ext4: fix NOWAIT semantic violation in DAX extending writes

When a DAX write starts before EOF but extends past i_disksize,
ext4_write_checks() skips the IOCB_NOWAIT check because
iocb->ki_pos <= old_size. However, ext4_dax_write_iter() later calls
ext4_journal_start() to prepare for inode extension, which can sleep
waiting for journal space or transaction commit.

This violates NOWAIT semantics and can stall asynchronous I/O frameworks
like io_uring that rely on non-blocking behavior.

Fix this by checking IOCB_NOWAIT before calling ext4_journal_start()
in the extending write path. If NOWAIT is set and extension is needed,
return -EAGAIN so the caller can retry in blocking context.

Example scenario:
  - File: i_size = 1000, i_disksize = 1000
  - DAX NOWAIT write: offset = 500, count = 2000
  - ext4_write_checks(): ki_pos (500) <= old_size (1000), skip NOWAIT check
  - ext4_dax_write_iter(): offset + count (2500) > i_disksize (1000)
  - ext4_journal_start() → may sleep → violates NOWAIT

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260618125735.4156639-1-libaokun@linux.alibaba.com?part=5
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20260629113827.4074335-7-libaokun@linux.alibaba.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

0 files changed, 0 insertions, 0 deletions