From 802b8aa8ff6b5877610a62b99b54d84980dcb8b0 Mon Sep 17 00:00:00 2001 From: Baokun Li Date: Mon, 29 Jun 2026 19:38:27 +0800 Subject: [PATCH] ext4: fix NOWAIT semantic violation in DAX extending writes MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Closes: https://sashiko.dev/#/patchset/20260618125735.4156639-1-libaokun@linux.alibaba.com?part=5 Reviewed-by: Jan Kara Signed-off-by: Baokun Li Reviewed-by: Zhang Yi Link: https://patch.msgid.link/20260629113827.4074335-7-libaokun@linux.alibaba.com Signed-off-by: Theodore Ts'o --- 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);