linux-stable/fs
Qu Wenruo 6ec9765d74 btrfs: introduce compressed_bio::pending_sectors to trace compressed bio
For btrfs_submit_compressed_read() and btrfs_submit_compressed_write(),
we have a pretty weird dance around compressed_bio::pending_bios:

  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
introducing a new member, compressed_bio::pending_sectors.

Now we use compressed_bio::pending_sectors to indicate whether we have
any pending sectors under IO or not yet submitted.

If pending_sectors == 0, we're definitely the last bio of compressed_bio,
and is OK to release the compressed bio.

Now the workflow looks like this:

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

	for (pg_index = 0; pg_index < cb->nr_pages; pg_index++) {
		if (submit) {
			refcount_inc(&cb->pending_bios);
			ret = btrfs_map_bio();
		}
	}

	/* Submit the last bio */
	refcount_inc(&cb->pending_bios);
	ret = btrfs_map_bio();
  }

For now we still need pending_bios for later error handling, but will
remove pending_bios eventually after properly handling the errors.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
2021-10-26 19:08:03 +02:00
..
9p
adfs
affs
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autofs autofs: fix wait name hash calculation in autofs_wait() 2021-10-20 21:09:02 -04:00
befs
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btrfs btrfs: introduce compressed_bio::pending_sectors to trace compressed bio 2021-10-26 19:08:03 +02:00
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ceph ceph: fix handling of "meta" errors 2021-10-19 09:36:06 +02:00
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