[PATCH] fuse: Fix VM_MAYSHARE and direct_io_allow_mmap

[Bernd Schubert <bschubert@ddn.com>]

There were multiple issues with direct_io_allow_mmap:
- fuse_link_write_file() was missing, resulting in warnings in
  fuse_write_file_get() and EIO from msync()
- "vma->vm_ops = &fuse_file_vm_ops" was not set, but especially
  fuse_page_mkwrite is needed.

The semantics of invalidate_inode_pages2() is so far not clearly defined
in fuse_file_mmap. It dates back to
commit 3121bfe76311 ("fuse: fix "direct_io" private mmap")
Though, as direct_io_allow_mmap is a new feature, that was for MAP_PRIVATE
only. As invalidate_inode_pages2() is calling into fuse_launder_folio()
and writes out dirty pages, it should be save to call
invalidate_inode_pages2 for MAP_PRIVATE and MAP_SHARED as well.

Cc: Hao Xu <howeyxu@tencent.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Dharmendra Singh <dsingh@ddn.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: linux-fsdevel@vger.kernel.org
Cc: stable@vger.kernel.org
Fixes: e78662e818f9 ("fuse: add a new fuse init flag to relax restrictions in no cache mode")
Signed-off-by: Bernd Schubert <bschubert@ddn.com>
---
 fs/fuse/file.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index a660f1f21540..174aa16407c4 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -2475,7 +2475,10 @@ static int fuse_file_mmap(struct file *file, struct vm_area_struct *vma)
 
 		invalidate_inode_pages2(file->f_mapping);
 
-		return generic_file_mmap(file, vma);
+		if (!(vma->vm_flags & VM_MAYSHARE)) {
+			/* MAP_PRIVATE */
+			return generic_file_mmap(file, vma);
+		}
 	}
 
 	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE))
-- 
2.40.1