swap: make swap discard async

swap can do cluster discard for SSD, which is good, but there are some problems here:
1. swap do the discard just before page reclaim gets a swap entry and writes the disk sectors. This is useless for high end SSD, because an overwrite to a sector implies a discard to original nand flash too. A discard + overwrite ==

overwrite.
2. the purpose of doing discard is to improve SSD firmware garbage collection. Doing discard just before write doesn't help, because the interval between discard and write is too short. Doing discard async and just after a swap entry is freed can make the interval longer, so SSD firmware has more time to do gc.
3. block discard is a sync API, which will delay scan_swap_map() significantly.
4. Write and discard command can be executed parallel in PCIe SSD. Making swap discard async can make execution more efficiently.

This patch makes swap discard async, and move discard to where swap entry is freed. Idealy we should do discard for any freed sectors, but some SSD discard

is very slow. This patch still does discard for a whole cluster.

My test does a several round of 'mmap, write, unmap', which will trigger a lot of swap discard. In a fusionio card, with this patch, the test runtime is reduced to 18% of the time without it, so around 5.5x faster.

Signed-off-by Shaohua Li <[email protected]>

---
 include/linux/swap.h |    3 
 mm/swapfile.c        |  177 +++++++++++++++++++++++++++------------------------
 2 files changed, 98 insertions(+), 82 deletions(-)

Index: linux/include/linux/swap.h
===================================================================

include/linux/swap.h +1 -2

@@ -192,8 +192,6 @@ struct swap_info_struct {
 	unsigned int inuse_pages;	/* number of those currently in use */
 	unsigned int cluster_next;	/* likely index for next allocation */
 	unsigned int cluster_nr;	/* countdown to next cluster search */
-	unsigned int lowest_alloc;	/* while preparing discard cluster */
-	unsigned int highest_alloc;	/* while preparing discard cluster */
 	struct swap_extent *curr_swap_extent;
 	struct swap_extent first_swap_extent;
 	struct block_device *bdev;	/* swap device or bdev of swap file */

@@ -203,6 +201,7 @@ struct swap_info_struct {
 	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
 	atomic_t frontswap_pages;	/* frontswap pages in-use counter */
 #endif
+	struct work_struct discard_work;
 };
 
 struct swap_list_t {

Index: linux/mm/swapfile.c
===================================================================

mm/swapfile.c +0 -4

@@ -173,15 +173,82 @@ static void discard_swap_cluster(struct
 	}
 }
 
-static int wait_for_discard(void *word)
-{
-	schedule();
-	return 0;

mm/swapfile.c +34 -0

+	     SWAPFILE_CLUSTER; i < round_down(si->highest_bit,
+	     SWAPFILE_CLUSTER) / SWAPFILE_CLUSTER; i++) {
+		if (counter[i] == CLUSTER_COUNT_DISCARDABLE) {
+			spin_lock(&swap_lock);
+			if (counter[i] != CLUSTER_COUNT_DISCARDABLE) {
+				spin_unlock(&swap_lock);
+				continue;
+			}
+			counter[i] = CLUSTER_COUNT_DISCARDING;
+			spin_unlock(&swap_lock);
+
+			discard_swap_cluster(si, i << SWAPFILE_CLUSTER_SHIFT,
+				SWAPFILE_CLUSTER);
+
+			spin_lock(&swap_lock);
+			counter[i] = 0;
+			memset(si->swap_map + (i << SWAPFILE_CLUSTER_SHIFT),
+					0, SWAPFILE_CLUSTER);
+			spin_unlock(&swap_lock);
+		}
+	}
+	for (i = round_up(si->lowest_bit, SWAPFILE_CLUSTER) /
+	     SWAPFILE_CLUSTER; i < round_down(si->cluster_next,
+	     SWAPFILE_CLUSTER) / SWAPFILE_CLUSTER; i++) {
+		if (counter[i] == CLUSTER_COUNT_DISCARDABLE) {
+			spin_lock(&swap_lock);
+			if (counter[i] != CLUSTER_COUNT_DISCARDABLE) {
+				spin_unlock(&swap_lock);
+				continue;
+			}
+			counter[i] = CLUSTER_COUNT_DISCARDING;
+			spin_unlock(&swap_lock);
+
+			discard_swap_cluster(si, i << SWAPFILE_CLUSTER_SHIFT,

Quoted 5 lines · Shaohua Li

swap can do cluster discard for SSD, which is good, but there are some problems here:
1. swap do the discard just before page reclaim gets a swap entry and writes the disk sectors. This is useless for high end SSD, because an overwrite to a sector implies a discard to original nand flash too. A discard + overwrite ==

Quoted 8 lines

overwrite.
2. the purpose of doing discard is to improve SSD firmware garbage collection. Doing discard just before write doesn't help, because the interval between discard and write is too short. Doing discard async and just after a swap entry is freed can make the interval longer, so SSD firmware has more time to do gc.
3. block discard is a sync API, which will delay scan_swap_map() significantly.
4. Write and discard command can be executed parallel in PCIe SSD. Making swap discard async can make execution more efficiently.

Great!

Quoted 3 lines

This patch makes swap discard async, and move discard to where swap entry is freed. Idealy we should do discard for any freed sectors, but some SSD discard

Yes. It's ideal but most of small storage(ex, eMMC) can't do it due to shortage of internal resource.

Quoted 1 line

is very slow. This patch still does discard for a whole cluster.

That's good for small nonration storage.

Quoted 4 lines

My test does a several round of 'mmap, write, unmap', which will trigger a lot of swap discard. In a fusionio card, with this patch, the test runtime is reduced to 18% of the time without it, so around 5.5x faster.

Could you share your test program?

Quoted 40 lines

Signed-off-by Shaohua Li <[email protected]>

---
 include/linux/swap.h |    3 
 mm/swapfile.c        |  177 +++++++++++++++++++++++++++------------------------
 2 files changed, 98 insertions(+), 82 deletions(-)

Index: linux/include/linux/swap.h
===================================================================

include/linux/swap.h +1 -2

@@ -192,8 +192,6 @@ struct swap_info_struct {
 	unsigned int inuse_pages;	/* number of those currently in use */
 	unsigned int cluster_next;	/* likely index for next allocation */
 	unsigned int cluster_nr;	/* countdown to next cluster search */
-	unsigned int lowest_alloc;	/* while preparing discard cluster */
-	unsigned int highest_alloc;	/* while preparing discard cluster */
 	struct swap_extent *curr_swap_extent;
 	struct swap_extent first_swap_extent;
 	struct block_device *bdev;	/* swap device or bdev of swap file */

@@ -203,6 +201,7 @@ struct swap_info_struct {
 	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
 	atomic_t frontswap_pages;	/* frontswap pages in-use counter */
 #endif
+	struct work_struct discard_work;
 };
 
 struct swap_list_t {

Index: linux/mm/swapfile.c
===================================================================

mm/swapfile.c +0 -4

@@ -173,15 +173,82 @@ static void discard_swap_cluster(struct
 	}
 }
 
-static int wait_for_discard(void *word)
-{
-	schedule();
-	return 0;

mm/swapfile.c static void discard_swap_cluster(struct · line 182

Quoted 10 lines

mm/swapfile.c +5 -3

-}
-
-#define SWAPFILE_CLUSTER	256
+#define SWAPFILE_CLUSTER_SHIFT	8
+#define SWAPFILE_CLUSTER	(1<<SWAPFILE_CLUSTER_SHIFT)
 #define LATENCY_LIMIT		256
 
+/* magic number to indicate the cluster is discardable */
+#define CLUSTER_COUNT_DISCARDABLE (SWAPFILE_CLUSTER * 2)
+#define CLUSTER_COUNT_DISCARDING (SWAPFILE_CLUSTER * 2 + 1)

#define CLUSTER_COUNT_DISCARDING (CLUSTER_COUNT_DISCARDABLE + 1)

mm/swapfile.c static void discard_swap_cluster(struct · line 195

Quoted 13 lines

mm/swapfile.c +13 -0

+static void swap_cluster_check_discard(struct swap_info_struct *si,
+		unsigned long offset)
+{
+	unsigned long cluster = offset/SWAPFILE_CLUSTER;
+
+	if (!(si->flags & SWP_DISCARDABLE))
+		return;
+	if (si->swap_cluster_count[cluster] > 0)
+		return;
+	si->swap_cluster_count[cluster] = CLUSTER_COUNT_DISCARDABLE;
+	/* Just mark the swap entries occupied */
+	memset(si->swap_map + (cluster << SWAPFILE_CLUSTER_SHIFT),
+			SWAP_MAP_BAD, SWAPFILE_CLUSTER);

You should explain why we need SWAP_MAP_BAD.

mm/swapfile.c static void discard_swap_cluster(struct · line 206

Quoted 11 lines

mm/swapfile.c +11 -0

+	schedule_work(&si->discard_work);
+}
+
+static void swap_discard_work(struct work_struct *work)
+{
+	struct swap_info_struct *si = container_of(work,
+		struct swap_info_struct, discard_work);
+	unsigned int *counter = si->swap_cluster_count;
+	int i;
+
+	for (i = round_up(si->cluster_next, SWAPFILE_CLUSTER) /

Why do we always start si->cluster_next?
IMHO, It would be better to start offset where swap_entry_free free.

Quoted 42 lines

mm/swapfile.c +42 -0

+	     SWAPFILE_CLUSTER; i < round_down(si->highest_bit,
+	     SWAPFILE_CLUSTER) / SWAPFILE_CLUSTER; i++) {
+		if (counter[i] == CLUSTER_COUNT_DISCARDABLE) {
+			spin_lock(&swap_lock);
+			if (counter[i] != CLUSTER_COUNT_DISCARDABLE) {
+				spin_unlock(&swap_lock);
+				continue;
+			}
+			counter[i] = CLUSTER_COUNT_DISCARDING;
+			spin_unlock(&swap_lock);
+
+			discard_swap_cluster(si, i << SWAPFILE_CLUSTER_SHIFT,
+				SWAPFILE_CLUSTER);
+
+			spin_lock(&swap_lock);
+			counter[i] = 0;
+			memset(si->swap_map + (i << SWAPFILE_CLUSTER_SHIFT),
+					0, SWAPFILE_CLUSTER);
+			spin_unlock(&swap_lock);
+		}
+	}
+	for (i = round_up(si->lowest_bit, SWAPFILE_CLUSTER) /
+	     SWAPFILE_CLUSTER; i < round_down(si->cluster_next,
+	     SWAPFILE_CLUSTER) / SWAPFILE_CLUSTER; i++) {
+		if (counter[i] == CLUSTER_COUNT_DISCARDABLE) {
+			spin_lock(&swap_lock);
+			if (counter[i] != CLUSTER_COUNT_DISCARDABLE) {
+				spin_unlock(&swap_lock);
+				continue;
+			}
+			counter[i] = CLUSTER_COUNT_DISCARDING;
+			spin_unlock(&swap_lock);
+
+			discard_swap_cluster(si, i << SWAPFILE_CLUSTER_SHIFT,
+				SWAPFILE_CLUSTER);
+
+			spin_lock(&swap_lock);
+			counter[i] = 0;
+			memset(si->swap_map + (i << SWAPFILE_CLUSTER_SHIFT),
+					0, SWAPFILE_CLUSTER);
+			spin_unlock(&swap_lock);
+		}

mm/swapfile.c static void discard_swap_cluster(struct · line 250

Quoted 2 lines

mm/swapfile.c +2 -0

+	}
+}

Whole searching for finding discardable cluster is rather overkill if we use big swap device.
Couldn't we make global discarable cluster counter and loop until it is zero? Anyway, it's just optimization point and could add up based on this patch. It shouldn't merge your patch. :)

Quoted 5 lines · Shaohua Li

swap can do cluster discard for SSD, which is good, but there are some problems here:
1. swap do the discard just before page reclaim gets a swap entry and writes the disk sectors. This is useless for high end SSD, because an overwrite to a sector implies a discard to original nand flash too. A discard + overwrite ==

Quoted 8 lines

overwrite.
2. the purpose of doing discard is to improve SSD firmware garbage collection. Doing discard just before write doesn't help, because the interval between discard and write is too short. Doing discard async and just after a swap entry is freed can make the interval longer, so SSD firmware has more time to do gc.
3. block discard is a sync API, which will delay scan_swap_map() significantly.
4. Write and discard command can be executed parallel in PCIe SSD. Making swap discard async can make execution more efficiently.

Great!

Quoted 3 lines

This patch makes swap discard async, and move discard to where swap entry is freed. Idealy we should do discard for any freed sectors, but some SSD discard

Yes. It's ideal but most of small storage(ex, eMMC) can't do it due to shortage of internal resource.

Quoted 1 line

is very slow. This patch still does discard for a whole cluster.

That's good for small nonration storage.

Quoted 4 lines

My test does a several round of 'mmap, write, unmap', which will trigger a lot of swap discard. In a fusionio card, with this patch, the test runtime is reduced to 18% of the time without it, so around 5.5x faster.

Could you share your test program?

mm/swapfile.c static void discard_swap_cluster(struct · line 182

Quoted 3 lines

mm/swapfile.c +3 -0

+/* magic number to indicate the cluster is discardable */
+#define CLUSTER_COUNT_DISCARDABLE (SWAPFILE_CLUSTER * 2)
+#define CLUSTER_COUNT_DISCARDING (SWAPFILE_CLUSTER * 2 + 1)

#define CLUSTER_COUNT_DISCARDING (CLUSTER_COUNT_DISCARDABLE + 1)

Quoted 7 lines

mm/swapfile.c +7 -0

+static void swap_cluster_check_discard(struct swap_info_struct *si,
+		unsigned long offset)
+{
+	unsigned long cluster = offset/SWAPFILE_CLUSTER;
+
+	if (!(si->flags & SWP_DISCARDABLE))
+		return;

mm/swapfile.c static void discard_swap_cluster(struct · line 195

Quoted 6 lines

mm/swapfile.c +6 -0

+	if (si->swap_cluster_count[cluster] > 0)
+		return;
+	si->swap_cluster_count[cluster] = CLUSTER_COUNT_DISCARDABLE;
+	/* Just mark the swap entries occupied */
+	memset(si->swap_map + (cluster << SWAPFILE_CLUSTER_SHIFT),
+			SWAP_MAP_BAD, SWAPFILE_CLUSTER);

You should explain why we need SWAP_MAP_BAD.

Quoted 6 lines

mm/swapfile.c +6 -0

+	schedule_work(&si->discard_work);
+}
+
+static void swap_discard_work(struct work_struct *work)
+{
+	struct swap_info_struct *si = container_of(work,

mm/swapfile.c static void discard_swap_cluster(struct · line 206

Quoted 5 lines

mm/swapfile.c +5 -0

+		struct swap_info_struct, discard_work);
+	unsigned int *counter = si->swap_cluster_count;
+	int i;
+
+	for (i = round_up(si->cluster_next, SWAPFILE_CLUSTER) /

Why do we always start si->cluster_next?
IMHO, It would be better to start offset where swap_entry_free free.

Quoted 10 lines

mm/swapfile.c +10 -0

+
+			discard_swap_cluster(si, i << SWAPFILE_CLUSTER_SHIFT,
+				SWAPFILE_CLUSTER);
+
+			spin_lock(&swap_lock);
+			counter[i] = 0;
+			memset(si->swap_map + (i << SWAPFILE_CLUSTER_SHIFT),
+					0, SWAPFILE_CLUSTER);
+			spin_unlock(&swap_lock);
+		}

Review

Quoted 2 lines

(quoted diff) +2 -0

+	}
+}

Whole searching for finding discardable cluster is rather overkill if we use big swap device.
Couldn't we make global discarable cluster counter and loop until it is zero? Anyway, it's just optimization point and could add up based on this patch. It shouldn't merge your patch. :)

swap can do cluster discard for SSD, which is good, but there are some problems here:
1. swap do the discard just before page reclaim gets a swap entry and writes the disk sectors. This is useless for high end SSD, because an overwrite to a sector implies a discard to original nand flash too. A discard + overwrite == overwrite.
2. the purpose of doing discard is to improve SSD firmware garbage collection.

Doing discard just before write doesn't help, because the interval between discard and write is too short. Doing discard async and just after a swap entry is freed can make the interval longer, so SSD firmware has more time to do gc.
3. block discard is a sync API, which will delay scan_swap_map() significantly.
4. Write and discard command can be executed parallel in PCIe SSD. Making swap discard async can make execution more efficiently.

This patch makes swap discard async, and move discard to where swap entry is freed. Idealy we should do discard for any freed sectors, but some SSD discard is very slow. This patch still does discard for a whole cluster.

My test does a several round of 'mmap, write, unmap', which will trigger a lot of swap discard. In a fusionio card, with this patch, the test runtime is reduced to 18% of the time without it, so around 5.5x faster.

Signed-off-by Shaohua Li <[email protected]>

---
  include/linux/swap.h |    3
  mm/swapfile.c        |  177 +++++++++++++++++++++++++++------------------------
  2 files changed, 98 insertions(+), 82 deletions(-)

Index: linux/include/linux/swap.h
===================================================================

include/linux/swap.h +1 -2

@@ -192,8 +192,6 @@ struct swap_info_struct {
  	unsigned int inuse_pages;	/* number of those currently in use */
  	unsigned int cluster_next;	/* likely index for next allocation */
  	unsigned int cluster_nr;	/* countdown to next cluster search */
-	unsigned int lowest_alloc;	/* while preparing discard cluster */
-	unsigned int highest_alloc;	/* while preparing discard cluster */
  	struct swap_extent *curr_swap_extent;
  	struct swap_extent first_swap_extent;
  	struct block_device *bdev;	/* swap device or bdev of swap file */

@@ -203,6 +201,7 @@ struct swap_info_struct {
  	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
  	atomic_t frontswap_pages;	/* frontswap pages in-use counter */
  #endif
+	struct work_struct discard_work;
  };
 
  struct swap_list_t {

Index: linux/mm/swapfile.c
===================================================================

mm/swapfile.c +97 -81

@@ -173,15 +173,82 @@ static void discard_swap_cluster(struct
  	}
  }
 
-static int wait_for_discard(void *word)
-{
-	schedule();
-	return 0;
-}
-
-#define SWAPFILE_CLUSTER	256
+#define SWAPFILE_CLUSTER_SHIFT	8
+#define SWAPFILE_CLUSTER	(1<<SWAPFILE_CLUSTER_SHIFT)
  #define LATENCY_LIMIT		256
 
+/* magic number to indicate the cluster is discardable */
+#define CLUSTER_COUNT_DISCARDABLE (SWAPFILE_CLUSTER * 2)
+#define CLUSTER_COUNT_DISCARDING (SWAPFILE_CLUSTER * 2 + 1)
+static void swap_cluster_check_discard(struct swap_info_struct *si,
+		unsigned long offset)
+{
+	unsigned long cluster = offset/SWAPFILE_CLUSTER;
+
+	if (!(si->flags & SWP_DISCARDABLE))
+		return;
+	if (si->swap_cluster_count[cluster] > 0)
+		return;
+	si->swap_cluster_count[cluster] = CLUSTER_COUNT_DISCARDABLE;
+	/* Just mark the swap entries occupied */
+	memset(si->swap_map + (cluster << SWAPFILE_CLUSTER_SHIFT),
+			SWAP_MAP_BAD, SWAPFILE_CLUSTER);
+	schedule_work(&si->discard_work);
+}
+
+static void swap_discard_work(struct work_struct *work)
+{
+	struct swap_info_struct *si = container_of(work,
+		struct swap_info_struct, discard_work);
+	unsigned int *counter = si->swap_cluster_count;
+	int i;
+
+	for (i = round_up(si->cluster_next, SWAPFILE_CLUSTER) /
+	     SWAPFILE_CLUSTER; i < round_down(si->highest_bit,
+	     SWAPFILE_CLUSTER) / SWAPFILE_CLUSTER; i++) {
+		if (counter[i] == CLUSTER_COUNT_DISCARDABLE) {
+			spin_lock(&swap_lock);
+			if (counter[i] != CLUSTER_COUNT_DISCARDABLE) {
+				spin_unlock(&swap_lock);
+				continue;
+			}
+			counter[i] = CLUSTER_COUNT_DISCARDING;
+			spin_unlock(&swap_lock);
+
+			discard_swap_cluster(si, i << SWAPFILE_CLUSTER_SHIFT,
+				SWAPFILE_CLUSTER);
+
+			spin_lock(&swap_lock);
+			counter[i] = 0;
+			memset(si->swap_map + (i << SWAPFILE_CLUSTER_SHIFT),
+					0, SWAPFILE_CLUSTER);
+			spin_unlock(&swap_lock);
+		}
+	}
+	for (i = round_up(si->lowest_bit, SWAPFILE_CLUSTER) /
+	     SWAPFILE_CLUSTER; i < round_down(si->cluster_next,
+	     SWAPFILE_CLUSTER) / SWAPFILE_CLUSTER; i++) {
+		if (counter[i] == CLUSTER_COUNT_DISCARDABLE) {
+			spin_lock(&swap_lock);
+			if (counter[i] != CLUSTER_COUNT_DISCARDABLE) {
+				spin_unlock(&swap_lock);
+				continue;
+			}
+			counter[i] = CLUSTER_COUNT_DISCARDING;
+			spin_unlock(&swap_lock);
+
+			discard_swap_cluster(si, i << SWAPFILE_CLUSTER_SHIFT,
+				SWAPFILE_CLUSTER);
+
+			spin_lock(&swap_lock);
+			counter[i] = 0;
+			memset(si->swap_map + (i << SWAPFILE_CLUSTER_SHIFT),
+					0, SWAPFILE_CLUSTER);
+			spin_unlock(&swap_lock);
+		}
+	}
+}
+
  static inline void inc_swap_cluster_count(unsigned int *swap_cluster_count,
  					unsigned long page_nr)
  {

@@ -203,9 +270,8 @@ static unsigned long scan_swap_map(struc
  {
  	unsigned long offset;
  	unsigned long scan_base;
-	unsigned long last_in_cluster = 0;
  	int latency_ration = LATENCY_LIMIT;
-	int found_free_cluster = 0;
+	bool has_discardable_cluster = false;
 
  	/*
  	 * We try to cluster swap pages by allocating them sequentially

@@ -228,21 +294,9 @@ static unsigned long scan_swap_map(struc
  			si->cluster_nr = SWAPFILE_CLUSTER - 1;
  			goto checks;
  		}
-		if (si->flags & SWP_DISCARDABLE) {
-			/*
-			 * Start range check on racing allocations, in case
-			 * they overlap the cluster we eventually decide on
-			 * (we scan without swap_lock to allow preemption).
-			 * It's hardly conceivable that cluster_nr could be
-			 * wrapped during our scan, but don't depend on it.
-			 */
-			if (si->lowest_alloc)
-				goto checks;
-			si->lowest_alloc = si->max;
-			si->highest_alloc = 0;
-		}
  		spin_unlock(&swap_lock);
 
+search_cluster:
  		/*
  		 * If seek is expensive, start searching for new cluster from
  		 * start of partition, to minimize the span of allocated swap.

@@ -259,13 +313,16 @@ static unsigned long scan_swap_map(struc
  		     base <= si->highest_bit; base += SWAPFILE_CLUSTER) {
  			if (!si->swap_cluster_count[base/SWAPFILE_CLUSTER]) {
  				offset = base;
-				last_in_cluster = offset + SWAPFILE_CLUSTER - 1;
  				spin_lock(&swap_lock);
  				si->cluster_next = offset;
  				si->cluster_nr = SWAPFILE_CLUSTER - 1;
-				found_free_cluster = 1;
  				goto checks;
  			}
+			if (si->swap_cluster_count[base/SWAPFILE_CLUSTER] >=
+			    CLUSTER_COUNT_DISCARDABLE) {
+				has_discardable_cluster = true;
+				schedule_work(&si->discard_work);
+			}
  			if (unlikely(--latency_ration < 0)) {
  				cond_resched();
  				latency_ration = LATENCY_LIMIT;

@@ -279,13 +336,16 @@ static unsigned long scan_swap_map(struc
  		     base < scan_base; base += SWAPFILE_CLUSTER) {
  			if (!si->swap_cluster_count[base/SWAPFILE_CLUSTER]) {
  				offset = base;
-				last_in_cluster = offset + SWAPFILE_CLUSTER - 1;
  				spin_lock(&swap_lock);
  				si->cluster_next = offset;
  				si->cluster_nr = SWAPFILE_CLUSTER - 1;
-				found_free_cluster = 1;
  				goto checks;
  			}
+			if (si->swap_cluster_count[base/SWAPFILE_CLUSTER] >=
+			    CLUSTER_COUNT_DISCARDABLE) {
+				has_discardable_cluster = true;
+				schedule_work(&si->discard_work);
+			}
  			if (unlikely(--latency_ration < 0)) {
  				cond_resched();
  				latency_ration = LATENCY_LIMIT;

@@ -293,9 +353,14 @@ static unsigned long scan_swap_map(struc
  		}
 
  		offset = scan_base;
+
+		if (has_discardable_cluster) {
+			flush_work(&si->discard_work);
+			goto search_cluster;
+		}
+
  		spin_lock(&swap_lock);
  		si->cluster_nr = SWAPFILE_CLUSTER - 1;
-		si->lowest_alloc = 0;
  	}
 
  checks:

@@ -335,59 +400,6 @@ checks:
  	si->cluster_next = offset + 1;
  	si->flags -= SWP_SCANNING;
 
-	if (si->lowest_alloc) {
-		/*
-		 * Only set when SWP_DISCARDABLE, and there's a scan
-		 * for a free cluster in progress or just completed.
-		 */
-		if (found_free_cluster) {
-			/*
-			 * To optimize wear-levelling, discard the
-			 * old data of the cluster, taking care not to
-			 * discard any of its pages that have already
-			 * been allocated by racing tasks (offset has
-			 * already stepped over any at the beginning).
-			 */
-			if (offset < si->highest_alloc &&
-			    si->lowest_alloc <= last_in_cluster)
-				last_in_cluster = si->lowest_alloc - 1;
-			si->flags |= SWP_DISCARDING;
-			spin_unlock(&swap_lock);
-
-			if (offset < last_in_cluster)
-				discard_swap_cluster(si, offset,
-					last_in_cluster - offset + 1);
-
-			spin_lock(&swap_lock);
-			si->lowest_alloc = 0;
-			si->flags &= ~SWP_DISCARDING;
-
-			smp_mb();	/* wake_up_bit advises this */
-			wake_up_bit(&si->flags, ilog2(SWP_DISCARDING));
-
-		} else if (si->flags & SWP_DISCARDING) {
-			/*
-			 * Delay using pages allocated by racing tasks
-			 * until the whole discard has been issued. We
-			 * could defer that delay until swap_writepage,
-			 * but it's easier to keep this self-contained.
-			 */
-			spin_unlock(&swap_lock);
-			wait_on_bit(&si->flags, ilog2(SWP_DISCARDING),
-				wait_for_discard, TASK_UNINTERRUPTIBLE);
-			spin_lock(&swap_lock);
-		} else {
-			/*
-			 * Note pages allocated by racing tasks while
-			 * scan for a free cluster is in progress, so
-			 * that its final discard can exclude them.
-			 */
-			if (offset < si->lowest_alloc)
-				si->lowest_alloc = offset;
-			if (offset > si->highest_alloc)
-				si->highest_alloc = offset;
-		}
-	}
  	return offset;
 
  scan:

@@ -583,6 +595,7 @@ static unsigned char swap_entry_free(str
  				disk->fops->swap_slot_free_notify(p->bdev,
  								  offset);
  		}
+		swap_cluster_check_discard(p, offset);
  	}
 
  	return usage;

@@ -1582,6 +1595,8 @@ SYSCALL_DEFINE1(swapoff, const char __us
  		goto out_dput;
  	}
 
+	flush_work(&p->discard_work);
+
  	destroy_swap_extents(p);
  	if (p->flags & SWP_CONTINUED)
  		free_swap_count_continuations(p);

@@ -1992,6 +2007,8 @@ SYSCALL_DEFINE2(swapon, const char __use
  	if (IS_ERR(p))
  		return PTR_ERR(p);
 
+	INIT_WORK(&p->discard_work, swap_discard_work);
+
  	name = getname(specialfile);
  	if (IS_ERR(name)) {
  		error = PTR_ERR(name);
 
--

To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to [email protected]. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"[email protected]"> [email protected] </a>