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)

---

Sending out this patch for early comments. This patch is STILL UNTESTED. Intention is to get comments and feedback on a couple of points

0. General approach taken by the patch. Are there any obvious considerations left out?

1. including "../read_write.h" and usage of its do_loop_read_write. There is a real good use of it inside ->direct_IO() but it looks like an ugly way to include the header for the prototypes. Should I be writing my own helper or extern'ing the prototype definition instead of #include or move the prototypes (or read_write.h) to include/linux/ ? Or just leave it like this?

 fs/fuse/dir.c  |    3 ---
 fs/fuse/file.c |   43 ++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 38 insertions(+), 8 deletions(-)

index d5016071..4271f04 100644
@@ -387,9 +387,6 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry, int mode,
 	if (fc->no_create)
 		return -ENOSYS;
 
-	if (flags & O_DIRECT)
-		return -EINVAL;
-
 	forget = fuse_alloc_forget();
 	if (!forget)
 		return -ENOMEM;

1.7.4.4