[RFC] fuse: O_DIRECT support for files

[RFC] fuse: O_DIRECT support for files

[Anand Avati]

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)

Signed-off-by: Anand Avati <avati@gluster.com>
---

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(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index d5016071..4271f04 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -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;
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 82a6646..205696a 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -14,6 +14,7 @@
 #include <linux/sched.h>
 #include <linux/module.h>
 #include <linux/compat.h>
+#include "../read_write.h" /* for io_fn_t and do_loop_readv_writev */
 
 static const struct file_operations fuse_direct_io_file_operations;
 
@@ -193,10 +194,6 @@ int fuse_open_common(struct inode *inode, struct file *file, bool isdir)
 	struct fuse_conn *fc = get_fuse_conn(inode);
 	int err;
 
-	/* VFS checks this, but only _after_ ->open() */
-	if (file->f_flags & O_DIRECT)
-		return -EINVAL;
-
 	err = generic_file_open(inode, file);
 	if (err)
 		return err;
@@ -979,6 +976,7 @@ static ssize_t fuse_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
 	struct file *file = iocb->ki_filp;
 	struct address_space *mapping = file->f_mapping;
 	size_t count = 0;
+	size_t ocount = 0;
 	ssize_t written = 0;
 	struct inode *inode = mapping->host;
 	ssize_t err;
@@ -986,10 +984,13 @@ static ssize_t fuse_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
 
 	WARN_ON(iocb->ki_pos != pos);
 
-	err = generic_segment_checks(iov, &nr_segs, &count, VERIFY_READ);
+	ocount = 0;
+	err = generic_segment_checks(iov, &nr_segs, &ocount, VERIFY_READ);
 	if (err)
 		return err;
 
+	count = ocount;
+
 	mutex_lock(&inode->i_mutex);
 	vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
 
@@ -1009,6 +1010,13 @@ static ssize_t fuse_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
 
 	file_update_time(file);
 
+	if (file->f_flags & O_DIRECT) {
+		written = generic_file_direct_write(iocb, iov, &nr_segs,
+						    pos, &iocb->ki_pos,
+						    count, ocount);
+		goto out;
+	}
+
 	iov_iter_init(&i, iov, nr_segs, count, 0);
 	written = fuse_perform_write(file, mapping, &i, pos);
 	if (written >= 0)
@@ -2132,6 +2140,30 @@ int fuse_notify_poll_wakeup(struct fuse_conn *fc,
 	return 0;
 }
 
+static ssize_t
+fuse_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
+			loff_t offset, unsigned long nr_segs)
+{
+	ssize_t ret = 0;
+	struct file *file = NULL;
+	loff_t pos = 0;
+	io_fn_t fn = NULL;
+	struct iov *iovp = NULL;
+
+	file = iocb->ki_filp;
+	pos = offset;
+	iovp = (struct iovec *) iov; /* non const */
+
+	if (rw == WRITE)
+		fn = (io_fn_t)fuse_direct_write;
+	else
+		fn = fuse_direct_read;
+
+	ret = do_loop_readv_writev(file, iovp, nr_segs, &pos, fn);
+
+	return ret;
+}
+
 static const struct file_operations fuse_file_operations = {
 	.llseek		= fuse_file_llseek,
 	.read		= do_sync_read,
@@ -2177,6 +2209,7 @@ static const struct address_space_operations fuse_file_aops  = {
 	.readpages	= fuse_readpages,
 	.set_page_dirty	= __set_page_dirty_nobuffers,
 	.bmap		= fuse_bmap,
+	.direct_IO	= fuse_direct_IO,
 };
 
 void fuse_init_file_inode(struct inode *inode)
-- 
1.7.4.4