Re: [PATCH 09/23] MM: submit multipage reads for SWP_FS_OPS swap-space

[Mark Hemment <markhemm@googlemail.com>]

On Mon, 24 Jan 2022 at 03:52, NeilBrown <neilb@suse.de> wrote:
>
> swap_readpage() is given one page at a time, but maybe 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 in a pointer-to-pointer when swap_readpage can
> store state between calls - much like the effect of blk_plug.  After
> calling swap_readpage() some number of times, the state will be passed
> to swap_read_unplug() which can submit the combined request.
>
> Some caller currently call blk_finish_plug() *before* the final call to
> swap_readpage(), so the last page cannot be included.  This patch moves
> blk_finish_plug() to after the last call, and calls swap_read_unplug()
> there too.
>
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  mm/madvise.c    |    8 +++-
>  mm/memory.c     |    2 +
>  mm/page_io.c    |  102 +++++++++++++++++++++++++++++++++++--------------------
>  mm/swap.h       |   16 +++++++--
>  mm/swap_state.c |   19 +++++++---
>  5 files changed, 98 insertions(+), 49 deletions(-)
>
...
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 6e32ca35d9b6..bcf655d650c8 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -390,46 +391,60 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
>  static void sio_read_complete(struct kiocb *iocb, long ret)
>  {
>         struct swap_iocb *sio = container_of(iocb, struct swap_iocb, iocb);
> -       struct page *page = sio->bvec.bv_page;
> -
> -       if (ret != 0 && ret != PAGE_SIZE) {
> -               SetPageError(page);
> -               ClearPageUptodate(page);
> -               pr_alert_ratelimited("Read-error on swap-device\n");
> -       } else {
> -               SetPageUptodate(page);
> -               count_vm_event(PSWPIN);
> +       int p;
> +
> +       for (p = 0; p < sio->pages; p++) {
> +               struct page *page = sio->bvec[p].bv_page;
> +               if (ret != 0 && ret != PAGE_SIZE * sio->pages) {
> +                       SetPageError(page);
> +                       ClearPageUptodate(page);
> +                       pr_alert_ratelimited("Read-error on swap-device\n");
> +               } else {
> +                       SetPageUptodate(page);
> +                       count_vm_event(PSWPIN);
> +               }
> +               unlock_page(page);
>         }
> -       unlock_page(page);
>         mempool_free(sio, sio_pool);
>  }

Trivial: on success, could be single call to count_vm_events(PSWPIN,
sio->pages).
Similar comment for PSWPOUT in sio_write_complete()

>
> -static int swap_readpage_fs(struct page *page)
> +static void swap_readpage_fs(struct page *page,
> +                            struct swap_iocb **plug)
>  {
>         struct swap_info_struct *sis = page_swap_info(page);
> -       struct file *swap_file = sis->swap_file;
> -       struct address_space *mapping = swap_file->f_mapping;
> -       struct iov_iter from;
> -       struct swap_iocb *sio;
> +       struct swap_iocb *sio = NULL;
>         loff_t pos = page_file_offset(page);
> -       int ret;
> -
> -       sio = mempool_alloc(sio_pool, GFP_KERNEL);
> -       init_sync_kiocb(&sio->iocb, swap_file);
> -       sio->iocb.ki_pos = pos;
> -       sio->iocb.ki_complete = sio_read_complete;
> -       sio->bvec.bv_page = page;
> -       sio->bvec.bv_len = PAGE_SIZE;
> -       sio->bvec.bv_offset = 0;
>
> -       iov_iter_bvec(&from, READ, &sio->bvec, 1, PAGE_SIZE);
> -       ret = mapping->a_ops->swap_rw(&sio->iocb, &from);
> -       if (ret != -EIOCBQUEUED)
> -               sio_read_complete(&sio->iocb, ret);
> -       return ret;
> +       if (*plug)
> +               sio = *plug;

'plug' can be NULL when called from do_swap_page();
        if (plug && *plug)

Cheers,
Mark