btrfs: determine synchronous writers from bio or writeback control

The writeback_control structure already passes down the information about
a writeback being synchronous from the core VM code, and thus information
is propagated into the bio REQ_SYNC flag through the wbc_to_write_flags
helper.

Use that information to decide if checksums calculation is offloaded to
a workqueue instead of btrfs_inode::sync_writers field that not only
bloats the inode but also has too wide scope, being inode wide instead
of limited to the actual writeback request.

The sync writes were set in:

- btrfs_do_write_iter - regular IO, sync status is set
- start_ordered_ops - ordered write start, writeback with WB_SYNC_ALL
  mode
- btrfs_write_marked_extents - write marked extents, writeback with
  WB_SYNC_ALL mode
Reviewed-by: Chris Mason <clm@fb.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/bio.c

@@ -579,11 +579,10 @@ static bool should_async_write(struct btrfs_bio *bbio)
 		return false;
 
 	/*
-	 * If the I/O is not issued by fsync and friends, (->sync_writers != 0),
-	 * then try to defer the submission to a workqueue to parallelize the
-	 * checksum calculation.
+	 * Try to defer the submission to a workqueue to parallelize the
+	 * checksum calculation unless the I/O is issued synchronously.
 	 */
-	if (atomic_read(&bbio->inode->sync_writers))
+	if (op_is_sync(bbio->bio.bi_opf))
 		return false;
 
 	/* Zoned devices require I/O to be submitted in order. */

fs/btrfs/btrfs_inode.h

@@ -116,9 +116,6 @@ struct btrfs_inode {
 
 	unsigned long runtime_flags;
 
-	/* Keep track of who's O_SYNC/fsyncing currently */
-	atomic_t sync_writers;
-
 	/* full 64 bit generation number, struct vfs_inode doesn't have a big
 	 * enough field for this.
 	 */

fs/btrfs/file.c

@@ -1651,7 +1651,6 @@ ssize_t btrfs_do_write_iter(struct kiocb *iocb, struct iov_iter *from,
 	struct file *file = iocb->ki_filp;
 	struct btrfs_inode *inode = BTRFS_I(file_inode(file));
 	ssize_t num_written, num_sync;
-	const bool sync = iocb_is_dsync(iocb);
 
 	/*
 	 * If the fs flips readonly due to some impossible error, although we
@@ -1664,9 +1663,6 @@ ssize_t btrfs_do_write_iter(struct kiocb *iocb, struct iov_iter *from,
 	if (encoded && (iocb->ki_flags & IOCB_NOWAIT))
 		return -EOPNOTSUPP;
 
-	if (sync)
-		atomic_inc(&inode->sync_writers);
-
 	if (encoded) {
 		num_written = btrfs_encoded_write(iocb, from, encoded);
 		num_sync = encoded->len;
@@ -1686,9 +1682,6 @@ ssize_t btrfs_do_write_iter(struct kiocb *iocb, struct iov_iter *from,
 			num_written = num_sync;
 	}
 
-	if (sync)
-		atomic_dec(&inode->sync_writers);
-
 	current->backing_dev_info = NULL;
 	return num_written;
 }
@@ -1733,9 +1726,7 @@ static int start_ordered_ops(struct inode *inode, loff_t start, loff_t end)
 	 * several segments of stripe length (currently 64K).
 	 */
 	blk_start_plug(&plug);
-	atomic_inc(&BTRFS_I(inode)->sync_writers);
 	ret = btrfs_fdatawrite_range(inode, start, end);
-	atomic_dec(&BTRFS_I(inode)->sync_writers);
 	blk_finish_plug(&plug);
 
 	return ret;

fs/btrfs/inode.c

@@ -8468,7 +8468,6 @@ struct inode *btrfs_alloc_inode(struct super_block *sb)
 	ei->io_tree.inode = ei;
 	extent_io_tree_init(fs_info, &ei->file_extent_tree,
 			    IO_TREE_INODE_FILE_EXTENT);
-	atomic_set(&ei->sync_writers, 0);
 	mutex_init(&ei->log_mutex);
 	btrfs_ordered_inode_tree_init(&ei->ordered_tree);
 	INIT_LIST_HEAD(&ei->delalloc_inodes);

fs/btrfs/transaction.c

@@ -1056,7 +1056,6 @@ int btrfs_write_marked_extents(struct btrfs_fs_info *fs_info,
 	u64 start = 0;
 	u64 end;
 
-	atomic_inc(&BTRFS_I(fs_info->btree_inode)->sync_writers);
 	while (!find_first_extent_bit(dirty_pages, start, &start, &end,
 				      mark, &cached_state)) {
 		bool wait_writeback = false;
@@ -1092,7 +1091,6 @@ int btrfs_write_marked_extents(struct btrfs_fs_info *fs_info,
 		cond_resched();
 		start = end + 1;
 	}
-	atomic_dec(&BTRFS_I(fs_info->btree_inode)->sync_writers);
 	return werr;
 }