Re: [PATCH 2/3] vfs_getdents/struct dir_context: add flags field

[Christian Brauner <brauner@kernel.org>]

On Tue, Jul 11, 2023 at 07:40:26PM +0800, Hao Xu wrote:
> From: Hao Xu <howeyxu@tencent.com>
> 
> The flags will allow passing DIR_CONTEXT_F_NOWAIT to iterate()
> implementations that support it (as signaled through FMODE_NWAIT
> in file->f_mode)
> 
> Notes:
> - considered using IOCB_NOWAIT but if we add more flags later it
> would be confusing to keep track of which values are valid, use
> dedicated flags
> - might want to check ctx.flags & DIR_CONTEXT_F_NOWAIT is only set
> when file->f_mode & FMODE_NOWAIT in iterate_dir() as e.g. WARN_ONCE?
> 
> Co-developed-by: Dominique Martinet <asmadeus@codewreck.org>
> Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> Signed-off-by: Hao Xu <howeyxu@tencent.com>
> ---
>  fs/internal.h      | 2 +-
>  fs/readdir.c       | 6 ++++--
>  include/linux/fs.h | 8 ++++++++
>  3 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/internal.h b/fs/internal.h
> index b1f66e52d61b..7508d485c655 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -311,4 +311,4 @@ void mnt_idmap_put(struct mnt_idmap *idmap);
>  struct linux_dirent64;
>  
>  int vfs_getdents(struct file *file, struct linux_dirent64 __user *dirent,
> -		 unsigned int count);
> +		 unsigned int count, unsigned long flags);
> diff --git a/fs/readdir.c b/fs/readdir.c
> index 9592259b7e7f..b80caf4c9321 100644
> --- a/fs/readdir.c
> +++ b/fs/readdir.c
> @@ -358,12 +358,14 @@ static bool filldir64(struct dir_context *ctx, const char *name, int namlen,
>   * @file    : pointer to file struct of directory
>   * @dirent  : pointer to user directory structure
>   * @count   : size of buffer
> + * @flags   : additional dir_context flags

Why do you need that flag argument. The ->iterate{_shared}() i_op gets
passed the file so the filesystem can check
@file->f_mode & FMODE_NOWAIT, no?

>   */
>  int vfs_getdents(struct file *file, struct linux_dirent64 __user *dirent,
> -		 unsigned int count)
> +		 unsigned int count, unsigned long flags)
>  {
>  	struct getdents_callback64 buf = {
>  		.ctx.actor = filldir64,
> +		.ctx.flags = flags,
>  		.count = count,
>  		.current_dir = dirent
>  	};
> @@ -395,7 +397,7 @@ SYSCALL_DEFINE3(getdents64, unsigned int, fd,
>  	if (!f.file)
>  		return -EBADF;
>  
> -	error = vfs_getdents(f.file, dirent, count);
> +	error = vfs_getdents(f.file, dirent, count, 0);
>  
>  	fdput_pos(f);
>  	return error;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 6867512907d6..f3e315e8efdd 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1719,8 +1719,16 @@ typedef bool (*filldir_t)(struct dir_context *, const char *, int, loff_t, u64,
>  struct dir_context {
>  	filldir_t actor;
>  	loff_t pos;
> +	unsigned long flags;
>  };
>  
> +/*
> + * flags for dir_context flags
> + * DIR_CONTEXT_F_NOWAIT: Request non-blocking iterate
> + *                       (requires file->f_mode & FMODE_NOWAIT)
> + */
> +#define DIR_CONTEXT_F_NOWAIT	(1 << 0)

Even if this should be needed, I don't think this needs to use a full
flags field.