fuse: add a new fuse init flag to relax restrictions in no cache mode

FOPEN_DIRECT_IO is usually set by fuse daemon to indicate need of strong
coherency, e.g. network filesystems.  Thus shared mmap is disabled since it
leverages page cache and may write to it, which may cause inconsistence.

But FOPEN_DIRECT_IO can be used not for coherency but to reduce memory
footprint as well, e.g. reduce guest memory usage with virtiofs.
Therefore, add a new fuse init flag FUSE_DIRECT_IO_RELAX to relax
restrictions in that mode, currently, it allows shared mmap.  One thing to
note is to make sure it doesn't break coherency in your use case.

Signed-off-by: Hao Xu <howeyxu@tencent.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 9b7fc7d3c7f1..d830c2360aef 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -792,6 +792,9 @@ struct fuse_conn {
 	/* Is tmpfile not implemented by fs? */
 	unsigned int no_tmpfile:1;
 
+	/* relax restrictions in FOPEN_DIRECT_IO mode */
+	unsigned int direct_io_relax:1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;