Re: [PATCH 2/8] swap: lock i_mutex for swap_writepage direct_IO

[Christoph Hellwig <hch@infradead.org>]

On Tue, Dec 16, 2014 at 12:56:24AM -0800, Omar Sandoval wrote:
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1728,6 +1728,9 @@ static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
>         }
>  
>         if (mapping->a_ops->swap_activate) {
> +               if (!mapping->a_ops->direct_IO)
> +                       return -EINVAL;
> +               swap_file->f_flags |= O_DIRECT;
>                 ret = mapping->a_ops->swap_activate(sis, swap_file, span);
>                 if (!ret) {
>                         sis->flags |= SWP_FILE;

This needs to hold swap_file->f_lock, but otherwise looks good.

> This seems to be more or less equivalent to doing a fcntl(F_SETFL) to
> add the O_DIRECT flag to swap_file (which is a struct file *). Swapoff
> calls filp_close on swap_file, so I don't see why it's necessary to
> clear the flag.

filp_lose doesn't nessecarily destroy the file structure, there might be
other reference to it, e.g. from dup() or descriptor passing.