[PATCH 1/2][SRU][Disco] UBUNTU: SAUCE: shiftfs: rework how shiftfs opens files

[Christian Brauner]

From: Christian Brauner <christian at brauner.io>

BugLink: https://bugs.launchpad.net/bugs/1846265

This commit simplifies how shiftfs open files, both regular files an
directories.

In the first iteration, we implemented a kmem cache for struct
shiftfs_file_info which stashed away a struct path and the struct file
for the underlay. The path however was never used anywhere so the struct
shiftfs_file_info and therefore the whole kmem cache can go away.
Instead we move to the same model as overlayfs and just stash away the
struct file for the underlay in file->private_data of the shiftfs struct
file.
Addtionally, we split the .open method for files and directories.
Similar to overlayfs .open for regular files uses open_with_fake_path()
which ensures that it doesn't contribute to the open file count (since
this would mean we'd count double). The .open method for directories
however used dentry_open() which contributes to the open file count.

The basic logic for opening files is unchanged. The main point is to
ensure that a reference to the underlay's dentry is kept through struct
path.

Various bits and pieces of this were cooked up in discussions Seth and I
had in Paris.

Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
---
 fs/shiftfs.c | 105 +++++++++++++++++++++++----------------------------
 1 file changed, 47 insertions(+), 58 deletions(-)

diff --git a/fs/shiftfs.c b/fs/shiftfs.c
index a21cb473e000..55bb32b611f2 100644
--- a/fs/shiftfs.c
+++ b/fs/shiftfs.c
@@ -31,13 +31,6 @@ struct shiftfs_super_info {
 	unsigned int passthrough_mark;
 };
 
-struct shiftfs_file_info {
-	struct path realpath;
-	struct file *realfile;
-};
-
-struct kmem_cache *shiftfs_file_info_cache;
-
 static void shiftfs_fill_inode(struct inode *inode, unsigned long ino,
 			       umode_t mode, dev_t dev, struct dentry *dentry);
 
@@ -1042,21 +1035,21 @@ static const struct inode_operations shiftfs_symlink_inode_operations = {
 };
 
 static struct file *shiftfs_open_realfile(const struct file *file,
-					  struct path *realpath)
+					  struct inode *realinode)
 {
-	struct file *lowerf;
-	const struct cred *oldcred;
+	struct file *realfile;
+	const struct cred *old_cred;
 	struct inode *inode = file_inode(file);
-	struct inode *loweri = realpath->dentry->d_inode;
+	struct dentry *lowerd = file->f_path.dentry->d_fsdata;
 	struct shiftfs_super_info *info = inode->i_sb->s_fs_info;
+	struct path realpath = { .mnt = info->mnt, .dentry = lowerd };
 
-	oldcred = shiftfs_override_creds(inode->i_sb);
-	/* XXX: open_with_fake_path() not gauranteed to stay around, if
-	 * removed use dentry_open() */
-	lowerf = open_with_fake_path(realpath, file->f_flags, loweri, info->creator_cred);
-	revert_creds(oldcred);
+	old_cred = shiftfs_override_creds(inode->i_sb);
+	realfile = open_with_fake_path(&realpath, file->f_flags, realinode,
+				       info->creator_cred);
+	revert_creds(old_cred);
 
-	return lowerf;
+	return realfile;
 }
 
 #define SHIFTFS_SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT)
@@ -1096,8 +1089,7 @@ static int shiftfs_change_flags(struct file *file, unsigned int flags)
 
 static int shiftfs_real_fdget(const struct file *file, struct fd *lowerfd)
 {
-	struct shiftfs_file_info *file_info = file->private_data;
-	struct file *realfile = file_info->realfile;
+	struct file *realfile = file->private_data;
 
 	lowerfd->flags = 0;
 	lowerfd->file = realfile;
@@ -1111,51 +1103,57 @@ static int shiftfs_real_fdget(const struct file *file, struct fd *lowerfd)
 
 static int shiftfs_open(struct inode *inode, struct file *file)
 {
-	struct shiftfs_super_info *ssi = inode->i_sb->s_fs_info;
-	struct shiftfs_file_info *file_info;
 	struct file *realfile;
-	struct path *realpath;
 
-	file_info = kmem_cache_zalloc(shiftfs_file_info_cache, GFP_KERNEL);
-	if (!file_info)
-		return -ENOMEM;
-
-	realpath = &file_info->realpath;
-	realpath->mnt = ssi->mnt;
-	realpath->dentry = file->f_path.dentry->d_fsdata;
-
-	realfile = shiftfs_open_realfile(file, realpath);
-	if (IS_ERR(realfile)) {
-		kmem_cache_free(shiftfs_file_info_cache, file_info);
+	realfile = shiftfs_open_realfile(file, inode->i_private);
+	if (IS_ERR(realfile))
 		return PTR_ERR(realfile);
-	}
 
-	file->private_data = file_info;
+	file->private_data = realfile;
 	/* For O_DIRECT dentry_open() checks f_mapping->a_ops->direct_IO. */
 	file->f_mapping = realfile->f_mapping;
 
-	file_info->realfile = realfile;
 	return 0;
 }
 
-static int shiftfs_release(struct inode *inode, struct file *file)
+static int shiftfs_dir_open(struct inode *inode, struct file *file)
 {
-	struct shiftfs_file_info *file_info = file->private_data;
+	struct file *realfile;
+	const struct cred *oldcred;
+	struct dentry *lowerd = file->f_path.dentry->d_fsdata;
+	struct shiftfs_super_info *info = inode->i_sb->s_fs_info;
+	struct path realpath = { .mnt = info->mnt, .dentry = lowerd };
+
+	oldcred = shiftfs_override_creds(file->f_path.dentry->d_sb);
+	realfile = dentry_open(&realpath, file->f_flags | O_NOATIME,
+			       info->creator_cred);
+	revert_creds(oldcred);
+	if (IS_ERR(realfile))
+		return PTR_ERR(realfile);
 
-	if (file_info) {
-		if (file_info->realfile)
-			fput(file_info->realfile);
+	file->private_data = realfile;
 
-		kmem_cache_free(shiftfs_file_info_cache, file_info);
-	}
+	return 0;
+}
+
+static int shiftfs_release(struct inode *inode, struct file *file)
+{
+	struct file *realfile = file->private_data;
+
+	if (realfile)
+		fput(realfile);
 
 	return 0;
 }
 
+static int shiftfs_dir_release(struct inode *inode, struct file *file)
+{
+	return shiftfs_release(inode, file);
+}
+
 static loff_t shiftfs_dir_llseek(struct file *file, loff_t offset, int whence)
 {
-	struct shiftfs_file_info *file_info = file->private_data;
-	struct file *realfile = file_info->realfile;
+	struct file *realfile = file->private_data;
 
 	return vfs_llseek(realfile, offset, whence);
 }
@@ -1274,8 +1272,7 @@ static int shiftfs_fsync(struct file *file, loff_t start, loff_t end,
 
 static int shiftfs_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	struct shiftfs_file_info *file_info = file->private_data;
-	struct file *realfile = file_info->realfile;
+	struct file *realfile = file->private_data;
 	const struct cred *oldcred;
 	int ret;
 
@@ -1671,8 +1668,7 @@ static int shiftfs_iterate_shared(struct file *file, struct dir_context *ctx)
 {
 	const struct cred *oldcred;
 	int err = -ENOTDIR;
-	struct shiftfs_file_info *file_info = file->private_data;
-	struct file *realfile = file_info->realfile;
+	struct file *realfile = file->private_data;
 
 	oldcred = shiftfs_override_creds(file->f_path.dentry->d_sb);
 	err = iterate_dir(realfile, ctx);
@@ -1698,13 +1694,13 @@ const struct file_operations shiftfs_file_operations = {
 };
 
 const struct file_operations shiftfs_dir_operations = {
+	.open			= shiftfs_dir_open,
+	.release		= shiftfs_dir_release,
 	.compat_ioctl		= shiftfs_compat_ioctl,
 	.fsync			= shiftfs_fsync,
 	.iterate_shared		= shiftfs_iterate_shared,
 	.llseek			= shiftfs_dir_llseek,
-	.open			= shiftfs_open,
 	.read			= generic_read_dir,
-	.release		= shiftfs_release,
 	.unlocked_ioctl		= shiftfs_ioctl,
 };
 
@@ -2106,19 +2102,12 @@ static struct file_system_type shiftfs_type = {
 
 static int __init shiftfs_init(void)
 {
-	shiftfs_file_info_cache = kmem_cache_create(
-		"shiftfs_file_info_cache", sizeof(struct shiftfs_file_info), 0,
-		SLAB_RECLAIM_ACCOUNT | SLAB_HWCACHE_ALIGN | SLAB_ACCOUNT | SLAB_MEM_SPREAD, NULL);
-	if (!shiftfs_file_info_cache)
-		return -ENOMEM;
-
 	return register_filesystem(&shiftfs_type);
 }
 
 static void __exit shiftfs_exit(void)
 {
 	unregister_filesystem(&shiftfs_type);
-	kmem_cache_destroy(shiftfs_file_info_cache);
 }
 
 MODULE_ALIAS_FS("shiftfs");
-- 
2.23.0