[PATCH v14 11/12] fuse: implement 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 | 16 ++++++++++++++++
 3 files changed, 20 insertions(+)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index d6fc99245a61..bae1137426a9 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -2499,6 +2499,9 @@ 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 (fuse_file_passthrough(ff))
+		return fuse_passthrough_mmap(file, vma);
+
 	if (ff->open_flags & FOPEN_DIRECT_IO) {
 		/* Can't provide the coherency needed for MAP_SHARED
 		 * if FUSE_DIRECT_IO_RELAX isn't set.
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 7dd770efceba..6fee4c33678f 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1403,5 +1403,6 @@ ssize_t fuse_passthrough_splice_read(struct file *in, loff_t *ppos,
 ssize_t fuse_passthrough_splice_write(struct pipe_inode_info *pipe,
 				      struct file *out, loff_t *ppos,
 				      size_t len, unsigned int flags);
+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 fa5b41a4cdc1..a05280ceba83 100644
--- a/fs/fuse/passthrough.c
+++ b/fs/fuse/passthrough.c
@@ -148,6 +148,22 @@ ssize_t fuse_passthrough_splice_write(struct pipe_inode_info *pipe,
 	return ret;
 }
 
+ssize_t fuse_passthrough_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct fuse_file *ff = file->private_data;
+	struct file *backing_file = fuse_file_passthrough(ff);
+	struct backing_file_ctx ctx = {
+		.cred = ff->cred,
+		.user_file = file,
+		.accessed = fuse_file_accessed,
+	};
+
+	pr_debug("%s: backing_file=0x%p, start=%lu, end=%lu\n", __func__,
+		 backing_file, vma->vm_start, vma->vm_end);
+
+	return backing_file_mmap(backing_file, vma, &ctx);
+}
+
 struct fuse_backing *fuse_backing_get(struct fuse_backing *fb)
 {
 	if (fb && refcount_inc_not_zero(&fb->count))
-- 
2.34.1