[PATCH v2 0/4] Misc dm writecache patches

[PATCH v2 0/4] Misc dm writecache patches

[Huaisheng Ye]

From: Huaisheng Ye <yehs1@lenovo.com>

This patch set is a collection about some patches for dm-writecache.

Patches 0001-0003 have got Acked-by or Signed-off-by from the author and
dm maintainer.

Patches 0004 is still under review. Any comments and suggestion are welcome.

Huaisheng Ye (4):
  dm writecache: remove needless dereferences in
    __writecache_writeback_pmem()
  dm writecache: add unlikely for returned value of rb_next/prev
  dm writecache: remove unused member page_offset in writeback_struct
  dm writecache: avoid unnecessary lookups in writecache_find_entry

 drivers/md/dm-writecache.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

-- 
1.8.3.1

[PATCH v2 1/4] dm writecache: remove needless dereferences in __writecache_writeback_pmem()

[Huaisheng Ye]

From: Huaisheng Ye <yehs1@lenovo.com>

bio is already available so there is no need to access it in terms of
the wb pointer.

Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm-writecache.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
index f782287..5b4d1c1 100644
--- a/drivers/md/dm-writecache.c
+++ b/drivers/md/dm-writecache.c
@@ -1478,9 +1478,9 @@ static void __writecache_writeback_pmem(struct dm_writecache *wc, struct writeba
 		bio = bio_alloc_bioset(GFP_NOIO, max_pages, &wc->bio_set);
 		wb = container_of(bio, struct writeback_struct, bio);
 		wb->wc = wc;
-		wb->bio.bi_end_io = writecache_writeback_endio;
-		bio_set_dev(&wb->bio, wc->dev->bdev);
-		wb->bio.bi_iter.bi_sector = read_original_sector(wc, e);
+		bio->bi_end_io = writecache_writeback_endio;
+		bio_set_dev(bio, wc->dev->bdev);
+		bio->bi_iter.bi_sector = read_original_sector(wc, e);
 		wb->page_offset = PAGE_SIZE;
 		if (max_pages <= WB_LIST_INLINE ||
 		    unlikely(!(wb->wc_list = kmalloc_array(max_pages, sizeof(struct wc_entry *),
@@ -1507,12 +1507,12 @@ static void __writecache_writeback_pmem(struct dm_writecache *wc, struct writeba
 			wb->wc_list[wb->wc_list_n++] = f;
 			e = f;
 		}
-		bio_set_op_attrs(&wb->bio, REQ_OP_WRITE, WC_MODE_FUA(wc) * REQ_FUA);
+		bio_set_op_attrs(bio, REQ_OP_WRITE, WC_MODE_FUA(wc) * REQ_FUA);
 		if (writecache_has_error(wc)) {
 			bio->bi_status = BLK_STS_IOERR;
-			bio_endio(&wb->bio);
+			bio_endio(bio);
 		} else {
-			submit_bio(&wb->bio);
+			submit_bio(bio);
 		}
 
 		__writeback_throttle(wc, wbl);
-- 
1.8.3.1

[PATCH v2 2/4] dm writecache: add unlikely for returned value of rb_next/prev

[Huaisheng Ye]

From: Huaisheng Ye <yehs1@lenovo.com>

In functions writecache_discard() and writecache_find_entry() there is a
high probablity that the pointer of structure rb_node won't equal NULL.
Add unlikely for the pointer node NULL.

Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm-writecache.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
index 5b4d1c1..cfbbfbc 100644
--- a/drivers/md/dm-writecache.c
+++ b/drivers/md/dm-writecache.c
@@ -571,7 +571,7 @@ static struct wc_entry *writecache_find_entry(struct dm_writecache *wc,
 			node = rb_prev(&e->rb_node);
 		else
 			node = rb_next(&e->rb_node);
-		if (!node)
+		if (unlikely(!node))
 			return e;
 		e2 = container_of(node, struct wc_entry, rb_node);
 		if (read_original_sector(wc, e2) != block)
@@ -804,7 +804,7 @@ static void writecache_discard(struct dm_writecache *wc, sector_t start, sector_
 			writecache_free_entry(wc, e);
 		}
 
-		if (!node)
+		if (unlikely(!node))
 			break;
 
 		e = container_of(node, struct wc_entry, rb_node);
-- 
1.8.3.1

[PATCH v2 3/4] dm writecache: remove unused member page_offset in writeback_struct

[Huaisheng Ye]

From: Huaisheng Ye <yehs1@lenovo.com>

The stucture member page_offset in writeback_struct never has been
used actually. Remove it.

Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
---
 drivers/md/dm-writecache.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
index cfbbfbc..ddf1732 100644
--- a/drivers/md/dm-writecache.c
+++ b/drivers/md/dm-writecache.c
@@ -190,7 +190,6 @@ struct writeback_struct {
 	struct dm_writecache *wc;
 	struct wc_entry **wc_list;
 	unsigned wc_list_n;
-	unsigned page_offset;
 	struct page *page;
 	struct wc_entry *wc_list_inline[WB_LIST_INLINE];
 	struct bio bio;
@@ -1481,7 +1480,6 @@ static void __writecache_writeback_pmem(struct dm_writecache *wc, struct writeba
 		bio->bi_end_io = writecache_writeback_endio;
 		bio_set_dev(bio, wc->dev->bdev);
 		bio->bi_iter.bi_sector = read_original_sector(wc, e);
-		wb->page_offset = PAGE_SIZE;
 		if (max_pages <= WB_LIST_INLINE ||
 		    unlikely(!(wb->wc_list = kmalloc_array(max_pages, sizeof(struct wc_entry *),
 							   GFP_NOIO | __GFP_NORETRY |
-- 
1.8.3.1

[PATCH v2 4/4] dm writecache: avoid unnecessary lookups in writecache_find_entry

[Huaisheng Ye]

From: Huaisheng Ye <yehs1@lenovo.com>

Only when entry has been found, that would only be necessary to check the
lowest or highest seq-count.

Add local variable "found" in writecache_find_entry, if no entry has been
found, it is meaningless that having a useless rb_prev or rb_next.

This patch is not designed for fixing logical error. That is used for
optimizing the behavior of writecache_find_entry.

Give an example to illustrate the point below.
Suppose that is the case, here is a normal READ bio comes to writecache_map.
And because of bio's direction is READ, writecache_find_entry would be called
with flags WFE_RETURN_FOLLOWING.

Now there are two scenarios,
1. writecache_find_entry successfully get an existing entry by searching
   rb_tree, we could call it HIT. Then the first 'while' will be finished by
   'break'. Next it will move to second 'while' loop, because of the flags
   hasn't been marked as WFE_LOWEST_SEQ. writecache_find_entry will try to
   return an entry with HIGHEST_SEQ, if there are other entries which has same
   original_sector in rb_tree.
   For this situation, the current code is okay to deal with that.

2. writecache_find_entry couldn't get an existing entry from rb_tree, we
   could call it MISS. Because of same flags WFE_RETURN_FOLLOWING,
   writecache_find_entry will get other entry, which's original_sector will
   slightly larger than input parameter block, with big probability.
   For this scenario, function writecache_find_entry doesn't need to enter
   second 'while' loop. But current code would still try to check there were
   other entry with same original_sector.
   So the additional rb_next or rb_prev is unnecessary by this case, also the
   code doesn't need to compare the original_sector of 'e2' with parameter
   'block'.

This patch is designed to optimize the second case. so it could skip the
second 'while' loop when the block is missed from rb_tree.

Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
---
 drivers/md/dm-writecache.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
index ddf1732..047ae09 100644
--- a/drivers/md/dm-writecache.c
+++ b/drivers/md/dm-writecache.c
@@ -537,14 +537,18 @@ static struct wc_entry *writecache_find_entry(struct dm_writecache *wc,
 {
 	struct wc_entry *e;
 	struct rb_node *node = wc->tree.rb_node;
+	bool found = false;
 
 	if (unlikely(!node))
 		return NULL;
 
 	while (1) {
 		e = container_of(node, struct wc_entry, rb_node);
-		if (read_original_sector(wc, e) == block)
+		if (read_original_sector(wc, e) == block) {
+			found = true;
 			break;
+		}
+
 		node = (read_original_sector(wc, e) >= block ?
 			e->rb_node.rb_left : e->rb_node.rb_right);
 		if (unlikely(!node)) {
@@ -564,7 +568,8 @@ static struct wc_entry *writecache_find_entry(struct dm_writecache *wc,
 		}
 	}
 
-	while (1) {
+	/* only need to check lowest or highest seq-count when entry has been found */
+	while (found) {
 		struct wc_entry *e2;
 		if (flags & WFE_LOWEST_SEQ)
 			node = rb_prev(&e->rb_node);
@@ -577,6 +582,9 @@ static struct wc_entry *writecache_find_entry(struct dm_writecache *wc,
 			return e;
 		e = e2;
 	}
+
+	/* no entry has been found, return the following entry */
+	return e;
 }
 
 static void writecache_insert_entry(struct dm_writecache *wc, struct wc_entry *ins)
-- 
1.8.3.1