[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 3e0fdb7..d2094e1 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1324,7 +1324,7 @@ EXPORT_SYMBOL_GPL(fuse_direct_io);
 
 static ssize_t __fuse_direct_read(struct file *file, const struct iovec *iov,
 				  unsigned long nr_segs, loff_t *ppos,
-				  struct kiocb *async)
+				  struct kiocb *async, size_t count)
 {
 	ssize_t res;
 	struct inode *inode = file->f_path.dentry->d_inode;
@@ -1332,8 +1332,7 @@ static ssize_t __fuse_direct_read(struct file *file, const struct iovec *iov,
 	if (is_bad_inode(inode))
 		return -EIO;
 
-	res = fuse_direct_io(file, iov, nr_segs, iov_length(iov, nr_segs),
-			     ppos, 0, async);
+	res = fuse_direct_io(file, iov, nr_segs, count, ppos, 0, async);
 
 	fuse_invalidate_attr(inode);
 
@@ -1344,7 +1343,7 @@ static ssize_t fuse_direct_read(struct file *file, char __user *buf,
 				     size_t count, loff_t *ppos)
 {
 	struct iovec iov = { .iov_base = buf, .iov_len = count };
-	return __fuse_direct_read(file, &iov, 1, ppos, NULL);
+	return __fuse_direct_read(file, &iov, 1, ppos, NULL, count);
 }
 
 static ssize_t __fuse_direct_write(struct file *file, const struct iovec *iov,
@@ -2405,8 +2404,15 @@ 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;
+	}
+
 	/* cannot write beyond eof asynchronously */
-	if (is_sync_kiocb(iocb) || (offset + count <= i_size) || rw != WRITE) {
+	if (is_sync_kiocb(iocb) || (offset + count <= i_size)) {
 		struct fuse_io_priv *io;
 
 		io = kmalloc(sizeof(struct fuse_io_priv), GFP_KERNEL);
@@ -2429,7 +2435,8 @@ fuse_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
 	if (rw == WRITE)
 		ret = __fuse_direct_write(file, iov, nr_segs, &pos, async_cb);
 	else
-		ret = __fuse_direct_read(file, iov, nr_segs, &pos, async_cb);
+		ret = __fuse_direct_read(file, iov, nr_segs, &pos, async_cb,
+					 count);
 
 	if (async_cb) {
 		fuse_aio_complete(async_cb->private, ret == count ? 0 : -EIO,