[PATCH] mm:swap: skip swapcache only if swapped page has no other reference

[Minchan Kim]

When 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.

Cc: Huang Ying <ying.hu...@intel.com>
Cc: Hugh Dickins <hu...@google.com>
Signed-off-by: Minchan Kim <minc...@kernel.org>
---
 include/linux/swap.h |  6 ++++++
 mm/memory.c          | 19 ++++++++++---------
 mm/swapfile.c        |  7 +++++++
 3 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index cd2f66fdfc2d..1f5c52313890 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -458,6 +458,7 @@ extern unsigned int count_swap_pages(int, int);
 extern sector_t map_swap_page(struct page *, struct block_device **);
 extern sector_t swapdev_block(int, pgoff_t);
 extern int page_swapcount(struct page *);
+extern int __swap_count(struct swap_info_struct *si, swp_entry_t entry);
 extern int __swp_swapcount(swp_entry_t entry);
 extern int swp_swapcount(swp_entry_t entry);
 extern struct swap_info_struct *page_swap_info(struct page *);
@@ -584,6 +585,11 @@ static inline int page_swapcount(struct page *page)
        return 0;
 }
 
+static inline int __swap_count(struct swap_info_struct *si, swp_entry_t entry)
+{
+       return 0;
+}
+
 static inline int __swp_swapcount(swp_entry_t entry)
 {
        return 0;
diff --git a/mm/memory.c b/mm/memory.c
index 163ab2062385..aff7e324564f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2890,15 +2890,8 @@ int do_swap_page(struct vm_fault *vmf)
        if (!page) {
                struct swap_info_struct *si = swp_swap_info(entry);
 
-               if (!(si->flags & SWP_SYNCHRONOUS_IO)) {
-                       if (vma_readahead)
-                               page = do_swap_page_readahead(entry,
-                                       GFP_HIGHUSER_MOVABLE, vmf, &swap_ra);
-                       else
-                               page = swapin_readahead(entry,
-                                       GFP_HIGHUSER_MOVABLE, vma, 
vmf->address);
-                       swapcache = page;
-               } else {
+               if (si->flags & SWP_SYNCHRONOUS_IO &&
+                               __swap_count(si, entry) == 1) {
                        /* skip swapcache */
                        page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, 
vmf->address);
                        if (page) {
@@ -2908,6 +2901,14 @@ int do_swap_page(struct vm_fault *vmf)
                                lru_cache_add_anon(page);
                                swap_readpage(page, true);
                        }
+               } else {
+                       if (vma_readahead)
+                               page = do_swap_page_readahead(entry,
+                                       GFP_HIGHUSER_MOVABLE, vmf, &swap_ra);
+                       else
+                               page = swapin_readahead(entry,
+                                       GFP_HIGHUSER_MOVABLE, vma, 
vmf->address);
+                       swapcache = page;
                }
 
                if (!page) {
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 64a3d85226ba..d67715ffc194 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;
-- 
2.7.4