[PATCH 6.1] backing-file: always end a queued write

[Amir Goldstein <amir73il@gmail.com>]

From: Chris Mason <mason@kernel.org>

commit 627cdcbc2a2eceb0452078a0c14370f9f1f17a91 upstream.

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>
[backport: no fs/backing-file.c, mark the write in struct ovl_aio_req and
 test it in ovl_aio_cleanup_handler()]
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/overlayfs/file.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -19,6 +19,7 @@ struct ovl_aio_req {
 	struct kiocb iocb;
 	refcount_t ref;
 	struct kiocb *orig_iocb;
+	bool is_write;
 };
 
 static struct kmem_cache *ovl_aio_request_cachep;
@@ -269,7 +270,7 @@ static void ovl_aio_cleanup_handler(struct ovl_aio_req *aio_req)
 	struct kiocb *iocb = &aio_req->iocb;
 	struct kiocb *orig_iocb = aio_req->orig_iocb;
 
-	if (iocb->ki_flags & IOCB_WRITE) {
+	if (aio_req->is_write) {
 		struct inode *inode = file_inode(orig_iocb->ki_filp);
 
 		/* Actually acquired in ovl_write_iter() */
@@ -396,6 +397,7 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 				     SB_FREEZE_WRITE);
 		real.flags = 0;
 		aio_req->orig_iocb = iocb;
+		aio_req->is_write = true;
 		kiocb_clone(&aio_req->iocb, iocb, get_file(real.file));
 		aio_req->iocb.ki_flags = ifl;
 		aio_req->iocb.ki_complete = ovl_aio_rw_complete;
-- 
backport