
2. Other writes, dirty the whole 64kb folio.
3. Writeback sees the whole folio dirty and atomic and tries to send a 64kb atomic write, which might exceed the allowed atomic write size and fail.

Patch 7 adds support for sub-page atomic write tracking to remove this restriction. We do this by adding 2 more bitmaps to ifs to track atomic write start and end.

Lastly, a non atomic write over an atomic write will remove the atomic guarantee. Userspace is expected to make sure to sync the data to disk after an atomic write before performing any overwrites.

This series has survived -g quick xfstests and I'll be continuing to test it. Just wanted to put out the RFC to get some reviews on the design and suggestions on any better approaches.

[1] https://lore.kernel.org/all/[email protected]/
[2] https://lore.kernel.org/all/[email protected]/

Thanks,
Ojaswin

John Garry (2):
fs: Rename STATX{_ATTR}_WRITE_ATOMIC -> STATX{_ATTR}_WRITE_ATOMIC_DIO
mm: Add PG_atomic

Ojaswin Mujoo (6):
fs: Add initial buffered atomic write support info to statx
iomap: buffered atomic write support
iomap: pin pages for RWF_ATOMIC buffered write
xfs: Report atomic write min and max for buf io as well
iomap: Add bs<ps buffered atomic writes support
xfs: Lift the bs == ps restriction for HW buffered atomic writes

 .../filesystems/ext4/atomic_writes.rst        |   4 +-
 block/bdev.c                                  |   7 +-
 fs/ext4/inode.c                               |   9 +-
 fs/iomap/buffered-io.c                        | 395 ++++++++++++++++--
 fs/iomap/ioend.c                              |  21 +-
 fs/iomap/trace.h                              |  12 +-
 fs/read_write.c                               |   3 -
 fs/stat.c                                     |  33 +-
 fs/xfs/xfs_file.c                             |   9 +-
 fs/xfs/xfs_iops.c                             | 127 +++---
 fs/xfs/xfs_iops.h                             |   6 +-
 include/linux/fs.h                            |   3 +-
 include/linux/iomap.h                         |   3 +
 include/linux/page-flags.h                    |   5 +
 include/trace/events/mmflags.h                |   3 +-
 include/trace/misc/fs.h                       |   3 +-
 include/uapi/linux/stat.h                     |  10 +-
 tools/include/uapi/linux/stat.h               |  10 +-
 .../trace/beauty/include/uapi/linux/stat.h    |  10 +-
 19 files changed, 551 insertions(+), 122 deletions(-)

2.51.0