[PATCH v4 4/9] mm/swap: skip read ahead for unreferenced swap slots

Tim Chen
2016-12-09T21:11:54Z

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.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Co-developed-by: "Huang, Ying" <ying.huang@intel.com>
---
include/linux/swap.h | 6 ++++++
mm/swap_state.c | 4 ++++
mm/swapfile.c | 47 +++++++++++++++++++++++++++++++++++++++++------
3 files changed, 51 insertions(+), 6 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 5f66c84..6b7a48b 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -424,6 +424,7 @@ 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 *);
@@ -518,6 +519,11 @@ 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 3863acd..3d76d80 100644
--- 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 eb6cba7..136c9dd 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -803,7 +803,7 @@ 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;
@@ -819,13 +819,8 @@ 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;
@@ -838,6 +833,24 @@ static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
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;
@@ -993,6 +1006,28 @@ int page_swapcount(struct page *page)

/*
* 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.
*/
int swp_swapcount(swp_entry_t entry)
--
2.5.5