mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference

When SWP_SYNCHRONOUS_IO swapped-in pages are shared by several
processes, it can cause unnecessary memory wastage by skipping swap
cache.  Because, with swapin fault by read, they could share a page if
the page were in swap cache.  Thus, it avoids allocating same content
new pages.

This patch makes the swapcache skipping work only if the swap pte is
non-sharable.

[akpm@linux-foundation.org: coding-style fixes]
Link: http://lkml.kernel.org/r/1507620825-5537-1-git-send-email-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Huang Ying <ying.huang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 85aff8a..3074b02 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1328,6 +1328,13 @@ int page_swapcount(struct page *page)
 	return count;
 }
 
+int __swap_count(struct swap_info_struct *si, swp_entry_t entry)
+{
+	pgoff_t offset = swp_offset(entry);
+
+	return swap_count(si->swap_map[offset]);
+}
+
 static int swap_swapcount(struct swap_info_struct *si, swp_entry_t entry)
 {
 	int count = 0;