[RFCv1 0/1] EXT4 support of multi-fsblock atomic write with bigalloc

["Ritesh Harjani (IBM)" <ritesh.list@gmail.com>]

This is an RFC patch before LSFMM to preview the change of how multi-fsblock atomic write
support with bigalloc look like. There is a scope of improvement in the
implementation, however this shows the general idea of the design. More details
are provided in the actual patch. There are still todos and more testing is
needed. But with iomap limitation of single fsblock atomic write now lifted,
the patch has definitely started to look better.

This is based out of vfs.all tree [1] for 6.15, which now has the necessary
iomap changes required for the bigalloc support in ext4.

TODOs:
1. Add better testcases to test atomic write support with bigalloc.
2. Discuss the approach of keeping the jbd2 txn open while zeroing the short
   underlying unwritten extents or short holes to create a single mapped type
   extent mapping. This anyway should be a non-perfomance critical path.
3. We use ext4_map_blocks() in loop instead of modifying the block allocator.
   Again since it's non-performance sensitive path, so hopefully it should ok?
   Because otherwise one can argue why take and release
   EXT4_I(inode)->i_data_sem multiple times. We won't take & release any group
   lock for this, since we know that with bigalloc the cluster is anyway
   available to us.
4. Once when we start supporting file/inode marked with atomic writes attribute,
   maybe we can add some optimizations like zero out the entire underlying
   cluster when someone forcefully wants to fzero or fpunch an underlying disk
   block, to keep the mapped extent intact.
5. Stress test of this is still pending through fsx and xfstests.

Reviews are appreciated.

[1]: https://web.git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/commit/?h=vfs.all&id=4f76518956c037517a4e4b120186075d3afb8266

Ritesh Harjani (IBM) (1):
  ext4: Add atomic write support for bigalloc

 fs/ext4/inode.c | 90 +++++++++++++++++++++++++++++++++++++++++++++++--
 fs/ext4/super.c |  8 +++--
 2 files changed, 93 insertions(+), 5 deletions(-)

--
2.48.1