Re: [PATCH 08/18] MM: Add AS_CAN_DIO mapping flag

[Mark Hemment]

On Thu, 16 Dec 2021 at 23:57, NeilBrown <neilb@xxxxxxx> wrote:
>
> Currently various places test if direct IO is possible on a file by
> checking for the existence of the direct_IO address space operation.
> This is a poor choice, as the direct_IO operation may not be used - it is
> only used if the generic_file_*_iter functions are called for direct IO
> and some filesystems - particularly NFS - don't do this.
>
> Instead, introduce a new mapping flag: AS_CAN_DIO and change the various
> places to check this (avoiding a pointer dereference).
> unlock_new_inode() will set this flag if ->direct_IO is present, so
> filesystems do not need to be changed.
...
> diff --git a/fs/inode.c b/fs/inode.c
> index 6b80a51129d5..bae65ccecdb1 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -1008,6 +1008,9 @@ EXPORT_SYMBOL(lockdep_annotate_inode_mutex_key);
>  void unlock_new_inode(struct inode *inode)
>  {
>         lockdep_annotate_inode_mutex_key(inode);
> +       if (inode->i_mapping->a_ops &&
> +           inode->i_mapping->a_ops->direct_IO)
> +               set_bit(AS_CAN_DIO, &inode->i_mapping->flags);
>         spin_lock(&inode->i_lock);
>         WARN_ON(!(inode->i_state & I_NEW));
>         inode->i_state &= ~I_NEW & ~I_CREATING;

Does d_instantiate_new() also need to set AS_CAN_DIO?

Mark