[PATCH v4 6/6] ext4: fix NOWAIT semantic violation in DAX extending writes

[Baokun Li <libaokun@linux.alibaba.com>]

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>
---
 fs/ext4/file.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 0e9448a110dc..9a16071b719d 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -725,6 +725,11 @@ ext4_dax_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	count = iov_iter_count(from);
 
 	if (offset + count > EXT4_I(inode)->i_disksize) {
+		if (iocb->ki_flags & IOCB_NOWAIT) {
+			ret = -EAGAIN;
+			goto out;
+		}
+
 		handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
 		if (IS_ERR(handle)) {
 			ret = PTR_ERR(handle);
-- 
2.43.7