[PATCH] fuse: enable caching of symlinks

[Dan Schatzberg <dschatzberg@fb.com>]

FUSE file reads are cached in the page cache, but symlink reads are
not. This patch enables FUSE READLINK operations to be cached which
can improve performance of some FUSE workloads.

In particular, I'm working on a FUSE filesystem for access to source
code and discovered that about a 10% improvement to build times is
achieved with this patch (there are a lot of symlinks in the source
tree).

Please provide feedback, I'm looking to flesh this out more.

Signed-off-by: Dan Schatzberg <dschatzberg@fb.com>
---
 fs/fuse/dir.c | 70 +++++++++++++++++++++++++++++++--------------------
 1 file changed, 43 insertions(+), 27 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 0979609d6eba..3c0a81ef5bca 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1398,38 +1398,46 @@ static int fuse_readdir(struct file *file, struct dir_context *ctx)
 	return err;
 }
 
-static const char *fuse_get_link(struct dentry *dentry,
-				 struct inode *inode,
-				 struct delayed_call *done)
+static int fuse_symlink_readpage(struct file *file, struct page *page)
 {
+	struct inode *inode = page->mapping->host;
 	struct fuse_conn *fc = get_fuse_conn(inode);
-	FUSE_ARGS(args);
-	char *link;
-	ssize_t ret;
-
-	if (!dentry)
-		return ERR_PTR(-ECHILD);
+	struct fuse_req *req;
+	int err;
+	size_t num_read;
 
-	link = kmalloc(PAGE_SIZE, GFP_KERNEL);
-	if (!link)
-		return ERR_PTR(-ENOMEM);
+	err = -EIO;
+	if (is_bad_inode(inode))
+		goto out;
 
-	args.in.h.opcode = FUSE_READLINK;
-	args.in.h.nodeid = get_node_id(inode);
-	args.out.argvar = 1;
-	args.out.numargs = 1;
-	args.out.args[0].size = PAGE_SIZE - 1;
-	args.out.args[0].value = link;
-	ret = fuse_simple_request(fc, &args);
-	if (ret < 0) {
-		kfree(link);
-		link = ERR_PTR(ret);
-	} else {
-		link[ret] = '\0';
-		set_delayed_call(done, kfree_link, link);
+	req = fuse_get_req(fc, 1);
+	if (IS_ERR(req)) {
+		err = PTR_ERR(req);
+		goto out;
 	}
+
+	req->out.page_zeroing = 1;
+	req->out.argpages = 1;
+	req->num_pages = 1;
+	req->pages[0] = page;
+	req->page_descs[0].length = PAGE_SIZE - 1;
+	req->in.h.opcode = FUSE_READLINK;
+	req->in.h.nodeid = get_node_id(inode);
+	req->out.argvar = 1;
+	req->out.numargs = 1;
+	req->out.args[0].size = PAGE_SIZE - 1;
+	fuse_request_send(fc, req);
+	num_read = req->out.args[0].size;
+	err = req->out.h.error;
+
+	if (!err)
+		SetPageUptodate(page);
+
+	fuse_put_request(fc, req);
 	fuse_invalidate_atime(inode);
-	return link;
+out:
+	unlock_page(page);
+	return err;
 }
 
 static int fuse_dir_open(struct inode *inode, struct file *file)
@@ -1855,7 +1863,7 @@ static const struct inode_operations fuse_common_inode_operations = {
 
 static const struct inode_operations fuse_symlink_inode_operations = {
 	.setattr	= fuse_setattr,
-	.get_link	= fuse_get_link,
+	.get_link	= page_get_link,
 	.getattr	= fuse_getattr,
 	.listxattr	= fuse_listxattr,
 };
@@ -1871,7 +1879,15 @@ void fuse_init_dir(struct inode *inode)
 	inode->i_fop = &fuse_dir_operations;
 }
 
+static const struct address_space_operations fuse_symlink_aops = {
+	.readpage       = fuse_symlink_readpage,
+};
+
 void fuse_init_symlink(struct inode *inode)
 {
 	inode->i_op = &fuse_symlink_inode_operations;
+	inode->i_data.a_ops = &fuse_symlink_aops;
+	mapping_set_gfp_mask(inode->i_mapping,
+			mapping_gfp_constraint(inode->i_mapping,
+					~(__GFP_FS | __GFP_HIGHMEM)));
 }
-- 
2.17.1