linux-kernel - [PATCH 4/8] mm/swap: skip read ahead for unreferenced swap slots

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Message-ID: <20160927171826.GA17884@linux.intel.com>
Date:   Tue, 27 Sep 2016 10:18:27 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     dave.hansen@...el.com, andi.kleen@...el.com, aaron.lu@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Huang Ying <ying.huang@...el.com>,
        Hugh Dickins <hughd@...gle.com>, Shaohua Li <shli@...nel.org>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vladimir Davydov <vdavydov@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>
Subject: [PATCH 4/8] mm/swap: skip read ahead 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.

Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
Signed-off-by: "Huang, Ying" <ying.huang@...el.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 5302cce..68ab90c 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -422,6 +422,7 @@ 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 *);
@@ -516,6 +517,11 @@ 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 102595c..579807d 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -316,6 +316,10 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 	*new_page_allocated = false;
 
 	do {
+		/* Just skip read ahead for unused swap slot */
+		if (!__swp_swapcount(entry))
+			return NULL;
+
 		/*
 		 * First check the swap cache.  Since this is normally
 		 * called after lookup_swap_cache() failed, re-calling
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 870f61f..980047d 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 @@ 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;
@@ -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