btrfs_submit_compressed_read/write()
   {
	cb = kmalloc()
	refcount_set(&cb->pending_bios, 0);
	bio = btrfs_alloc_bio();

	/* NOTE here, we haven't yet submitted any bio */
	refcount_set(&cb->pending_bios, 1);

	for (pg_index = 0; pg_index < cb->nr_pages; pg_index++) {
		if (submit) {
			/* Here we submit bio, but we always have one
			 * extra pending_bios */
			refcount_inc(&cb->pending_bios);
			ret = btrfs_map_bio();
		}
	}

	/* Submit the last bio */
	ret = btrfs_map_bio();
   }

There are two reasons why we do this:

- compressed_bio::pending_bios is a refcount
   Thus if it's reduced to 0, it can not be increased again.

- To ensure the compressed_bio is not freed by some submitted bios
   If the submitted bio is finished before the next bio submitted,
   we can free the compressed_bio completely.

But the above code is sometimes confusing, and we can do it better by
just introduce a new member, compressed_bio::io_sectors.

With that member, we can easily distinguish if we're really the last
bio at endio time, and even allows us to remove some BUG_ON() later,
as we now know how many bytes are not yet submitted.

With this new member, now compressed_bio::pending_bios really indicates
the pending bios, without any special handling needed.

Signed-off-by: Qu Wenruo <redacted>
---
  fs/btrfs/compression.c | 71 ++++++++++++++++++++++--------------------
  fs/btrfs/compression.h | 10 +++++-
  2 files changed, 47 insertions(+), 34 deletions(-)

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index fc4f37adb7b7..c9dbe306f6ba 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c

@@ -193,6 +193,34 @@ static int check_compressed_csum(struct btrfs_inode *inode, struct bio *bio,
  	return 0;
  }
  
+/*
+ * Reduce bio and io accounting for a compressed_bio with its coresponding bio.
+ *
+ * Return true if there is no pending bio nor io.
+ * Return false otherwise.
+ */
+static bool dec_and_test_compressed_bio(struct compressed_bio *cb,
+					struct bio *bio)
+{
+	struct btrfs_fs_info *fs_info = btrfs_sb(cb->inode->i_sb);
+	unsigned int bi_size = bio->bi_iter.bi_size;
+	bool last_bio = false;
+	bool last_io = false;
+
+	if (bio->bi_status)
+		cb->errors = 1;
+
+	last_bio = atomic_dec_and_test(&cb->pending_bios);
+	last_io = atomic_sub_and_test(bi_size >> fs_info->sectorsize_bits,
+				       &cb->io_sectors);
+
+	/*
+	 * We can only finish the compressed bio if no pending bio and all io
+	 * submitted.
+	 */
+	return last_bio && last_io;
+}
+
  /* when we finish reading compressed pages from the disk, we
   * decompress them and then run the bio end_io routines on the
   * decompressed pages (in the inode address space).

@@ -212,13 +240,7 @@ static void end_compressed_bio_read(struct bio *bio)
  	unsigned int mirror = btrfs_io_bio(bio)->mirror_num;
  	int ret = 0;
  
-	if (bio->bi_status)
-		cb->errors = 1;
-
-	/* if there are more bios still pending for this compressed
-	 * extent, just exit
-	 */
-	if (!refcount_dec_and_test(&cb->pending_bios))
+	if (!dec_and_test_compressed_bio(cb, bio))
  		goto out;
  
  	/*

@@ -336,13 +358,7 @@ static void end_compressed_bio_write(struct bio *bio)
  	struct page *page;
  	unsigned long index;
  
-	if (bio->bi_status)
-		cb->errors = 1;
-
-	/* if there are more bios still pending for this compressed
-	 * extent, just exit
-	 */
-	if (!refcount_dec_and_test(&cb->pending_bios))
+	if (!dec_and_test_compressed_bio(cb, bio))
  		goto out;
  
  	/* ok, we're the last bio for this extent, step one is to

@@ -408,7 +424,8 @@ blk_status_t btrfs_submit_compressed_write(struct btrfs_inode *inode, u64 start,
  	cb = kmalloc(compressed_bio_size(fs_info, compressed_len), GFP_NOFS);
  	if (!cb)
  		return BLK_STS_RESOURCE;
-	refcount_set(&cb->pending_bios, 0);
+	atomic_set(&cb->pending_bios, 0);
+	atomic_set(&cb->io_sectors, compressed_len >> fs_info->sectorsize_bits);
  	cb->errors = 0;
  	cb->inode = &inode->vfs_inode;
  	cb->start = start;

@@ -441,7 +458,6 @@ blk_status_t btrfs_submit_compressed_write(struct btrfs_inode *inode, u64 start,
  		bio->bi_opf |= REQ_CGROUP_PUNT;
  		kthread_associate_blkcg(blkcg_css);
  	}
-	refcount_set(&cb->pending_bios, 1);
  
  	/* create and submit bios for the compressed pages */
  	bytes_left = compressed_len;

@@ -462,13 +478,7 @@ blk_status_t btrfs_submit_compressed_write(struct btrfs_inode *inode, u64 start,
  
  		page->mapping = NULL;
  		if (submit || len < PAGE_SIZE) {
-			/*
-			 * inc the count before we submit the bio so
-			 * we know the end IO handler won't happen before
-			 * we inc the count.  Otherwise, the cb might get
-			 * freed before we're done setting it up
-			 */
-			refcount_inc(&cb->pending_bios);
+			atomic_inc(&cb->pending_bios);
  			ret = btrfs_bio_wq_end_io(fs_info, bio,
  						  BTRFS_WQ_ENDIO_DATA);
  			BUG_ON(ret); /* -ENOMEM */

@@ -506,6 +516,7 @@ blk_status_t btrfs_submit_compressed_write(struct btrfs_inode *inode, u64 start,
  		cond_resched();
  	}
  
+	atomic_inc(&cb->pending_bios);
  	ret = btrfs_bio_wq_end_io(fs_info, bio, BTRFS_WQ_ENDIO_DATA);
  	BUG_ON(ret); /* -ENOMEM */
  

@@ -689,7 +700,8 @@ blk_status_t btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
  	if (!cb)
  		goto out;
  
-	refcount_set(&cb->pending_bios, 0);
+	atomic_set(&cb->pending_bios, 0);
+	atomic_set(&cb->io_sectors, compressed_len >> fs_info->sectorsize_bits);
  	cb->errors = 0;
  	cb->inode = inode;
  	cb->mirror_num = mirror_num;

@@ -734,7 +746,6 @@ blk_status_t btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
  	comp_bio->bi_opf = REQ_OP_READ;
  	comp_bio->bi_private = cb;
  	comp_bio->bi_end_io = end_compressed_bio_read;
-	refcount_set(&cb->pending_bios, 1);
  
  	for (pg_index = 0; pg_index < nr_pages; pg_index++) {
  		u32 pg_len = PAGE_SIZE;

@@ -763,18 +774,11 @@ blk_status_t btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
  		if (submit || bio_add_page(comp_bio, page, pg_len, 0) < pg_len) {
  			unsigned int nr_sectors;
  
+			atomic_inc(&cb->pending_bios);
  			ret = btrfs_bio_wq_end_io(fs_info, comp_bio,
  						  BTRFS_WQ_ENDIO_DATA);
  			BUG_ON(ret); /* -ENOMEM */
  
-			/*
-			 * inc the count before we submit the bio so
-			 * we know the end IO handler won't happen before
-			 * we inc the count.  Otherwise, the cb might get
-			 * freed before we're done setting it up
-			 */
-			refcount_inc(&cb->pending_bios);
-
  			ret = btrfs_lookup_bio_sums(inode, comp_bio, sums);
  			BUG_ON(ret); /* -ENOMEM */
  

@@ -798,6 +802,7 @@ blk_status_t btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
  		cur_disk_byte += pg_len;
  	}
  
+	atomic_inc(&cb->pending_bios);
  	ret = btrfs_bio_wq_end_io(fs_info, comp_bio, BTRFS_WQ_ENDIO_DATA);
  	BUG_ON(ret); /* -ENOMEM */
  

diff --git a/fs/btrfs/compression.h b/fs/btrfs/compression.h
index 8001b700ea3a..3df3262fedcd 100644
--- a/fs/btrfs/compression.h
+++ b/fs/btrfs/compression.h

@@ -29,7 +29,15 @@ struct btrfs_inode;
  
  struct compressed_bio {
  	/* number of bios pending for this compressed extent */
-	refcount_t pending_bios;
+	atomic_t pending_bios;
+
+	/*
+	 * Number of sectors which hasn't finished.
+	 *
+	 * Combined with pending_bios, we can manually finish the compressed_bio
+	 * if we hit some error while there is still some pages not added.
+	 */
+	atomic_t io_sectors;
  
  	/* the pages with the compressed data on them */
  	struct page **compressed_pages;