ext4: Add atomic writes support for DIO

v3 -> v4:
=========
1. Patch-1 changes the helper function from ext4_can_atomic_write() to ext4_inode_can_atomic_write() based on suggestions from Darrick and John. Patch-3 then goes and uses it.
2. Patch-4 adds an inline helper ext4_want_directio_fallback() which simplifies the logic checks and inherently fixes condition on when to return -ENOTBLK which otherwise was always returning true for any write or directio in ext4_iomap_end(). It was ok since ext4 only supports direct-io via iomap.
[v3]: https://lore.kernel.org/linux-xfs/[email protected]/

(Note as mentioned below as well this is built on top of John XFS atomic series [2]) [2]: https://lore.kernel.org/linux-xfs/[email protected]/

v2 -> v3:
==========
1. Patch-1 adds an "experimental" string in dmesg log during mount when EXT4 detects that it is capable of doing DIO atomic writes on a given device with min and max unit details.
2. Patch-4 has been updated to avoid returning -ENOTBLK (in ext4_iomap_end) if the request belongs to atomic write. This patch also adds a WARN_ON_ONCE() if atomic write ever fallback to buffered-io (to catch any unwanted bugs in the future). More details in the commit log of patch-4.
3. Collected RBs tag from John for Patch 2 & 3.

[v2]: https://lore.kernel.org/linux-ext4/[email protected]/

Previous cover letter log:

In v2, we had split the series and this one only takes care of atomic writes for single fsblock.
That means for now this gets only enabled on bs < ps systems on ext4. Enablement of atomic writes for bigalloc (multi-fsblock support) is still under discussion and may require general consensus within the filesystem community [1].

This series adds the base feature support to enable atomic writes in direct-io path for ext4. We advertise the minimum and the maximum atomic write unit sizes via statx on a regular file.

This series allows users to utilize atomic write support using -
1. on bs < ps systems via - mkfs.ext4 -F -b 16384 /dev/sda

This can then be utilized using -

	xfs_io -fdc "pwrite -V 1 -A -b16k 0 16k" /mnt/f1

This is built on top of John's DIO atomic write series for XFS [2]. The VFS and block layer enablement for atomic writes were merged already.

[1]: https://lore.kernel.org/linux-ext4/[email protected]
[2]: https://lore.kernel.org/linux-xfs/[email protected]/

Changelogs:
===========
PATCH -> PATCH v2:
- addressed review comments from John and Darrick.
- renamed ext4_sb_info variables names: fs_awu* -> s_awu*
- [PATCH]: https://lore.kernel.org/linux-ext4/[email protected]/

RFC -> PATCH:
- Dropped RFC tag
- Last RFC was posted a while ago but back then a lot of VFS and block layer interfaces were still not merged. Those are now merged, thanks to John and everyone else.
- [RFC] - https://lore.kernel.org/linux-ext4/[email protected]/

Ritesh Harjani (IBM) (4):
ext4: Add statx support for atomic writes
ext4: Check for atomic writes support in write iter
ext4: Support setting FMODE_CAN_ATOMIC_WRITE
ext4: Do not fallback to buffered-io for DIO atomic write

 fs/ext4/ext4.h  | 10 ++++++++++
 fs/ext4/file.c  | 24 ++++++++++++++++++++++++
 fs/ext4/inode.c | 39 ++++++++++++++++++++++++++++++++++-----
 fs/ext4/super.c | 31 +++++++++++++++++++++++++++++++
 4 files changed, 99 insertions(+), 5 deletions(-)

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2.46.0