[PATCH v2] 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).
---
Changes in v2:
  - Gated behind INIT flag for backwards compatibility

 fs/fuse/dir.c             | 67 +++++++++++++++++++++++++++++++++++++--
 fs/fuse/fuse_i.h          |  3 ++
 fs/fuse/inode.c           |  4 ++-
 include/uapi/linux/fuse.h |  7 +++-
 4 files changed, 76 insertions(+), 5 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 0979609d6eba..ce23fb6ac5f4 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1398,11 +1398,53 @@ 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);
+	struct fuse_req *req;
+	int err;
+	size_t num_read;
+
+	err = -EIO;
+	if (is_bad_inode(inode))
+		goto out;
+
+	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);
+out:
+	unlock_page(page);
+	return err;
+}
+
+static const char *fuse_get_link_nocache(struct dentry *dentry,
+					 struct inode *inode,
+					 struct delayed_call *done,
+					 struct fuse_conn *fc)
+{
 	FUSE_ARGS(args);
 	char *link;
 	ssize_t ret;
@@ -1432,6 +1474,17 @@ static const char *fuse_get_link(struct dentry *dentry,
 	return link;
 }
 
+static const char *fuse_get_link(struct dentry *dentry,
+				 struct inode *inode,
+				 struct delayed_call *done)
+{
+	struct fuse_conn *fc = get_fuse_conn(inode);
+	if (fc->cache_symlinks)
+		return page_get_link(dentry, inode, done);
+	else
+		return fuse_get_link_nocache(dentry, inode, done, fc);
+}
+
 static int fuse_dir_open(struct inode *inode, struct file *file)
 {
 	return fuse_open_common(inode, file, true);
@@ -1871,7 +1924,15 @@ 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)));
 }
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index f78e9614bb5f..50f1d1e96011 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -551,6 +551,9 @@ struct fuse_conn {
 	/** handle fs handles killing suid/sgid/cap on write/chown/trunc */
 	unsigned handle_killpriv:1;
 
+	/** cache READLINK responses in page cache */
+	unsigned cache_symlinks:1;
+
 	/*
 	 * The following bitfields are only for optimization purposes
 	 * and hence races in setting them will not cause malfunction
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index db9e60b7eb69..0d8c2ee01308 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -926,6 +926,8 @@ static void process_init_reply(struct fuse_conn *fc, struct fuse_req *req)
 			}
 			if (arg->flags & FUSE_ABORT_ERROR)
 				fc->abort_err = 1;
+			if (arg->flags & FUSE_CACHE_SYMLINKS)
+				fc->cache_symlinks = 1;
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -957,7 +959,7 @@ static void fuse_send_init(struct fuse_conn *fc, struct fuse_req *req)
 		FUSE_DO_READDIRPLUS | FUSE_READDIRPLUS_AUTO | FUSE_ASYNC_DIO |
 		FUSE_WRITEBACK_CACHE | FUSE_NO_OPEN_SUPPORT |
 		FUSE_PARALLEL_DIROPS | FUSE_HANDLE_KILLPRIV | FUSE_POSIX_ACL |
-		FUSE_ABORT_ERROR;
+		FUSE_ABORT_ERROR | FUSE_CACHE_SYMLINKS;
 	req->in.h.opcode = FUSE_INIT;
 	req->in.numargs = 1;
 	req->in.args[0].size = sizeof(*arg);
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 92fa24c24c92..348a38019646 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -116,6 +116,9 @@
  *
  *  7.27
  *  - add FUSE_ABORT_ERROR
+ *
+ *  7.28
+ *  - add FUSE_CACHE_SYMLINKS
  */
 
 #ifndef _LINUX_FUSE_H
@@ -151,7 +154,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 27
+#define FUSE_KERNEL_MINOR_VERSION 28
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -249,6 +252,7 @@ struct fuse_file_lock {
  * FUSE_HANDLE_KILLPRIV: fs handles killing suid/sgid/cap on write/chown/trunc
  * FUSE_POSIX_ACL: filesystem supports posix acls
  * FUSE_ABORT_ERROR: reading the device after abort returns ECONNABORTED
+ * FUSE_CACHE_SYMLINKS: READLINK responses are cached in the page cache
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -272,6 +276,7 @@ struct fuse_file_lock {
 #define FUSE_HANDLE_KILLPRIV	(1 << 19)
 #define FUSE_POSIX_ACL		(1 << 20)
 #define FUSE_ABORT_ERROR	(1 << 21)
+#define FUSE_CACHE_SYMLINKS     (1 << 22)
 
 /**
  * CUSE INIT request/reply flags
-- 
2.17.1