fuse: O_DIRECT support for files

Implement ->direct_IO() method in aops. The ->direct_IO() method combines the existing fuse_direct_read/fuse_direct_write with the generic do_loop_readv/writev to neatly re-use the existing direct_io framework.

Reaching ->direct_IO() in the read path via generic_file_aio_read ensures proper synchronization with page cache with its existing framework.

Reaching ->direct_IO() in the write path via fuse_file_aio_write is made to come via generic_file_direct_write() which makes it play nice with the page cache w.r.t other mmap pages etc.

On files marked 'direct_io' by the filesystem server, IO always follows the fuse_direct_read/write path. There is no effect of fcntl(O_DIRECT) and it always succeeds.

On files not marked with 'direct_io' by the filesystem server, the IO path depends on O_DIRECT flag by the application. This can be passed at the time of open() as well as via fcntl().

Note that asynchronous O_DIRECT iocb jobs are completed synchronously always (this has been the case with FUSE even before this patch)

---
 fs/fuse/dir.c  |    3 --
 fs/fuse/file.c |   73 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
 2 files changed, 68 insertions(+), 8 deletions(-)

index 2066328..7e5dbd0 100644
@@ -387,9 +387,6 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
 	if (fc->no_create)
 		return -ENOSYS;
 
-	if (flags & O_DIRECT)
-		return -EINVAL;
-
 	forget = fuse_alloc_forget();
 	if (!forget)
 		return -ENOMEM;

1.7.6.4