[PATCH 2/2] mm,swap: skip swap readahead if page was obtained instantaneously

[Rik van Riel]

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)
 {
+	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 */
-- 
2.25.4