On Mon, Jan 24, 2022 at 02:48:32PM +1100, NeilBrown 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 | 103 +++++++++++++++++++++++++++++----------------
> mm/swap.h | 1
> mm/vmscan.c | 9 +++-
> 4 files changed, 82 insertions(+), 38 deletions(-)
>
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index fec248ab1fec..6dcaa0639c0d 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -80,6 +80,13 @@ struct writeback_control {
>
> unsigned punt_to_cgroup:1; /* cgrp punting, see __REQ_CGROUP_PUNT */
>
> + /* To enable batching of swap writes to non-block-device backends,
> + * "plug" can be set point to a 'struct swap_iocb *'. When all swap
> + * writes have been submitted, if with swap_iocb is not NULL,
> + * swap_write_unplug() should be called.
> + */
> + struct swap_iocb **plug;
Mayb plug isn't really the best name for something swap-specific in this
generic structure?
Also the above does not fit the normal kernel comment style with an
otherwise empty
/*
line.
> + for (p = 0; p < sio->pages; p++) {
> + struct page *page = sio->bvec[p].bv_page;
> +
> + if (ret != 0 && ret != PAGE_SIZE * sio->pages) {
> + /*
> + * In the case of swap-over-nfs, this can be a
> + * temporary failure if the system has limited
> + * memory for allocating transmit buffers.
> + * Mark the page dirty and avoid
> + * folio_rotate_reclaimable but rate-limit the
> + * messages but do not flag PageError like
> + * the normal direct-to-bio case as it could
> + * be temporary.
> + */
> + set_page_dirty(page);
> + ClearPageReclaim(page);
> + pr_err_ratelimited("Write error %ld on dio swapfile (%llu)\n",
> + ret, page_file_offset(page));
> + } else
> + count_vm_event(PSWPOUT);
I'd rather check for the error condition ones and have separate loops
forthe success vs error cases instead of checking the condition again
and again.
Otherwise looks good:
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
