[RFC PATCH 1/2] fs: Add dirreadahead syscall and VFS hooks

[Abhi Das]

Also adds a void *opaque field to struct dir_context that can be
used by filesystems to temporarily store any context as this
struct gets passed around in the fs.

Signed-off-by: Abhi Das <adas@xxxxxxxxxx>
---
 arch/x86/syscalls/syscall_32.tbl |  1 +
 arch/x86/syscalls/syscall_64.tbl |  1 +
 fs/readdir.c                     | 49 ++++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h               |  3 +++
 4 files changed, 54 insertions(+)

diff --git a/arch/x86/syscalls/syscall_32.tbl b/arch/x86/syscalls/syscall_32.tbl
index d6b8679..3e0ef85 100644
--- a/arch/x86/syscalls/syscall_32.tbl
+++ b/arch/x86/syscalls/syscall_32.tbl
@@ -360,3 +360,4 @@
 351	i386	sched_setattr		sys_sched_setattr
 352	i386	sched_getattr		sys_sched_getattr
 353	i386	renameat2		sys_renameat2
+354	i386	dirreadahead		sys_dirreadahead
diff --git a/arch/x86/syscalls/syscall_64.tbl b/arch/x86/syscalls/syscall_64.tbl
index ec255a1..2ec0991 100644
--- a/arch/x86/syscalls/syscall_64.tbl
+++ b/arch/x86/syscalls/syscall_64.tbl
@@ -323,6 +323,7 @@
 314	common	sched_setattr		sys_sched_setattr
 315	common	sched_getattr		sys_sched_getattr
 316	common	renameat2		sys_renameat2
+317	common	dirreadahead		sys_dirreadahead
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/fs/readdir.c b/fs/readdir.c
index 33fd922..d216db7 100644
--- a/fs/readdir.c
+++ b/fs/readdir.c
@@ -198,6 +198,7 @@ SYSCALL_DEFINE3(getdents, unsigned int, fd,
 	struct linux_dirent __user * lastdirent;
 	struct getdents_callback buf = {
 		.ctx.actor = filldir,
+		.ctx.opaque = NULL,
 		.count = count,
 		.current_dir = dirent
 	};
@@ -278,6 +279,7 @@ SYSCALL_DEFINE3(getdents64, unsigned int, fd,
 	struct linux_dirent64 __user * lastdirent;
 	struct getdents_callback64 buf = {
 		.ctx.actor = filldir64,
+		.ctx.opaque = NULL,
 		.count = count,
 		.current_dir = dirent
 	};
@@ -304,3 +306,50 @@ SYSCALL_DEFINE3(getdents64, unsigned int, fd,
 	fdput(f);
 	return error;
 }
+
+SYSCALL_DEFINE3(dirreadahead, unsigned int, fd,
+		loff_t __user *, offset, unsigned int, count)
+{
+	struct fd f;
+	struct inode *inode;
+	int error = -ENOTDIR;
+	loff_t off = 0;
+	struct dir_context ctx = {.actor = NULL, .opaque = NULL};
+
+	if (!count)
+		return -EINVAL;
+
+	f = fdget(fd);
+	if (!f.file)
+		return -EBADF;
+
+	inode = f.file->f_path.dentry->d_inode;
+
+	error = -ENOTSUPP;
+	if (!f.file->f_op || !f.file->f_op->dir_readahead)
+		goto out;
+
+	error = security_file_permission(f.file, MAY_READ);
+	if (error)
+		goto out;
+
+	error = -EFAULT;
+	if (__get_user(ctx.pos, offset))
+		goto out;
+
+	error = mutex_lock_killable(&inode->i_mutex);
+	if (error)
+		goto out;
+
+	error = -ENOENT;
+	if (!IS_DEADDIR(inode)) {
+		error = f.file->f_op->dir_readahead(f.file, &ctx, count);
+		if (__put_user(ctx.pos, offset))
+			error = -EFAULT;
+		file_accessed(f.file);
+	}
+	mutex_unlock(&inode->i_mutex);
+out:
+	fdput(f);
+	return error;
+}
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 338e6f7..fae4a6e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1438,9 +1438,11 @@ int fiemap_check_flags(struct fiemap_extent_info *fieinfo, u32 fs_flags);
  * to have different dirent layouts depending on the binary type.
  */
 typedef int (*filldir_t)(void *, const char *, int, loff_t, u64, unsigned);
+
 struct dir_context {
 	const filldir_t actor;
 	loff_t pos;
+	void *opaque;
 };
 
 struct block_device_operations;
@@ -1463,6 +1465,7 @@ struct file_operations {
 	ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
 	ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
 	int (*iterate) (struct file *, struct dir_context *);
+	int (*dir_readahead) (struct file *, struct dir_context *, unsigned int);
 	unsigned int (*poll) (struct file *, struct poll_table_struct *);
 	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
 	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
-- 
1.8.1.4

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