mm/swap: skip readahead for unreferenced swap slots

We can avoid needlessly allocating page for swap slots that are not used
by anyone.  No pages have to be read in for these slots.

Link: http://lkml.kernel.org/r/0784b3f20b9bd3aa5552219624cb78dc4ae710c9.1484082593.git.tim.c.chen@linux.intel.com
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Aaron Lu <aaron.lu@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net> escreveu:
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 648a32b58e3eff7d26af5ae2dd3b0487ad5c0782..3116382067cd8b8bd2f754c62d2276d3c943a415 100644 (file)
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -398,6 +398,7 @@ 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 __swp_swapcount(swp_entry_t entry);
 extern int swp_swapcount(swp_entry_t entry);
 extern struct swap_info_struct *page_swap_info(struct page *);
 extern bool reuse_swap_page(struct page *, int *);
@@ -492,6 +493,11 @@ static inline int page_swapcount(struct page *page)
        return 0;
 }
 
+static inline int __swp_swapcount(swp_entry_t entry)
+{
+       return 0;
+}
+
 static inline int swp_swapcount(swp_entry_t entry)
 {
        return 0;

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 3863acd6189cba35cb0d2a811169b6b69f37981f..3d76d80c07d6d61c3feb3b8a959e34c70f862a80 100644 (file)
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -323,6 +323,10 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                if (found_page)
                        break;
 
+               /* Just skip read ahead for unused swap slot */
+               if (!__swp_swapcount(entry))
+                       return NULL;
+
                /*
                 * Get a new page to read into from swap.
                 */

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 66e95eb730407ec02604262de296278aab7b5880..7e888de35c41a62ade94ef7401d0c07b47943a32 100644 (file)
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -798,7 +798,7 @@ swp_entry_t get_swap_page_of_type(int type)
        return (swp_entry_t) {0};
 }
 
-static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
+static struct swap_info_struct *__swap_info_get(swp_entry_t entry)
 {
        struct swap_info_struct *p;
        unsigned long offset, type;
@@ -814,13 +814,8 @@ static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
        offset = swp_offset(entry);
        if (offset >= p->max)
                goto bad_offset;
-       if (!p->swap_map[offset])
-               goto bad_free;
        return p;
 
-bad_free:
-       pr_err("swap_info_get: %s%08lx\n", Unused_offset, entry.val);
-       goto out;
 bad_offset:
        pr_err("swap_info_get: %s%08lx\n", Bad_offset, entry.val);
        goto out;
@@ -833,6 +828,24 @@ out:
        return NULL;
 }
 
+static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
+{
+       struct swap_info_struct *p;
+
+       p = __swap_info_get(entry);
+       if (!p)
+               goto out;
+       if (!p->swap_map[swp_offset(entry)])
+               goto bad_free;
+       return p;
+
+bad_free:
+       pr_err("swap_info_get: %s%08lx\n", Unused_offset, entry.val);
+       goto out;
+out:
+       return NULL;
+}
+
 static struct swap_info_struct *swap_info_get(swp_entry_t entry)
 {
        struct swap_info_struct *p;
@@ -986,6 +999,28 @@ int page_swapcount(struct page *page)
        return count;
 }
 
+/*
+ * How many references to @entry are currently swapped out?
+ * This does not give an exact answer when swap count is continued,
+ * but does include the high COUNT_CONTINUED flag to allow for that.
+ */
+int __swp_swapcount(swp_entry_t entry)
+{
+       int count = 0;
+       pgoff_t offset;
+       struct swap_info_struct *si;
+       struct swap_cluster_info *ci;
+
+       si = __swap_info_get(entry);
+       if (si) {
+               offset = swp_offset(entry);
+               ci = lock_cluster_or_swap_info(si, offset);
+               count = swap_count(si->swap_map[offset]);
+               unlock_cluster_or_swap_info(si, ci);
+       }
+       return count;
+}
+
 /*
  * How many references to @entry are currently swapped out?
  * This considers COUNT_CONTINUED so it returns exact answer.