[PATCH v2] fuse: use copy_splice_read() for FOPEN_DIRECT_IO splice read

[Joanne Koong <joannelkoong@gmail.com>]

When FOPEN_DIRECT_IO is set, fuse_splice_read() calls
filemap_splice_read(), which populates the pipe with pages from the fuse
inode's page cache.

This is incorrect for files opened with FOPEN_DIRECT_IO, where reads
should fetch fresh data from the server rather than using cached (and
potentially stale) data.

Use copy_splice_read() instead, which will invoke ->read_iter() on each
splice read, sending a FUSE_READ to the server every time.

We do not need to add checking for O_DIRECT since this is already
handled at the vfs layer in do_splice_read().

Fixes: 2cb1e08985e3 ("splice: Use filemap_splice_read() instead of generic_file_splice_read()")
Cc: stable@vger.kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
v1 -> v2: 
- Reword commit message to be more accurate, per Amir's feedback

v1: https://lore.kernel.org/fuse-devel/20260512200448.3818665-1-joannelkoong@gmail.com/
---
 fs/fuse/file.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index b8fd148dc109..7a35ac3e0023 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1858,7 +1858,9 @@ static ssize_t fuse_splice_read(struct file *in, loff_t *ppos,
 	struct fuse_file *ff = in->private_data;
 
 	/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
-	if (fuse_file_passthrough(ff) && !(ff->open_flags & FOPEN_DIRECT_IO))
+	if (ff->open_flags & FOPEN_DIRECT_IO)
+		return copy_splice_read(in, ppos, pipe, len, flags);
+	else if (fuse_file_passthrough(ff))
 		return fuse_passthrough_splice_read(in, ppos, pipe, len, flags);
 	else
 		return filemap_splice_read(in, ppos, pipe, len, flags);
-- 
2.52.0