[PATCH v13 07/10] fuse: Introduce passthrough for mmap

[Amir Goldstein <amir73il@gmail.com>]

Enabling FUSE passthrough for mmap-ed operations not only affects
performance, but has also been shown as mandatory for the correct
functioning of FUSE passthrough.

yanwu noticed [1] that a FUSE file with passthrough enabled may suffer
data inconsistencies if the same file is also accessed with mmap.
What happens is that read/write operations are directly applied to the
backing file's page cache, while mmap-ed operations are affecting the
FUSE file's page cache.

Extend the FUSE passthrough implementation to also handle memory-mapped
FUSE file, to both fix the cache inconsistencies and extend the
passthrough performance benefits to mmap-ed operations.

[1] https://lore.kernel.org/lkml/20210119110654.11817-1-wu-yan@tcl.com/

Signed-off-by: Alessio Balsini <balsini@android.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/fuse/file.c        |  3 +++
 fs/fuse/fuse_i.h      |  1 +
 fs/fuse/passthrough.c | 27 +++++++++++++++++++++++++++
 3 files changed, 31 insertions(+)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 24d37681ddcd..80e20bae569f 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -2521,6 +2521,9 @@ static int fuse_file_mmap(struct file *file, struct vm_area_struct *vma)
 	if (FUSE_IS_DAX(file_inode(file)))
 		return fuse_dax_mmap(file, vma);
 
+	if (ff->passthrough)
+		return fuse_passthrough_mmap(file, vma);
+
 	if (ff->open_flags & FOPEN_DIRECT_IO) {
 		/* Can't provide the coherency needed for MAP_SHARED */
 		if (vma->vm_flags & VM_MAYSHARE)
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 61a3968cfc8f..238a43349298 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1356,5 +1356,6 @@ void fuse_passthrough_free(struct fuse_passthrough *passthrough);
 
 ssize_t fuse_passthrough_read_iter(struct kiocb *iocb, struct iov_iter *to);
 ssize_t fuse_passthrough_write_iter(struct kiocb *iocb, struct iov_iter *from);
+ssize_t fuse_passthrough_mmap(struct file *file, struct vm_area_struct *vma);
 
 #endif /* _FS_FUSE_I_H */
diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
index 06c6926aa85a..10b370bcc423 100644
--- a/fs/fuse/passthrough.c
+++ b/fs/fuse/passthrough.c
@@ -129,6 +129,33 @@ ssize_t fuse_passthrough_write_iter(struct kiocb *iocb_fuse,
 	return ret;
 }
 
+ssize_t fuse_passthrough_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	int ret;
+	const struct cred *old_cred;
+	struct fuse_file *ff = file->private_data;
+	struct file *passthrough_filp = ff->passthrough->filp;
+
+	if (!passthrough_filp->f_op->mmap)
+		return -ENODEV;
+
+	if (WARN_ON(file != vma->vm_file))
+		return -EIO;
+
+	vma->vm_file = get_file(passthrough_filp);
+
+	old_cred = override_creds(ff->passthrough->cred);
+	ret = call_mmap(vma->vm_file, vma);
+	revert_creds(old_cred);
+
+	if (ret)
+		fput(passthrough_filp);
+	else
+		fput(file);
+
+	return ret;
+}
+
 /*
  * Returns passthrough_fh id that can be passed with FOPEN_PASSTHROUGH
  * open response and needs to be released with fuse_passthrough_close().
-- 
2.34.1