Re: [PATCH] qnx4: convert qnx4 to use the new mount api

[Christian Brauner <brauner@kernel.org>]

On Mon, Feb 26, 2024 at 04:46:28PM -0600, Bill O'Donnell wrote:
> Convert the qnx4 filesystem to use the new mount API.
> 
> Tested mount, umount, and remount using a qnx4 boot image.
> 
> Signed-off-by: Bill O'Donnell <bodonnel@redhat.com>
> ---
>  fs/qnx4/inode.c | 49 +++++++++++++++++++++++++++++++------------------
>  1 file changed, 31 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/qnx4/inode.c b/fs/qnx4/inode.c
> index 6eb9bb369b57..c36fbe45a0e9 100644
> --- a/fs/qnx4/inode.c
> +++ b/fs/qnx4/inode.c
> @@ -21,6 +21,7 @@
>  #include <linux/buffer_head.h>
>  #include <linux/writeback.h>
>  #include <linux/statfs.h>
> +#include <linux/fs_context.h>
>  #include "qnx4.h"
>  
>  #define QNX4_VERSION  4
> @@ -30,28 +31,33 @@ static const struct super_operations qnx4_sops;
>  
>  static struct inode *qnx4_alloc_inode(struct super_block *sb);
>  static void qnx4_free_inode(struct inode *inode);
> -static int qnx4_remount(struct super_block *sb, int *flags, char *data);
>  static int qnx4_statfs(struct dentry *, struct kstatfs *);
> +static int qnx4_get_tree(struct fs_context *fc);
>  
>  static const struct super_operations qnx4_sops =
>  {
>  	.alloc_inode	= qnx4_alloc_inode,
>  	.free_inode	= qnx4_free_inode,
>  	.statfs		= qnx4_statfs,
> -	.remount_fs	= qnx4_remount,
>  };
>  
> -static int qnx4_remount(struct super_block *sb, int *flags, char *data)
> +static int qnx4_reconfigure(struct fs_context *fc)
>  {
> -	struct qnx4_sb_info *qs;
> +	struct super_block *sb = fc->root->d_sb;
> +	struct qnx4_sb_info *qs = sb->s_fs_info;
>  
>  	sync_filesystem(sb);
>  	qs = qnx4_sb(sb);
>  	qs->Version = QNX4_VERSION;
> -	*flags |= SB_RDONLY;
> +	fc->sb_flags |= SB_RDONLY;

This confused me to no end because setting SB_RDONLY here
unconditionally would be wrong if it's not requested from userspace
during a remount. Because in that case the vfs wouldn't know that an
actual read-only remount request had been made which means that we don't
take the necessary protection steps to transition from read-write to
read-only. But qnx{4,6} are read-only so this is actually correct even
though it seems pretty weird.