On Tue, Sep 22, 2020 at 10:02 AM Rik van Riel <riel@xxxxxxxxxxx> wrote:
>
> Check whether a swap page was obtained instantaneously, for example
> because it is in zswap, or on a very fast IO device which uses busy
> waiting, and we did not wait on IO to swap in this page.
> If no IO was needed to get the swap page we want, kicking off readahead
> on surrounding swap pages is likely to be counterproductive, because the
> extra loads will cause additional latency, use up extra memory, and chances
> are the surrounding pages in swap are just as fast to load as this one,
> making readahead pointless.
>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
> ---
> mm/swap_state.c | 14 +++++++++++---
> 1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index aacb9ba53f63..6919f9d5fe88 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -637,6 +637,7 @@ static struct page *swap_cluster_read_one(swp_entry_t entry,
> struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
> struct vm_fault *vmf)
Why not do this for swap_vma_readahead() too? swap_cluster_read_one()
can be used in swap_vma_readahead() too.
> {
> + struct page *page;
> unsigned long entry_offset = swp_offset(entry);
> unsigned long offset = entry_offset;
> unsigned long start_offset, end_offset;
> @@ -668,11 +669,18 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
> end_offset = si->max - 1;
>
> blk_start_plug(&plug);
> + /* If we read the page without waiting on IO, skip readahead. */
> + page = swap_cluster_read_one(entry, offset, gfp_mask, vma, addr, false);
> + if (page && PageUptodate(page))
> + goto skip_unplug;
> +
> + /* Ok, do the async read-ahead now. */
> for (offset = start_offset; offset <= end_offset ; offset++) {
> - /* Ok, do the async read-ahead now */
> - swap_cluster_read_one(entry, offset, gfp_mask, vma, addr,
> - offset != entry_offset);
> + if (offset == entry_offset)
> + continue;
> + swap_cluster_read_one(entry, offset, gfp_mask, vma, addr, true);
> }
> +skip_unplug:
> blk_finish_plug(&plug);
>
> lru_add_drain(); /* Push any new pages onto the LRU now */
Best Regards,
Huang, Ying
