RAID1: a new I/O barrier implementation to remove resync window

'Commit 79ef3a8aa1cb ("raid1: Rewrite the implementation of iobarrier.")'
introduces a sliding resync window for raid1 I/O barrier, this idea limits
I/O barriers to happen only inside a slidingresync window, for regular
I/Os out of this resync window they don't need to wait for barrier any
more. On large raid1 device, it helps a lot to improve parallel writing
I/O throughput when there are background resync I/Os performing at
same time.

The idea of sliding resync widow is awesome, but code complexity is a
challenge. Sliding resync window requires several variables to work
collectively, this is complexed and very hard to make it work correctly.
Just grep "Fixes: 79ef3a8aa1" in kernel git log, there are 8 more patches
to fix the original resync window patch. This is not the end, any further
related modification may easily introduce more regreassion.

Therefore I decide to implement a much simpler raid1 I/O barrier, by
removing resync window code, I believe life will be much easier.

The brief idea of the simpler barrier is,
 - Do not maintain a global unique resync window
 - Use multiple hash buckets to reduce I/O barrier conflicts, regular
   I/O only has to wait for a resync I/O when both them have same barrier
   bucket index, vice versa.
 - I/O barrier can be reduced to an acceptable number if there are enough
   barrier buckets

Here I explain how the barrier buckets are designed,
 - BARRIER_UNIT_SECTOR_SIZE
   The whole LBA address space of a raid1 device is divided into multiple
   barrier units, by the size of BARRIER_UNIT_SECTOR_SIZE.
   Bio requests won't go across border of barrier unit size, that means
   maximum bio size is BARRIER_UNIT_SECTOR_SIZE<<9 (64MB) in bytes.
   For random I/O 64MB is large enough for both read and write requests,
   for sequential I/O considering underlying block layer may merge them
   into larger requests, 64MB is still good enough.
   Neil also points out that for resync operation, "we want the resync to
   move from region to region fairly quickly so that the slowness caused
   by having to synchronize with the resync is averaged out over a fairly
   small time frame". For full speed resync, 64MB should take less then 1
   second. When resync is competing with other I/O, it could take up a few
   minutes. Therefore 64MB size is fairly good range for resync.

 - BARRIER_BUCKETS_NR
   There are BARRIER_BUCKETS_NR buckets in total, which is defined by,
        #define BARRIER_BUCKETS_NR_BITS   (PAGE_SHIFT - 2)
        #define BARRIER_BUCKETS_NR        (1<<BARRIER_BUCKETS_NR_BITS)
   this patch makes the bellowed members of struct r1conf from integer
   to array of integers,
        -       int                     nr_pending;
        -       int                     nr_waiting;
        -       int                     nr_queued;
        -       int                     barrier;
        +       int                     *nr_pending;
        +       int                     *nr_waiting;
        +       int                     *nr_queued;
        +       int                     *barrier;
   number of the array elements is defined as BARRIER_BUCKETS_NR. For 4KB
   kernel space page size, (PAGE_SHIFT - 2) indecates there are 1024 I/O
   barrier buckets, and each array of integers occupies single memory page.
   1024 means for a request which is smaller than the I/O barrier unit size
   has ~0.1% chance to wait for resync to pause, which is quite a small
   enough fraction. Also requesting single memory page is more friendly to
   kernel page allocator than larger memory size.

 - I/O barrier bucket is indexed by bio start sector
   If multiple I/O requests hit different I/O barrier units, they only need
   to compete I/O barrier with other I/Os which hit the same I/O barrier
   bucket index with each other. The index of a barrier bucket which a
   bio should look for is calculated by sector_to_idx() which is defined
   in raid1.h as an inline function,
        static inline int sector_to_idx(sector_t sector)
        {
                return hash_long(sector >> BARRIER_UNIT_SECTOR_BITS,
                                BARRIER_BUCKETS_NR_BITS);
        }
   Here sector_nr is the start sector number of a bio.

 - Single bio won't go across boundary of a I/O barrier unit
   If a request goes across boundary of barrier unit, it will be split. A
   bio may be split in raid1_make_request() or raid1_sync_request(), if
   sectors returned by align_to_barrier_unit_end() is smaller than
   original bio size.

Comparing to single sliding resync window,
 - Currently resync I/O grows linearly, therefore regular and resync I/O
   will conflict within a single barrier units. So the I/O behavior is
   similar to single sliding resync window.
 - But a barrier unit bucket is shared by all barrier units with identical
   barrier uinit index, the probability of conflict might be higher
   than single sliding resync window, in condition that writing I/Os
   always hit barrier units which have identical barrier bucket indexs with
   the resync I/Os. This is a very rare condition in real I/O work loads,
   I cannot imagine how it could happen in practice.
 - Therefore we can achieve a good enough low conflict rate with much
   simpler barrier algorithm and implementation.

There are two changes should be noticed,
 - In raid1d(), I change the code to decrease conf->nr_pending[idx] into
   single loop, it looks like this,
        spin_lock_irqsave(&conf->device_lock, flags);
        conf->nr_queued[idx]--;
        spin_unlock_irqrestore(&conf->device_lock, flags);
   This change generates more spin lock operations, but in next patch of
   this patch set, it will be replaced by a single line code,
        atomic_dec(&conf->nr_queueud[idx]);
   So we don't need to worry about spin lock cost here.
 - Mainline raid1 code split original raid1_make_request() into
   raid1_read_request() and raid1_write_request(). If the original bio
   goes across an I/O barrier unit size, this bio will be split before
   calling raid1_read_request() or raid1_write_request(),  this change
   the code logic more simple and clear.
 - In this patch wait_barrier() is moved from raid1_make_request() to
   raid1_write_request(). In raid_read_request(), original wait_barrier()
   is replaced by raid1_read_request().
   The differnece is wait_read_barrier() only waits if array is frozen,
   using different barrier function in different code path makes the code
   more clean and easy to read.
Changelog
V4:
- Add alloc_r1bio() to remove redundant r1bio memory allocation code.
- Fix many typos in patch comments.
- Use (PAGE_SHIFT - ilog2(sizeof(int))) to define BARRIER_BUCKETS_NR_BITS.
V3:
- Rebase the patch against latest upstream kernel code.
- Many fixes by review comments from Neil,
  - Back to use pointers to replace arraries in struct r1conf
  - Remove total_barriers from struct r1conf
  - Add more patch comments to explain how/why the values of
    BARRIER_UNIT_SECTOR_SIZE and BARRIER_BUCKETS_NR are decided.
  - Use get_unqueued_pending() to replace get_all_pendings() and
    get_all_queued()
  - Increase bucket number from 512 to 1024
- Change code comments format by review from Shaohua.
V2:
- Use bio_split() to split the orignal bio if it goes across barrier unit
  bounday, to make the code more simple, by suggestion from Shaohua and
  Neil.
- Use hash_long() to replace original linear hash, to avoid a possible
  confilict between resync I/O and sequential write I/O, by suggestion from
  Shaohua.
- Add conf->total_barriers to record barrier depth, which is used to
  control number of parallel sync I/O barriers, by suggestion from Shaohua.
- In V1 patch the bellowed barrier buckets related members in r1conf are
  allocated in memory page. To make the code more simple, V2 patch moves
  the memory space into struct r1conf, like this,
        -       int                     nr_pending;
        -       int                     nr_waiting;
        -       int                     nr_queued;
        -       int                     barrier;
        +       int                     nr_pending[BARRIER_BUCKETS_NR];
        +       int                     nr_waiting[BARRIER_BUCKETS_NR];
        +       int                     nr_queued[BARRIER_BUCKETS_NR];
        +       int                     barrier[BARRIER_BUCKETS_NR];
  This change is by the suggestion from Shaohua.
- Remove some inrelavent code comments, by suggestion from Guoqing.
- Add a missing wait_barrier() before jumping to retry_write, in
  raid1_make_write_request().
V1:
- Original RFC patch for comments

Signed-off-by: Coly Li <colyli@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Guoqing Jiang <gqjiang@suse.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Signed-off-by: Shaohua Li <shli@fb.com>

2 files changed, 294 insertions, 236 deletions

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index ad5c948..40297fd 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -71,9 +71,8 @@
  */
 static int max_queued_requests = 1024;
 
-static void allow_barrier(struct r1conf *conf, sector_t start_next_window,
-			  sector_t bi_sector);
-static void lower_barrier(struct r1conf *conf);
+static void allow_barrier(struct r1conf *conf, sector_t sector_nr);
+static void lower_barrier(struct r1conf *conf, sector_t sector_nr);
 
 #define raid1_log(md, fmt, args...)				\
 	do { if ((md)->queue) blk_add_trace_msg((md)->queue, "raid1 " fmt, ##args); } while (0)
@@ -100,7 +99,6 @@ static void r1bio_pool_free(void *r1_bio, void *data)
 #define RESYNC_WINDOW_SECTORS (RESYNC_WINDOW >> 9)
 #define CLUSTER_RESYNC_WINDOW (16 * RESYNC_WINDOW)
 #define CLUSTER_RESYNC_WINDOW_SECTORS (CLUSTER_RESYNC_WINDOW >> 9)
-#define NEXT_NORMALIO_DISTANCE (3 * RESYNC_WINDOW_SECTORS)
 
 static void * r1buf_pool_alloc(gfp_t gfp_flags, void *data)
 {
@@ -215,7 +213,7 @@ static void put_buf(struct r1bio *r1_bio)
 
 	mempool_free(r1_bio, conf->r1buf_pool);
 
-	lower_barrier(conf);
+	lower_barrier(conf, r1_bio->sector);
 }
 
 static void reschedule_retry(struct r1bio *r1_bio)
@@ -223,10 +221,12 @@ static void reschedule_retry(struct r1bio *r1_bio)
 	unsigned long flags;
 	struct mddev *mddev = r1_bio->mddev;
 	struct r1conf *conf = mddev->private;
+	int idx;
 
+	idx = sector_to_idx(r1_bio->sector);
 	spin_lock_irqsave(&conf->device_lock, flags);
 	list_add(&r1_bio->retry_list, &conf->retry_list);
-	conf->nr_queued ++;
+	conf->nr_queued[idx]++;
 	spin_unlock_irqrestore(&conf->device_lock, flags);
 
 	wake_up(&conf->wait_barrier);
@@ -243,7 +243,6 @@ static void call_bio_endio(struct r1bio *r1_bio)
 	struct bio *bio = r1_bio->master_bio;
 	int done;
 	struct r1conf *conf = r1_bio->mddev->private;
-	sector_t start_next_window = r1_bio->start_next_window;
 	sector_t bi_sector = bio->bi_iter.bi_sector;
 
 	if (bio->bi_phys_segments) {
@@ -269,7 +268,7 @@ static void call_bio_endio(struct r1bio *r1_bio)
 		 * Wake up any possible resync thread that waits for the device
 		 * to go idle.
 		 */
-		allow_barrier(conf, start_next_window, bi_sector);
+		allow_barrier(conf, bi_sector);
 	}
 }
 
@@ -517,6 +516,25 @@ static void raid1_end_write_request(struct bio *bio)
 		bio_put(to_put);
 }
 
+static sector_t align_to_barrier_unit_end(sector_t start_sector,
+					  sector_t sectors)
+{
+	sector_t len;
+
+	WARN_ON(sectors == 0);
+	/*
+	 * len is the number of sectors from start_sector to end of the
+	 * barrier unit which start_sector belongs to.
+	 */
+	len = round_up(start_sector + 1, BARRIER_UNIT_SECTOR_SIZE) -
+	      start_sector;
+
+	if (len > sectors)
+		len = sectors;
+
+	return len;
+}
+
 /*
  * This routine returns the disk from which the requested read should
  * be done. There is a per-array 'next expected sequential IO' sector
@@ -813,168 +831,168 @@ static void flush_pending_writes(struct r1conf *conf)
  */
 static void raise_barrier(struct r1conf *conf, sector_t sector_nr)
 {
+	int idx = sector_to_idx(sector_nr);
+
 	spin_lock_irq(&conf->resync_lock);
 
 	/* Wait until no block IO is waiting */
-	wait_event_lock_irq(conf->wait_barrier, !conf->nr_waiting,
+	wait_event_lock_irq(conf->wait_barrier, !conf->nr_waiting[idx],
 			    conf->resync_lock);
 
 	/* block any new IO from starting */
-	conf->barrier++;
-	conf->next_resync = sector_nr;
+	conf->barrier[idx]++;
 
 	/* For these conditions we must wait:
 	 * A: while the array is in frozen state
-	 * B: while barrier >= RESYNC_DEPTH, meaning resync reach
-	 *    the max count which allowed.
-	 * C: next_resync + RESYNC_SECTORS > start_next_window, meaning
-	 *    next resync will reach to the window which normal bios are
-	 *    handling.
-	 * D: while there are any active requests in the current window.
+	 * B: while conf->nr_pending[idx] is not 0, meaning regular I/O
+	 *    existing in corresponding I/O barrier bucket.
+	 * C: while conf->barrier[idx] >= RESYNC_DEPTH, meaning reaches
+	 *    max resync count which allowed on current I/O barrier bucket.
 	 */
 	wait_event_lock_irq(conf->wait_barrier,
 			    !conf->array_frozen &&
-			    conf->barrier < RESYNC_DEPTH &&
-			    conf->current_window_requests == 0 &&
-			    (conf->start_next_window >=
-			     conf->next_resync + RESYNC_SECTORS),
+			     !conf->nr_pending[idx] &&
+			     conf->barrier[idx] < RESYNC_DEPTH,
 			    conf->resync_lock);
 
-	conf->nr_pending++;
+	conf->nr_pending[idx]++;
 	spin_unlock_irq(&conf->resync_lock);
 }
 
-static void lower_barrier(struct r1conf *conf)
+static void lower_barrier(struct r1conf *conf, sector_t sector_nr)
 {
 	unsigned long flags;
-	BUG_ON(conf->barrier <= 0);
+	int idx = sector_to_idx(sector_nr);
+
+	BUG_ON(conf->barrier[idx] <= 0);
+
 	spin_lock_irqsave(&conf->resync_lock, flags);
-	conf->barrier--;
-	conf->nr_pending--;
+	conf->barrier[idx]--;
+	conf->nr_pending[idx]--;
 	spin_unlock_irqrestore(&conf->resync_lock, flags);
 	wake_up(&conf->wait_barrier);
 }
 
-static bool need_to_wait_for_sync(struct r1conf *conf, struct bio *bio)
+static void _wait_barrier(struct r1conf *conf, int idx)
 {
-	bool wait = false;
-
-	if (conf->array_frozen || !bio)
-		wait = true;
-	else if (conf->barrier && bio_data_dir(bio) == WRITE) {
-		if ((conf->mddev->curr_resync_completed
-		     >= bio_end_sector(bio)) ||
-		    (conf->start_next_window + NEXT_NORMALIO_DISTANCE
-		     <= bio->bi_iter.bi_sector))
-			wait = false;
-		else
-			wait = true;
+	spin_lock_irq(&conf->resync_lock);
+	if (conf->array_frozen || conf->barrier[idx]) {
+		conf->nr_waiting[idx]++;
+		/* Wait for the barrier to drop. */
+		wait_event_lock_irq(
+			conf->wait_barrier,
+			!conf->array_frozen && !conf->barrier[idx],
+			conf->resync_lock);
+		conf->nr_waiting[idx]--;
 	}
 
-	return wait;
+	conf->nr_pending[idx]++;
+	spin_unlock_irq(&conf->resync_lock);
 }
 
-static sector_t wait_barrier(struct r1conf *conf, struct bio *bio)
+static void wait_read_barrier(struct r1conf *conf, sector_t sector_nr)
 {
-	sector_t sector = 0;
+	int idx = sector_to_idx(sector_nr);
 
 	spin_lock_irq(&conf->resync_lock);
-	if (need_to_wait_for_sync(conf, bio)) {
-		conf->nr_waiting++;
-		/* Wait for the barrier to drop.
-		 * However if there are already pending
-		 * requests (preventing the barrier from
-		 * rising completely), and the
-		 * per-process bio queue isn't empty,
-		 * then don't wait, as we need to empty
-		 * that queue to allow conf->start_next_window
-		 * to increase.
-		 */
-		raid1_log(conf->mddev, "wait barrier");
-		wait_event_lock_irq(conf->wait_barrier,
-				    !conf->array_frozen &&
-				    (!conf->barrier ||
-				     ((conf->start_next_window <
-				       conf->next_resync + RESYNC_SECTORS) &&
-				      current->bio_list &&
-				      !bio_list_empty(current->bio_list))),
-				    conf->resync_lock);
-		conf->nr_waiting--;
-	}
-
-	if (bio && bio_data_dir(bio) == WRITE) {
-		if (bio->bi_iter.bi_sector >= conf->next_resync) {
-			if (conf->start_next_window == MaxSector)
-				conf->start_next_window =
-					conf->next_resync +
-					NEXT_NORMALIO_DISTANCE;
-
-			if ((conf->start_next_window + NEXT_NORMALIO_DISTANCE)
-			    <= bio->bi_iter.bi_sector)
-				conf->next_window_requests++;
-			else
-				conf->current_window_requests++;
-			sector = conf->start_next_window;
-		}
+	if (conf->array_frozen) {
+		conf->nr_waiting[idx]++;
+		/* Wait for array to unfreeze */
+		wait_event_lock_irq(
+			conf->wait_barrier,
+			!conf->array_frozen,
+			conf->resync_lock);
+		conf->nr_waiting[idx]--;
 	}
 
-	conf->nr_pending++;
+	conf->nr_pending[idx]++;
 	spin_unlock_irq(&conf->resync_lock);
-	return sector;
 }
 
-static void allow_barrier(struct r1conf *conf, sector_t start_next_window,
-			  sector_t bi_sector)
+static void wait_barrier(struct r1conf *conf, sector_t sector_nr)
+{
+	int idx = sector_to_idx(sector_nr);
+
+	_wait_barrier(conf, idx);
+}
+
+static void wait_all_barriers(struct r1conf *conf)
+{
+	int idx;
+
+	for (idx = 0; idx < BARRIER_BUCKETS_NR; idx++)
+		_wait_barrier(conf, idx);
+}
+
+static void _allow_barrier(struct r1conf *conf, int idx)
 {
 	unsigned long flags;
 
 	spin_lock_irqsave(&conf->resync_lock, flags);
-	conf->nr_pending--;
-	if (start_next_window) {
-		if (start_next_window == conf->start_next_window) {
-			if (conf->start_next_window + NEXT_NORMALIO_DISTANCE
-			    <= bi_sector)
-				conf->next_window_requests--;
-			else
-				conf->current_window_requests--;
-		} else
-			conf->current_window_requests--;
-
-		if (!conf->current_window_requests) {
-			if (conf->next_window_requests) {
-				conf->current_window_requests =
-					conf->next_window_requests;
-				conf->next_window_requests = 0;
-				conf->start_next_window +=
-					NEXT_NORMALIO_DISTANCE;
-			} else
-				conf->start_next_window = MaxSector;
-		}
-	}
+	conf->nr_pending[idx]--;
 	spin_unlock_irqrestore(&conf->resync_lock, flags);
 	wake_up(&conf->wait_barrier);
 }
 
+static void allow_barrier(struct r1conf *conf, sector_t sector_nr)
+{
+	int idx = sector_to_idx(sector_nr);
+
+	_allow_barrier(conf, idx);
+}
+
+static void allow_all_barriers(struct r1conf *conf)
+{
+	int idx;
+
+	for (idx = 0; idx < BARRIER_BUCKETS_NR; idx++)
+		_allow_barrier(conf, idx);
+}
+
+/* conf->resync_lock should be held */
+static int get_unqueued_pending(struct r1conf *conf)
+{
+	int idx, ret;
+
+	for (ret = 0, idx = 0; idx < BARRIER_BUCKETS_NR; idx++)
+		ret += conf->nr_pending[idx] - conf->nr_queued[idx];
+
+	return ret;
+}
+
 static void freeze_array(struct r1conf *conf, int extra)
 {
-	/* stop syncio and normal IO and wait for everything to
+	/* Stop sync I/O and normal I/O and wait for everything to
 	 * go quite.
-	 * We wait until nr_pending match nr_queued+extra
-	 * This is called in the context of one normal IO request
-	 * that has failed. Thus any sync request that might be pending
-	 * will be blocked by nr_pending, and we need to wait for
-	 * pending IO requests to complete or be queued for re-try.
-	 * Thus the number queued (nr_queued) plus this request (extra)
-	 * must match the number of pending IOs (nr_pending) before
-	 * we continue.
+	 * This is called in two situations:
+	 * 1) management command handlers (reshape, remove disk, quiesce).
+	 * 2) one normal I/O request failed.
+
+	 * After array_frozen is set to 1, new sync IO will be blocked at
+	 * raise_barrier(), and new normal I/O will blocked at _wait_barrier()
+	 * or wait_read_barrier(). The flying I/Os will either complete or be
+	 * queued. When everything goes quite, there are only queued I/Os left.
+
+	 * Every flying I/O contributes to a conf->nr_pending[idx], idx is the
+	 * barrier bucket index which this I/O request hits. When all sync and
+	 * normal I/O are queued, sum of all conf->nr_pending[] will match sum
+	 * of all conf->nr_queued[]. But normal I/O failure is an exception,
+	 * in handle_read_error(), we may call freeze_array() before trying to
+	 * fix the read error. In this case, the error read I/O is not queued,
+	 * so get_unqueued_pending() == 1.
+	 *
+	 * Therefore before this function returns, we need to wait until
+	 * get_unqueued_pendings(conf) gets equal to extra. For
+	 * normal I/O context, extra is 1, in rested situations extra is 0.
 	 */
 	spin_lock_irq(&conf->resync_lock);
 	conf->array_frozen = 1;
 	raid1_log(conf->mddev, "wait freeze");
-	wait_event_lock_irq_cmd(conf->wait_barrier,
-				conf->nr_pending == conf->nr_queued+extra,
-				conf->resync_lock,
-				flush_pending_writes(conf));
+	wait_event_lock_irq_cmd(
+		conf->wait_barrier,
+		get_unqueued_pending(conf) == extra,
+		conf->resync_lock,
+		flush_pending_writes(conf));
 	spin_unlock_irq(&conf->resync_lock);
 }
 static void unfreeze_array(struct r1conf *conf)
@@ -1070,11 +1088,28 @@ static void raid1_unplug(struct blk_plug_cb *cb, bool from_schedule)
 	kfree(plug);
 }
 
-static void raid1_read_request(struct mddev *mddev, struct bio *bio,
-				 struct r1bio *r1_bio)
+static inline struct r1bio *
+alloc_r1bio(struct mddev *mddev, struct bio *bio, sector_t sectors_handled)
+{
+	struct r1conf *conf = mddev->private;
+	struct r1bio *r1_bio;
+
+	r1_bio = mempool_alloc(conf->r1bio_pool, GFP_NOIO);
+
+	r1_bio->master_bio = bio;
+	r1_bio->sectors = bio_sectors(bio) - sectors_handled;
+	r1_bio->state = 0;
+	r1_bio->mddev = mddev;
+	r1_bio->sector = bio->bi_iter.bi_sector + sectors_handled;
+
+	return r1_bio;
+}
+
+static void raid1_read_request(struct mddev *mddev, struct bio *bio)
 {
 	struct r1conf *conf = mddev->private;
 	struct raid1_info *mirror;
+	struct r1bio *r1_bio;
 	struct bio *read_bio;
 	struct bitmap *bitmap = mddev->bitmap;
 	const int op = bio_op(bio);
@@ -1083,8 +1118,29 @@ static void raid1_read_request(struct mddev *mddev, struct bio *bio,
 	int max_sectors;
 	int rdisk;
 
-	wait_barrier(conf, bio);
+	/*
+	 * Still need barrier for READ in case that whole
+	 * array is frozen.
+	 */
+	wait_read_barrier(conf, bio->bi_iter.bi_sector);
+
+	r1_bio = alloc_r1bio(mddev, bio, 0);
 
+	/*
+	 * We might need to issue multiple reads to different
+	 * devices if there are bad blocks around, so we keep
+	 * track of the number of reads in bio->bi_phys_segments.
+	 * If this is 0, there is only one r1_bio and no locking
+	 * will be needed when requests complete.  If it is
+	 * non-zero, then it is the number of not-completed requests.
+	 */
+	bio->bi_phys_segments = 0;
+	bio_clear_flag(bio, BIO_SEG_VALID);
+
+	/*
+	 * make_request() can abort the operation when read-ahead is being
+	 * used and no empty request is available.
+	 */
 read_again:
 	rdisk = read_balance(conf, r1_bio, &max_sectors);
 
@@ -1106,7 +1162,6 @@ read_again:
 			   atomic_read(&bitmap->behind_writes) == 0);
 	}
 	r1_bio->read_disk = rdisk;
-	r1_bio->start_next_window = 0;
 
 	read_bio = bio_clone_fast(bio, GFP_NOIO, mddev->bio_set);
 	bio_trim(read_bio, r1_bio->sector - bio->bi_iter.bi_sector,
@@ -1151,22 +1206,16 @@ read_again:
 		 */
 		reschedule_retry(r1_bio);
 
-		r1_bio = mempool_alloc(conf->r1bio_pool, GFP_NOIO);
-
-		r1_bio->master_bio = bio;
-		r1_bio->sectors = bio_sectors(bio) - sectors_handled;
-		r1_bio->state = 0;
-		r1_bio->mddev = mddev;
-		r1_bio->sector = bio->bi_iter.bi_sector + sectors_handled;
+		r1_bio = alloc_r1bio(mddev, bio, sectors_handled);
 		goto read_again;
 	} else
 		generic_make_request(read_bio);
 }
 
-static void raid1_write_request(struct mddev *mddev, struct bio *bio,
-				struct r1bio *r1_bio)
+static void raid1_write_request(struct mddev *mddev, struct bio *bio)
 {
 	struct r1conf *conf = mddev->private;
+	struct r1bio *r1_bio;
 	int i, disks;
 	struct bitmap *bitmap = mddev->bitmap;
 	unsigned long flags;
@@ -1180,7 +1229,6 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
 	int first_clone;
 	int sectors_handled;
 	int max_sectors;
-	sector_t start_next_window;
 
 	/*
 	 * Register the new request and wait if the reconstruction
@@ -1216,7 +1264,19 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
 		}
 		finish_wait(&conf->wait_barrier, &w);
 	}
-	start_next_window = wait_barrier(conf, bio);
+	wait_barrier(conf, bio->bi_iter.bi_sector);
+
+	r1_bio = alloc_r1bio(mddev, bio, 0);
+
+	/* We might need to issue multiple writes to different
+	 * devices if there are bad blocks around, so we keep
+	 * track of the number of writes in bio->bi_phys_segments.
+	 * If this is 0, there is only one r1_bio and no locking
+	 * will be needed when requests complete.  If it is
+	 * non-zero, then it is the number of not-completed requests.
+	 */
+	bio->bi_phys_segments = 0;
+	bio_clear_flag(bio, BIO_SEG_VALID);
 
 	if (conf->pending_count >= max_queued_requests) {
 		md_wakeup_thread(mddev->thread);
@@ -1237,7 +1297,6 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
 
 	disks = conf->raid_disks * 2;
  retry_write:
-	r1_bio->start_next_window = start_next_window;
 	blocked_rdev = NULL;
 	rcu_read_lock();
 	max_sectors = r1_bio->sectors;
@@ -1304,25 +1363,15 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
 	if (unlikely(blocked_rdev)) {
 		/* Wait for this device to become unblocked */
 		int j;
-		sector_t old = start_next_window;
 
 		for (j = 0; j < i; j++)
 			if (r1_bio->bios[j])
 				rdev_dec_pending(conf->mirrors[j].rdev, mddev);
 		r1_bio->state = 0;
-		allow_barrier(conf, start_next_window, bio->bi_iter.bi_sector);
+		allow_barrier(conf, bio->bi_iter.bi_sector);
 		raid1_log(mddev, "wait rdev %d blocked", blocked_rdev->raid_disk);
 		md_wait_for_blocked_rdev(blocked_rdev, mddev);
-		start_next_window = wait_barrier(conf, bio);
-		/*
-		 * We must make sure the multi r1bios of bio have
-		 * the same value of bi_phys_segments
-		 */
-		if (bio->bi_phys_segments && old &&
-		    old != start_next_window)
-			/* Wait for the former r1bio(s) to complete */
-			wait_event(conf->wait_barrier,
-				   bio->bi_phys_segments == 1);
+		wait_barrier(conf, bio->bi_iter.bi_sector);
 		goto retry_write;
 	}
 
@@ -1440,12 +1489,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
 		/* We need another r1_bio.  It has already been counted
 		 * in bio->bi_phys_segments
 		 */
-		r1_bio = mempool_alloc(conf->r1bio_pool, GFP_NOIO);
-		r1_bio->master_bio = bio;
-		r1_bio->sectors = bio_sectors(bio) - sectors_handled;
-		r1_bio->state = 0;
-		r1_bio->mddev = mddev;
-		r1_bio->sector = bio->bi_iter.bi_sector + sectors_handled;
+		r1_bio = alloc_r1bio(mddev, bio, sectors_handled);
 		goto retry_write;
 	}
 
@@ -1457,36 +1501,25 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
 
 static void raid1_make_request(struct mddev *mddev, struct bio *bio)
 {
-	struct r1conf *conf = mddev->private;
-	struct r1bio *r1_bio;
+	struct bio *split;
+	sector_t sectors;
 
-	/*
-	 * make_request() can abort the operation when read-ahead is being
-	 * used and no empty request is available.
-	 *
-	 */
-	r1_bio = mempool_alloc(conf->r1bio_pool, GFP_NOIO);
-
-	r1_bio->master_bio = bio;
-	r1_bio->sectors = bio_sectors(bio);
-	r1_bio->state = 0;
-	r1_bio->mddev = mddev;
-	r1_bio->sector = bio->bi_iter.bi_sector;
-
-	/*
-	 * We might need to issue multiple reads to different devices if there
-	 * are bad blocks around, so we keep track of the number of reads in
-	 * bio->bi_phys_segments.  If this is 0, there is only one r1_bio and
-	 * no locking will be needed when requests complete.  If it is
-	 * non-zero, then it is the number of not-completed requests.
-	 */
-	bio->bi_phys_segments = 0;
-	bio_clear_flag(bio, BIO_SEG_VALID);
+	/* if bio exceeds barrier unit boundary, split it */
+	do {
+		sectors = align_to_barrier_unit_end(
+				bio->bi_iter.bi_sector, bio_sectors(bio));
+		if (sectors < bio_sectors(bio)) {
+			split = bio_split(bio, sectors, GFP_NOIO, fs_bio_set);
+			bio_chain(split, bio);
+		} else {
+			split = bio;
+		}
 
-	if (bio_data_dir(bio) == READ)
-		raid1_read_request(mddev, bio, r1_bio);
-	else
-		raid1_write_request(mddev, bio, r1_bio);
+		if (bio_data_dir(split) == READ)
+			raid1_read_request(mddev, split);
+		else
+			raid1_write_request(mddev, split);
+	} while (split != bio);
 }
 
 static void raid1_status(struct seq_file *seq, struct mddev *mddev)
@@ -1577,19 +1610,11 @@ static void print_conf(struct r1conf *conf)
 
 static void close_sync(struct r1conf *conf)
 {
-	wait_barrier(conf, NULL);
-	allow_barrier(conf, 0, 0);
+	wait_all_barriers(conf);
+	allow_all_barriers(conf);
 
 	mempool_destroy(conf->r1buf_pool);
 	conf->r1buf_pool = NULL;
-
-	spin_lock_irq(&conf->resync_lock);
-	conf->next_resync = MaxSector - 2 * NEXT_NORMALIO_DISTANCE;
-	conf->start_next_window = MaxSector;
-	conf->current_window_requests +=
-		conf->next_window_requests;
-	conf->next_window_requests = 0;
-	spin_unlock_irq(&conf->resync_lock);
 }
 
 static int raid1_spare_active(struct mddev *mddev)
@@ -2337,8 +2362,9 @@ static void handle_sync_write_finished(struct r1conf *conf, struct r1bio *r1_bio
 
 static void handle_write_finished(struct r1conf *conf, struct r1bio *r1_bio)
 {
-	int m;
+	int m, idx;
 	bool fail = false;
+
 	for (m = 0; m < conf->raid_disks * 2 ; m++)
 		if (r1_bio->bios[m] == IO_MADE_GOOD) {
 			struct md_rdev *rdev = conf->mirrors[m].rdev;
@@ -2364,7 +2390,8 @@ static void handle_write_finished(struct r1conf *conf, struct r1bio *r1_bio)
 	if (fail) {
 		spin_lock_irq(&conf->device_lock);
 		list_add(&r1_bio->retry_list, &conf->bio_end_io_list);
-		conf->nr_queued++;
+		idx = sector_to_idx(r1_bio->sector);
+		conf->nr_queued[idx]++;
 		spin_unlock_irq(&conf->device_lock);
 		md_wakeup_thread(conf->mddev->thread);
 	} else {
@@ -2460,15 +2487,8 @@ read_more:
 			generic_make_request(bio);
 			bio = NULL;
 
-			r1_bio = mempool_alloc(conf->r1bio_pool, GFP_NOIO);
-
-			r1_bio->master_bio = mbio;
-			r1_bio->sectors = bio_sectors(mbio) - sectors_handled;
-			r1_bio->state = 0;
+			r1_bio = alloc_r1bio(mddev, mbio, sectors_handled);
 			set_bit(R1BIO_ReadError, &r1_bio->state);
-			r1_bio->mddev = mddev;
-			r1_bio->sector = mbio->bi_iter.bi_sector +
-				sectors_handled;
 
 			goto read_more;
 		} else {
@@ -2487,6 +2507,7 @@ static void raid1d(struct md_thread *thread)
 	struct r1conf *conf = mddev->private;
 	struct list_head *head = &conf->retry_list;
 	struct blk_plug plug;
+	int idx;
 
 	md_check_recovery(mddev);
 
@@ -2494,17 +2515,17 @@ static void raid1d(struct md_thread *thread)
 	    !test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags)) {
 		LIST_HEAD(tmp);
 		spin_lock_irqsave(&conf->device_lock, flags);
-		if (!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags)) {
-			while (!list_empty(&conf->bio_end_io_list)) {
-				list_move(conf->bio_end_io_list.prev, &tmp);
-				conf->nr_queued--;
-			}
-		}
+		if (!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags))
+			list_splice_init(&conf->bio_end_io_list, &tmp);
 		spin_unlock_irqrestore(&conf->device_lock, flags);
 		while (!list_empty(&tmp)) {
 			r1_bio = list_first_entry(&tmp, struct r1bio,
 						  retry_list);
 			list_del(&r1_bio->retry_list);
+			idx = sector_to_idx(r1_bio->sector);
+			spin_lock_irqsave(&conf->device_lock, flags);
+			conf->nr_queued[idx]--;
+			spin_unlock_irqrestore(&conf->device_lock, flags);
 			if (mddev->degraded)
 				set_bit(R1BIO_Degraded, &r1_bio->state);
 			if (test_bit(R1BIO_WriteError, &r1_bio->state))
@@ -2525,7 +2546,8 @@ static void raid1d(struct md_thread *thread)
 		}
 		r1_bio = list_entry(head->prev, struct r1bio, retry_list);
 		list_del(head->prev);
-		conf->nr_queued--;
+		idx = sector_to_idx(r1_bio->sector);
+		conf->nr_queued[idx]--;
 		spin_unlock_irqrestore(&conf->device_lock, flags);
 
 		mddev = r1_bio->mddev;
@@ -2564,7 +2586,6 @@ static int init_resync(struct r1conf *conf)
 					  conf->poolinfo);
 	if (!conf->r1buf_pool)
 		return -ENOMEM;
-	conf->next_resync = 0;
 	return 0;
 }
 
@@ -2593,6 +2614,7 @@ static sector_t raid1_sync_request(struct mddev *mddev, sector_t sector_nr,
 	int still_degraded = 0;
 	int good_sectors = RESYNC_SECTORS;
 	int min_bad = 0; /* number of sectors that are bad in all devices */
+	int idx = sector_to_idx(sector_nr);
 
 	if (!conf->r1buf_pool)
 		if (init_resync(conf))
@@ -2642,7 +2664,7 @@ static sector_t raid1_sync_request(struct mddev *mddev, sector_t sector_nr,
 	 * If there is non-resync activity waiting for a turn, then let it
 	 * though before starting on this new sync request.
 	 */
-	if (conf->nr_waiting)
+	if (conf->nr_waiting[idx])
 		schedule_timeout_uninterruptible(1);
 
 	/* we are incrementing sector_nr below. To be safe, we check against
@@ -2669,6 +2691,8 @@ static sector_t raid1_sync_request(struct mddev *mddev, sector_t sector_nr,
 	r1_bio->sector = sector_nr;
 	r1_bio->state = 0;
 	set_bit(R1BIO_IsSync, &r1_bio->state);
+	/* make sure good_sectors won't go across barrier unit boundary */
+	good_sectors = align_to_barrier_unit_end(sector_nr, good_sectors);
 
 	for (i = 0; i < conf->raid_disks * 2; i++) {
 		struct md_rdev *rdev;
@@ -2899,6 +2923,26 @@ static struct r1conf *setup_conf(struct mddev *mddev)
 	if (!conf)
 		goto abort;
 
+	conf->nr_pending = kcalloc(BARRIER_BUCKETS_NR,
+				   sizeof(int), GFP_KERNEL);
+	if (!conf->nr_pending)
+		goto abort;
+
+	conf->nr_waiting = kcalloc(BARRIER_BUCKETS_NR,
+				   sizeof(int), GFP_KERNEL);
+	if (!conf->nr_waiting)
+		goto abort;
+
+	conf->nr_queued = kcalloc(BARRIER_BUCKETS_NR,
+				  sizeof(int), GFP_KERNEL);
+	if (!conf->nr_queued)
+		goto abort;
+
+	conf->barrier = kcalloc(BARRIER_BUCKETS_NR,
+				sizeof(int), GFP_KERNEL);
+	if (!conf->barrier)
+		goto abort;
+
 	conf->mirrors = kzalloc(sizeof(struct raid1_info)
 				* mddev->raid_disks * 2,
 				 GFP_KERNEL);
@@ -2954,9 +2998,6 @@ static struct r1conf *setup_conf(struct mddev *mddev)
 	conf->pending_count = 0;
 	conf->recovery_disabled = mddev->recovery_disabled - 1;
 
-	conf->start_next_window = MaxSector;
-	conf->current_window_requests = conf->next_window_requests = 0;
-
 	err = -EIO;
 	for (i = 0; i < conf->raid_disks * 2; i++) {
 
@@ -2999,6 +3040,10 @@ static struct r1conf *setup_conf(struct mddev *mddev)
 		kfree(conf->mirrors);
 		safe_put_page(conf->tmppage);
 		kfree(conf->poolinfo);
+		kfree(conf->nr_pending);
+		kfree(conf->nr_waiting);
+		kfree(conf->nr_queued);
+		kfree(conf->barrier);
 		kfree(conf);
 	}
 	return ERR_PTR(err);
@@ -3100,6 +3145,10 @@ static void raid1_free(struct mddev *mddev, void *priv)
 	kfree(conf->mirrors);
 	safe_put_page(conf->tmppage);
 	kfree(conf->poolinfo);
+	kfree(conf->nr_pending);
+	kfree(conf->nr_waiting);
+	kfree(conf->nr_queued);
+	kfree(conf->barrier);
 	kfree(conf);
 }
 
diff --git a/drivers/md/raid1.h b/drivers/md/raid1.h
index c52ef42..3442e8f 100644
--- a/drivers/md/raid1.h
+++ b/drivers/md/raid1.h
@@ -1,6 +1,29 @@
 #ifndef _RAID1_H
 #define _RAID1_H
 
+/*
+ * each barrier unit size is 64MB fow now
+ * note: it must be larger than RESYNC_DEPTH
+ */
+#define BARRIER_UNIT_SECTOR_BITS	17
+#define BARRIER_UNIT_SECTOR_SIZE	(1<<17)
+/*
+ * In struct r1conf, the following members are related to I/O barrier
+ * buckets,
+ *	int	*nr_pending;
+ *	int	*nr_waiting;
+ *	int	*nr_queued;
+ *	int	*barrier;
+ * Each of them points to array of integers, each array is designed to
+ * have BARRIER_BUCKETS_NR elements and occupy a single memory page. The
+ * data width of integer variables is 4, equal to 1<<(ilog2(sizeof(int))),
+ * BARRIER_BUCKETS_NR_BITS is defined as (PAGE_SHIFT - ilog2(sizeof(int)))
+ * to make sure an array of integers with BARRIER_BUCKETS_NR elements just
+ * exactly occupies a single memory page.
+ */
+#define BARRIER_BUCKETS_NR_BITS		(PAGE_SHIFT - ilog2(sizeof(int)))
+#define BARRIER_BUCKETS_NR		(1<<BARRIER_BUCKETS_NR_BITS)
+
 struct raid1_info {
 	struct md_rdev	*rdev;
 	sector_t	head_position;
@@ -35,25 +58,6 @@ struct r1conf {
 						 */
 	int			raid_disks;
 
-	/* During resync, read_balancing is only allowed on the part
-	 * of the array that has been resynced.  'next_resync' tells us
-	 * where that is.
-	 */
-	sector_t		next_resync;
-
-	/* When raid1 starts resync, we divide array into four partitions
-	 * |---------|--------------|---------------------|-------------|
-	 *        next_resync   start_next_window       end_window
-	 * start_next_window = next_resync + NEXT_NORMALIO_DISTANCE
-	 * end_window = start_next_window + NEXT_NORMALIO_DISTANCE
-	 * current_window_requests means the count of normalIO between
-	 *   start_next_window and end_window.
-	 * next_window_requests means the count of normalIO after end_window.
-	 * */
-	sector_t		start_next_window;
-	int			current_window_requests;
-	int			next_window_requests;
-
 	spinlock_t		device_lock;
 
 	/* list of 'struct r1bio' that need to be processed by raid1d,
@@ -79,10 +83,10 @@ struct r1conf {
 	 */
 	wait_queue_head_t	wait_barrier;
 	spinlock_t		resync_lock;
-	int			nr_pending;
-	int			nr_waiting;
-	int			nr_queued;
-	int			barrier;
+	int			*nr_pending;
+	int			*nr_waiting;
+	int			*nr_queued;
+	int			*barrier;
 	int			array_frozen;
 
 	/* Set to 1 if a full sync is needed, (fresh device added).
@@ -135,7 +139,6 @@ struct r1bio {
 						 * in this BehindIO request
 						 */
 	sector_t		sector;
-	sector_t		start_next_window;
 	int			sectors;
 	unsigned long		state;
 	struct mddev		*mddev;
@@ -185,4 +188,10 @@ enum r1bio_state {
 	R1BIO_WriteError,
 	R1BIO_FailFast,
 };
+
+static inline int sector_to_idx(sector_t sector)
+{
+	return hash_long(sector >> BARRIER_UNIT_SECTOR_BITS,
+			 BARRIER_BUCKETS_NR_BITS);
+}
 #endif