[PATCH 1/2] fuse: fix FOPEN_PARALLEL_DIRECT_WRITES being ignored for passthrough writes

[Russ Fellows <russ.fellows@gmail.com>]

From: Russ Fellows <russ.fellows@gmail.com>

FOPEN_PARALLEL_DIRECT_WRITES has no effect on passthrough-backed FUSE
files due to two independent bugs that each prevent it from working.
Both must be fixed to restore parallel write concurrency.

Bug 1: fuse_passthrough_write_iter() acquires the exclusive inode lock
directly:

    inode_lock(inode);
    ret = backing_file_write_iter(...);
    inode_unlock(inode);

This serializes all concurrent writers regardless of whether the server
set FOPEN_PARALLEL_DIRECT_WRITES.  The flag is checked by
fuse_dio_wr_exclusive_lock(), called from fuse_dio_lock(), called from
fuse_direct_write_iter() -- the non-passthrough O_DIRECT path.
fuse_file_write_iter() routes passthrough opens to
fuse_passthrough_write_iter() instead, bypassing the flag check entirely.

Bug 2: fuse_file_io_open() in iomode.c strips FOPEN_PARALLEL_DIRECT_WRITES
from any open that lacks FOPEN_DIRECT_IO:

    if (!(ff->open_flags & FOPEN_DIRECT_IO))
        ff->open_flags &= ~FOPEN_PARALLEL_DIRECT_WRITES;

This is correct for regular direct-IO opens where FOPEN_DIRECT_IO ensures
O_DIRECT is actually in effect.  It is wrong for passthrough opens: a
passthrough file already bypasses the FUSE page cache by definition, so
FOPEN_DIRECT_IO is redundant and should not be required to preserve the
parallel-writes flag.

Note: adding FOPEN_DIRECT_IO to the daemon's open flags is not a valid
workaround.  fuse_file_write_iter() checks FOPEN_DIRECT_IO before
FOPEN_PASSTHROUGH, so setting both causes writes to be routed through
fuse_direct_write_iter() (requiring a userspace round-trip) instead of
fuse_passthrough_write_iter() (zero-copy kernel path).

Combined effect: a daemon that opens with FOPEN_PASSTHROUGH |
FOPEN_PARALLEL_DIRECT_WRITES (without FOPEN_DIRECT_IO) has the parallel
flag stripped by Bug 2 before Bug 1 is even reached.  Both bugs must be
fixed together.

Fix Bug 1: make fuse_dio_lock() and fuse_dio_unlock() non-static and call
them from fuse_passthrough_write_iter(), replacing the open-coded
inode_lock/inode_unlock.  This reuses the existing logic that handles
FOPEN_PARALLEL_DIRECT_WRITES, append writes, writes past EOF, and
page-cache IO mode transitions.

Fix Bug 2: skip the FOPEN_PARALLEL_DIRECT_WRITES strip when
FOPEN_PASSTHROUGH is set.  The flag remains stripped for non-passthrough
opens without FOPEN_DIRECT_IO, preserving existing behaviour.

Safety: backing_file_write_iter() calls into the backing filesystem's
write_iter (e.g. xfs_file_write_iter), which acquires the backing inode's
own lock independently.  The FUSE inode lock and the backing inode lock are
entirely separate; using inode_lock_shared on the FUSE inode does not
affect the backing filesystem's concurrency control.

Fixes: 4d99ff8f6b85 ("fuse: implement open/create with FOPEN_PASSTHROUGH")
Cc: stable@vger.kernel.org
Signed-off-by: Russ Fellows <russ.fellows@gmail.com>
---
 fs/fuse/file.c        |  6 +++---
 fs/fuse/fuse_i.h      |  2 ++
 fs/fuse/iomode.c      |  8 ++++++--
 fs/fuse/passthrough.c |  6 +++---
 4 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index f94f3dc082c6..602c3f18676e 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1428,8 +1428,8 @@ static bool fuse_dio_wr_exclusive_lock(struct kiocb *iocb, struct iov_iter *from
 	return false;
 }
 
-static void fuse_dio_lock(struct kiocb *iocb, struct iov_iter *from,
-			  bool *exclusive)
+void fuse_dio_lock(struct kiocb *iocb, struct iov_iter *from,
+		   bool *exclusive)
 {
 	struct inode *inode = file_inode(iocb->ki_filp);
 	struct fuse_inode *fi = get_fuse_inode(inode);
@@ -1455,7 +1455,7 @@ static void fuse_dio_lock(struct kiocb *iocb, struct iov_iter *from,
 	}
 }
 
-static void fuse_dio_unlock(struct kiocb *iocb, bool exclusive)
+void fuse_dio_unlock(struct kiocb *iocb, bool exclusive)
 {
 	struct inode *inode = file_inode(iocb->ki_filp);
 	struct fuse_inode *fi = get_fuse_inode(inode);
@@ -1469,7 +1469,7 @@ static void fuse_dio_unlock(struct kiocb *iocb, bool exclusive)
 	}
 }
 
-static const struct iomap_write_ops fuse_iomap_write_ops = {
+static const struct iomap_write_ops fuse_iomap_write_ops = {	/* unchanged */
 	.read_folio_range = fuse_iomap_read_folio_range,
 };
 
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 17423d4e3cfa..120de517cea0 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1541,6 +1541,8 @@ int fuse_file_io_open(struct file *file, struct inode *inode);
 void fuse_file_io_release(struct fuse_file *ff, struct inode *inode);
 
 /* file.c */
+void fuse_dio_lock(struct kiocb *iocb, struct iov_iter *from, bool *exclusive);
+void fuse_dio_unlock(struct kiocb *iocb, bool exclusive);
 struct fuse_file *fuse_file_open(struct fuse_mount *fm, u64 nodeid,
 				 unsigned int open_flags, bool isdir);
 void fuse_file_release(struct inode *inode, struct fuse_file *ff,
diff --git a/fs/fuse/iomode.c b/fs/fuse/iomode.c
index c99e285f3..b3f51e3d1 100644
--- a/fs/fuse/iomode.c
+++ b/fs/fuse/iomode.c
@@ -214,10 +214,14 @@ int fuse_file_io_open(struct file *file, struct inode *inode)
 	if (fuse_inode_backing(fi) && !(ff->open_flags & FOPEN_PASSTHROUGH))
 		goto fail;
 
-	/*
-	 * FOPEN_PARALLEL_DIRECT_WRITES requires FOPEN_DIRECT_IO.
-	 */
-	if (!(ff->open_flags & FOPEN_DIRECT_IO))
+	/*
+	 * FOPEN_PARALLEL_DIRECT_WRITES requires FOPEN_DIRECT_IO, except for
+	 * passthrough opens which bypass the page cache regardless and do not
+	 * need FOPEN_DIRECT_IO to guarantee direct I/O semantics.
+	 */
+	if (!(ff->open_flags & FOPEN_DIRECT_IO) &&
+	    !(ff->open_flags & FOPEN_PASSTHROUGH))
 		ff->open_flags &= ~FOPEN_PARALLEL_DIRECT_WRITES;
 
 	/*
diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
index f2d08ac2459b..f83d0a27cfb9 100644
--- a/fs/fuse/passthrough.c
+++ b/fs/fuse/passthrough.c
@@ -54,11 +54,11 @@ ssize_t fuse_passthrough_write_iter(struct kiocb *iocb,
 				    struct iov_iter *iter)
 {
 	struct file *file = iocb->ki_filp;
-	struct inode *inode = file_inode(file);
 	struct fuse_file *ff = file->private_data;
 	struct file *backing_file = fuse_file_passthrough(ff);
 	size_t count = iov_iter_count(iter);
 	ssize_t ret;
+	bool exclusive;
 	struct backing_file_ctx ctx = {
 		.cred = ff->cred,
 		.end_write = fuse_passthrough_end_write,
@@ -70,10 +70,10 @@ ssize_t fuse_passthrough_write_iter(struct kiocb *iocb,
 	if (!count)
 		return 0;
 
-	inode_lock(inode);
+	fuse_dio_lock(iocb, iter, &exclusive);
 	ret = backing_file_write_iter(backing_file, iter, iocb, iocb->ki_flags,
 				      &ctx);
-	inode_unlock(inode);
+	fuse_dio_unlock(iocb, exclusive);
 
 	return ret;
 }
-- 
2.51.0