[PATCH v2 05/21] fuse: prepare for long lived reference on backing file

[Joanne Koong <joannelkoong@gmail.com>]

From: Amir Goldstein <amir73il@gmail.com>

Currently backing file is attached to fuse inode on the first
passthrough open of the inode and detached on last passthrough close.

In preparation for attaching a backing file to inode with no open file,
allow attaching a single long lived reference on fuse inode backing file
that will be used for passthrough of inode operations and detached on
fuse inode evict.

Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/fuse/file.c   |  3 ++-
 fs/fuse/fuse_i.h |  5 +++--
 fs/fuse/inode.c  |  5 +++++
 fs/fuse/iomode.c | 16 ++++++++++++----
 4 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 828e3fc0ec31..1173811b2ea7 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1442,6 +1442,7 @@ static void fuse_dio_lock(struct kiocb *iocb, struct iov_iter *from,
 			  bool *exclusive)
 {
 	struct inode *inode = file_inode(iocb->ki_filp);
+	struct fuse_file *ff = iocb->ki_filp->private_data;
 
 	*exclusive = fuse_dio_wr_exclusive_lock(iocb, from);
 	if (*exclusive) {
@@ -1456,7 +1457,7 @@ static void fuse_dio_lock(struct kiocb *iocb, struct iov_iter *from,
 		 * have raced, so check it again.
 		 */
 		if (fuse_io_past_eof(iocb, from) ||
-		    fuse_inode_uncached_io_start(inode, NULL) != 0) {
+		    fuse_inode_uncached_io_start(inode, ff, NULL) != 0) {
 			inode_unlock_shared(inode);
 			inode_lock(inode);
 			*exclusive = true;
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 23d2510f8e93..502b3b3f7e89 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -258,7 +258,7 @@ enum {
 	 * or the fuse server has an exclusive "lease" on distributed fs
 	 */
 	FUSE_I_EXCLUSIVE,
-	/* Has backing file for inode ops passthrough */
+	/* Has long lived backing file for inode ops passthrough */
 	FUSE_I_PASSTHROUGH,
 };
 
@@ -1246,7 +1246,8 @@ int fuse_fileattr_set(struct mnt_idmap *idmap,
 
 /* iomode.c */
 int fuse_file_cached_io_open(struct inode *inode, struct fuse_file *ff);
-int fuse_inode_uncached_io_start(struct inode *inode, struct fuse_backing *fb);
+int fuse_inode_uncached_io_start(struct inode *inode, struct fuse_file *ff,
+				 struct fuse_backing *fb);
 void fuse_inode_uncached_io_end(struct inode *inode);
 
 int fuse_file_io_open(struct file *file, struct inode *inode);
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index cef8b853b3d8..9e9c20e846e7 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -183,6 +183,11 @@ static void fuse_evict_inode(struct inode *inode)
 		WARN_ON(!list_empty(&fi->write_files));
 		WARN_ON(!list_empty(&fi->queued_writes));
 	}
+	/* fuse inode may have a long lived reference to backing file */
+	if (fuse_inode_backing(fi)) {
+		WARN_ON(!test_bit(FUSE_I_PASSTHROUGH, &fi->state));
+		fuse_inode_uncached_io_end(inode);
+	}
 	WARN_ON(fi->iocachectr != 0);
 }
 
diff --git a/fs/fuse/iomode.c b/fs/fuse/iomode.c
index c83a070cd834..6e2ddfe384b3 100644
--- a/fs/fuse/iomode.c
+++ b/fs/fuse/iomode.c
@@ -87,7 +87,8 @@ static void fuse_file_cached_io_release(struct fuse_file *ff,
 }
 
 /* Start strictly uncached io mode where cache access is not allowed */
-int fuse_inode_uncached_io_start(struct inode *inode, struct fuse_backing *fb)
+int fuse_inode_uncached_io_start(struct inode *inode, struct fuse_file *ff,
+				 struct fuse_backing *fb)
 {
 	struct fuse_inode *fi = get_fuse_inode(inode);
 	struct fuse_conn *fc = get_fuse_conn(inode);
@@ -116,12 +117,19 @@ int fuse_inode_uncached_io_start(struct inode *inode, struct fuse_backing *fb)
 		err = -ETXTBSY;
 		goto unlock;
 	}
-	fi->iocachectr--;
+	/* every open file holds a single refcount of backing file... */
+	if (ff)
+		fi->iocachectr--;
 
-	/* fuse inode holds a single refcount of backing file */
 	if (fb && !oldfb) {
 		oldfb = fuse_inode_backing_set(fi, fb);
 		WARN_ON_ONCE(oldfb != NULL);
+		/* ...and an optional extra refcount for inode ops */
+		if ((fb->ops_mask & FUSE_PASSTHROUGH_INODE_OPS)) {
+			WARN_ON_ONCE(test_bit(FUSE_I_PASSTHROUGH, &fi->state));
+			set_bit(FUSE_I_PASSTHROUGH, &fi->state);
+			fi->iocachectr--;
+		}
 	} else {
 		fuse_backing_put(fb);
 	}
@@ -137,7 +145,7 @@ static int fuse_file_uncached_io_open(struct inode *inode,
 {
 	int err;
 
-	err = fuse_inode_uncached_io_start(inode, fb);
+	err = fuse_inode_uncached_io_start(inode, ff, fb);
 	if (err)
 		return err;
 
-- 
2.52.0