mm, swap: fix swap readahead marking

In the original implementation, it is possible that the existing pages
in the swap cache (not newly readahead) could be marked as the readahead
pages.  This will cause the statistics of swap readahead be wrong and
influence the swap readahead algorithm too.

This is fixed via marking a page as the readahead page only if it is
newly allocated and read from the disk.

When testing with linpack, after the fixing the swap readahead hit rate
increased from ~66% to ~86%.

Link: http://lkml.kernel.org/r/20170807054038.1843-3-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Cc: Tim Chen <tim.c.chen@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/swap_state.c b/mm/swap_state.c
index d1bdb31cab13179bd9b3ff6e2d9f67d927239b95..a901afe9da61e418aaca3f4a6816a9632ae5d261 100644 (file)
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -498,7 +498,7 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
        unsigned long start_offset, end_offset;
        unsigned long mask;
        struct blk_plug plug;
-       bool do_poll = true;
+       bool do_poll = true, page_allocated;
 
        mask = swapin_nr_pages(offset) - 1;
        if (!mask)
@@ -514,14 +514,18 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
        blk_start_plug(&plug);
        for (offset = start_offset; offset <= end_offset ; offset++) {
                /* Ok, do the async read-ahead now */
-               page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
-                                               gfp_mask, vma, addr, false);
+               page = __read_swap_cache_async(
+                       swp_entry(swp_type(entry), offset),
+                       gfp_mask, vma, addr, &page_allocated);
                if (!page)
                        continue;
-               if (offset != entry_offset &&
-                   likely(!PageTransCompound(page))) {
-                       SetPageReadahead(page);
-                       count_vm_event(SWAP_RA);
+               if (page_allocated) {
+                       swap_readpage(page, false);
+                       if (offset != entry_offset &&
+                           likely(!PageTransCompound(page))) {
+                               SetPageReadahead(page);
+                               count_vm_event(SWAP_RA);
+                       }
                }
                put_page(page);
        }