backing-file: always end a queued write

Writes to a loop device with direct I/O on an overlay file make the
next freeze of the upper filesystem hang for ever.

Every kiocb_start_write() needs a kiocb_end_write(). For a write that
is queued, vfs_iocb_iter_write() leaves the end to the completion, and
backing_aio_rw_complete() only calls kiocb_end_write() when IOCB_WRITE
is set in the flags copied from the submitter. In-kernel submitters
such as loop do not set that flag, so each queued write keeps the
freeze protection of the backing filesystem.

Direct writes to a loop device with direct I/O backed by an overlay
file left fsfreeze of the ext4 upper filesystem, and a later touch
there, blocked in D state. It takes write access to the node of the
loop device, which root has by default, to attach the file and to write.

The fix used here is to mark a write in struct backing_aio and to call
kiocb_end_write() on completion of every marked write.

Reproduced under qemu without KVM, on v7.3-rc6-21-g602042bf29f6 with
earlier fixes of this series applied, with and without this change,
once each. The kernel was not changed for the run. With this change
fsfreeze finished in about a second. Not run: the nvmet and LIO file
backends, which do not set IOCB_WRITE either, FUSE passthrough, and an
xfs upper filesystem.

Fixes: 2406a307ac7d ("ovl: implement async IO routines")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
[amir: use aio->is_write to explicitly mark a write aio]
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://patch.msgid.link/20261029180319.2852090-1-amir73il@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions