[PATCH v2] exfat: improve performance of exfat_free_cluster when using dirsync mount option

[Hyeongseok Kim]

There are stressful update of cluster allocation bitmap when using
dirsync mount option which is doing sync buffer on every cluster bit
clearing. This could result in performance degradation when deleting
big size file.
Fix to update only when the bitmap buffer index is changed would make
less disk access, improving performance especially for truncate operation.

Testing with Samsung 256GB sdcard, mounted with dirsync option
(mount -t exfat /dev/block/mmcblk0p1 /temp/mount -o dirsync)

Remove 4GB file, blktrace result.
[Before] : 39 secs.
Total (blktrace):
 Reads Queued:      0,        0KiB   Writes Queued:      32775,    16387KiB
 Read Dispatches:   0,        0KiB   Write Dispatches:   32775,    16387KiB
 Reads Requeued:    0                Writes Requeued:        0
 Reads Completed:   0,        0KiB   Writes Completed:   32775,    16387KiB
 Read Merges:       0,        0KiB   Write Merges:           0,        0KiB
 IO unplugs:        2                Timer unplugs:          0

[After] : 1 sec.
Total (blktrace):
 Reads Queued:      0,        0KiB   Writes Queued:         13,        6KiB
 Read Dispatches:   0,        0KiB   Write Dispatches:      13,        6KiB
 Reads Requeued:    0                Writes Requeued:        0
 Reads Completed:   0,        0KiB   Writes Completed:      13,        6KiB
 Read Merges:       0,        0KiB   Write Merges:           0,        0KiB
 IO unplugs:        1                Timer unplugs:          0

Signed-off-by: Hyeongseok Kim <hyeongseok@xxxxxxxxx>
---
 fs/exfat/balloc.c   |  4 ++--
 fs/exfat/exfat_fs.h |  2 +-
 fs/exfat/fatent.c   | 43 +++++++++++++++++++++++++++++++++++++------
 3 files changed, 40 insertions(+), 9 deletions(-)

diff --git a/fs/exfat/balloc.c b/fs/exfat/balloc.c
index a987919686c0..761c79c3a4ba 100644
--- a/fs/exfat/balloc.c
+++ b/fs/exfat/balloc.c
@@ -166,7 +166,7 @@ int exfat_set_bitmap(struct inode *inode, unsigned int clu)
  * If the value of "clu" is 0, it means cluster 2 which is the first cluster of
  * the cluster heap.
  */
-void exfat_clear_bitmap(struct inode *inode, unsigned int clu)
+void exfat_clear_bitmap(struct inode *inode, unsigned int clu, bool sync)
 {
 	int i, b;
 	unsigned int ent_idx;
@@ -180,7 +180,7 @@ void exfat_clear_bitmap(struct inode *inode, unsigned int clu)
 	b = BITMAP_OFFSET_BIT_IN_SECTOR(sb, ent_idx);
 
 	clear_bit_le(b, sbi->vol_amap[i]->b_data);
-	exfat_update_bh(sbi->vol_amap[i], IS_DIRSYNC(inode));
+	exfat_update_bh(sbi->vol_amap[i], sync);
 
 	if (opts->discard) {
 		int ret_discard;
diff --git a/fs/exfat/exfat_fs.h b/fs/exfat/exfat_fs.h
index b8f0e829ecbd..764bc645241e 100644
--- a/fs/exfat/exfat_fs.h
+++ b/fs/exfat/exfat_fs.h
@@ -408,7 +408,7 @@ int exfat_count_num_clusters(struct super_block *sb,
 int exfat_load_bitmap(struct super_block *sb);
 void exfat_free_bitmap(struct exfat_sb_info *sbi);
 int exfat_set_bitmap(struct inode *inode, unsigned int clu);
-void exfat_clear_bitmap(struct inode *inode, unsigned int clu);
+void exfat_clear_bitmap(struct inode *inode, unsigned int clu, bool sync);
 unsigned int exfat_find_free_bitmap(struct super_block *sb, unsigned int clu);
 int exfat_count_used_clusters(struct super_block *sb, unsigned int *ret_count);
 
diff --git a/fs/exfat/fatent.c b/fs/exfat/fatent.c
index c3c9afee7418..7b2e8af17193 100644
--- a/fs/exfat/fatent.c
+++ b/fs/exfat/fatent.c
@@ -157,6 +157,7 @@ int exfat_free_cluster(struct inode *inode, struct exfat_chain *p_chain)
 	unsigned int clu;
 	struct super_block *sb = inode->i_sb;
 	struct exfat_sb_info *sbi = EXFAT_SB(sb);
+	int cur_cmap_i, next_cmap_i;
 
 	/* invalid cluster number */
 	if (p_chain->dir == EXFAT_FREE_CLUSTER ||
@@ -176,21 +177,51 @@ int exfat_free_cluster(struct inode *inode, struct exfat_chain *p_chain)
 
 	clu = p_chain->dir;
 
+	cur_cmap_i = next_cmap_i =
+		BITMAP_OFFSET_SECTOR_INDEX(sb, CLUSTER_TO_BITMAP_ENT(clu));
+
 	if (p_chain->flags == ALLOC_NO_FAT_CHAIN) {
+		unsigned int last_cluster = p_chain->dir + p_chain->size - 1;
 		do {
-			exfat_clear_bitmap(inode, clu);
-			clu++;
+			bool sync = false;
+
+			if (clu < last_cluster)
+				next_cmap_i =
+				  BITMAP_OFFSET_SECTOR_INDEX(sb, CLUSTER_TO_BITMAP_ENT(clu+1));
 
+			/* flush bitmap only if index would be changed or for last cluster */
+			if (clu == last_cluster || cur_cmap_i != next_cmap_i) {
+				sync = true;
+				cur_cmap_i = next_cmap_i;
+			}
+
+			exfat_clear_bitmap(inode, clu, (sync && IS_DIRSYNC(inode)));
+			clu++;
 			num_clusters++;
 		} while (num_clusters < p_chain->size);
 	} else {
 		do {
-			exfat_clear_bitmap(inode, clu);
-
-			if (exfat_get_next_cluster(sb, &clu))
-				goto dec_used_clus;
+			bool sync = false;
+			unsigned int n_clu = clu;
+			int err = exfat_get_next_cluster(sb, &n_clu);
+
+			if (err || n_clu == EXFAT_EOF_CLUSTER)
+				sync = true;
+			else
+				next_cmap_i =
+				  BITMAP_OFFSET_SECTOR_INDEX(sb, CLUSTER_TO_BITMAP_ENT(n_clu));
+
+			if (cur_cmap_i != next_cmap_i) {
+				sync = true;
+				cur_cmap_i = next_cmap_i;
+			}
 
+			exfat_clear_bitmap(inode, clu, (sync && IS_DIRSYNC(inode)));
+			clu = n_clu;
 			num_clusters++;
+
+			if (err)
+				goto dec_used_clus;
 		} while (clu != EXFAT_EOF_CLUSTER);
 	}
 
-- 
2.27.0.83.g0313f36