[Devel] [PATCH 6/6] fuse: optimize short direct reads

[Maxim V. Patlasov]

If user requested direct read beyond EOF, we can skip sending fuse requests
for positions beyond EOF because userspace would ACK them with zero bytes read
anyway. We can trust to i_size in fuse_direct_IO for such cases because it's
called from fuse_file_aio_read() and the latter updates fuse attributes
including i_size.

Signed-off-by: Maxim Patlasov <mpatlasov at parallels.com>
---
 fs/fuse/file.c |   19 +++++++++++++------
 1 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index b6e9b8d..ceacd20 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1322,7 +1322,8 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, const struct iovec *iov,
 EXPORT_SYMBOL_GPL(fuse_direct_io);
 
 static ssize_t __fuse_direct_read(struct fuse_io_priv *io, const struct iovec *iov,
-				  unsigned long nr_segs, loff_t *ppos)
+				  unsigned long nr_segs, loff_t *ppos,
+				  size_t count)
 {
 	ssize_t res;
 	struct file *file = io->file;
@@ -1331,8 +1332,7 @@ static ssize_t __fuse_direct_read(struct fuse_io_priv *io, const struct iovec *i
 	if (is_bad_inode(inode))
 		return -EIO;
 
-	res = fuse_direct_io(io, iov, nr_segs, iov_length(iov, nr_segs),
-			     ppos, 0);
+	res = fuse_direct_io(io, iov, nr_segs, count, ppos, 0);
 
 	fuse_invalidate_attr(inode);
 
@@ -1344,7 +1344,7 @@ static ssize_t fuse_direct_read(struct file *file, char __user *buf,
 {
 	struct fuse_io_priv io = { .async = 0, .file = file };
 	struct iovec iov = { .iov_base = buf, .iov_len = count };
-	return __fuse_direct_read(&io, &iov, 1, ppos);
+	return __fuse_direct_read(&io, &iov, 1, ppos, count);
 }
 
 static ssize_t __fuse_direct_write(struct fuse_io_priv *io, const struct iovec *iov,
@@ -2404,6 +2404,13 @@ fuse_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
 	inode = file->f_mapping->host;
 	i_size = i_size_read(inode);
 
+	/* optimization for short read */
+	if (rw != WRITE && offset + count > i_size) {
+		if (offset >= i_size)
+			return 0;
+		count = i_size - offset;
+	}
+
 	io = kmalloc(sizeof(struct fuse_io_priv), GFP_KERNEL);
 	if (!io)
 		return -ENOMEM;
@@ -2427,13 +2434,13 @@ fuse_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
 	 * to wait on real async I/O requests, so we must submit this request
 	 * synchronously.
 	 */
-	if (!is_sync_kiocb(iocb) && (offset + count > i_size) && rw == WRITE)
+	if (!is_sync_kiocb(iocb) && (offset + count > i_size))
 		io->async = 0;
 
 	if (rw == WRITE)
 		ret = __fuse_direct_write(io, iov, nr_segs, &pos);
 	else
-		ret = __fuse_direct_read(io, iov, nr_segs, &pos);
+		ret = __fuse_direct_read(io, iov, nr_segs, &pos, count);
 
 	if (io->async) {
 		fuse_aio_complete(io, ret == count ? 0 : -EIO, -1);