Re: [PATCH 07/18] MM: submit multipage write for SWP_FS_OPS swap-space

[Mark Hemment]

On Thu, 16 Dec 2021 at 23:56, NeilBrown <neilb@xxxxxxx> wrote:
>
> swap_writepage() is given one page at a time, but may be called repeatedly
> in succession.
> For block-device swapspace, the blk_plug functionality allows the
> multiple pages to be combined together at lower layers.
> That cannot be used for SWP_FS_OPS as blk_plug may not exist - it is
> only active when CONFIG_BLOCK=y.  Consequently all swap reads over NFS
> are single page reads.
>
> With this patch we pass a pointer-to-pointer via the wbc.
> swap_writepage can store state between calls - much like the pointer
> passed explicitly to swap_readpage.  After calling swap_writepage() some
> number of times, the state will be passed to swap_write_unplug() which
> can submit the combined request.
>
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
>  include/linux/writeback.h |    7 +++
>  mm/page_io.c              |   98 ++++++++++++++++++++++++++++++---------------
>  mm/swap.h                 |    1
>  mm/vmscan.c               |    9 +++-
>  4 files changed, 80 insertions(+), 35 deletions(-)
...
> +void swap_write_unplug(struct swap_iocb *sio)
> +{
> +       struct iov_iter from;
> +       struct address_space *mapping = sio->iocb.ki_filp->f_mapping;
> +       int ret;
> +
> +       iov_iter_bvec(&from, WRITE, sio->bvec, sio->pages,
> +                     PAGE_SIZE * sio->pages);
> +       ret = mapping->a_ops->swap_rw(&sio->iocb, &from);
> +       if (ret != -EIOCBQUEUED)
> +               sio_write_complete(&sio->iocb, ret);
> +}
> +

As swap_write_unplug() is called from vmscan.c, need an 'no-op'
version (in "swap.h") for when !CONFIG_SWAP

Mark