Re: [RFC] JBD ordered mode rewrite

[Mingming Cao <cmm@us.ibm.com>]

On Thu, 2008-03-06 at 18:42 +0100, Jan Kara wrote:
>   Hi,
> 
Hi Jan,

>   Below is my rewrite of ordered mode in JBD. Now we don't have a list of
> data buffers that need syncing on transaction commit but a list of inodes
> that need writeout during commit. This brings all sorts of advantages such
> as possibility to get rid of journal heads and buffer heads for data
> buffers in ordered mode, better ordering of writes on transaction commit,
> simplification of some JBD code, no more anonymous pages when truncate of
> data being committed happens. The patch has survived some light testing
> but it still has some potential of eating your data so beware :) I've run
> dbench to see whether we didn't decrease performance by different handling
> of truncate and the throughput I'm getting on my machine is the same (OK,
> is lower by 0.5%) if I disable the code in truncate waiting for commit to
> finish... Also the throughput of dbench is about 2% better with my patch
> than with current JBD.


I know ext4 is keep changing that it's a bit hard to create patch
against ext4, but I feel features like especially rewrite the default
ordered mode should done in ext4/jbd2. I could port to current ext4 and
JBD2 if you agrees with this.

Also, would it make sense to create a new ordered mode writepage
routines, and keep the old ordered mode code there for a while, to allow
easy comparison? This could a good transition for people to start
experiment this ordered mode without worrying about put data in danger
by default.

>   Any comments or testing most welcom

[...]
>  /*
>   * Note that we always start a transaction even if we're not journalling
>   * data.  This is to preserve ordering: any hole instantiation within
> @@ -1465,15 +1444,11 @@ static int journal_dirty_data_fn(handle_t *handle, struct buffer_head *bh)
>   * We don't honour synchronous mounts for writepage().  That would be
>   * disastrous.  Any write() or metadata operation will sync the fs for
>   * us.
> - *
> - * AKPM2: if all the page's buffers are mapped to disk and !data=journal,
> - * we don't need to open a transaction here.
>   */
>  static int ext3_ordered_writepage(struct page *page,
>  				struct writeback_control *wbc)
>  {
>  	struct inode *inode = page->mapping->host;
> -	struct buffer_head *page_bufs;
>  	handle_t *handle = NULL;
>  	int ret = 0;
>  	int err;
> @@ -1487,46 +1462,49 @@ static int ext3_ordered_writepage(struct page *page,
>  	if (ext3_journal_current_handle())
>  		goto out_fail;
> 
> -	handle = ext3_journal_start(inode, ext3_writepage_trans_blocks(inode));
> -
> -	if (IS_ERR(handle)) {
> -		ret = PTR_ERR(handle);
> -		goto out_fail;
> +	/*
> +	 * Now there are two different reasons why we can be called:
> +	 *   1) write out during commit
> +	 *   2) fsync / writeout to free memory
> +	 *
> +	 * In the first case, we just need to write the buffer to disk, in the
> +	 * second case we may need to do hole filling and attach the inode to
> +	 * the transaction. Note that even in the first case, we may get an
> +	 * unmapped buffer (hole fill with data via mmap) but we don't have to
> +	 * write it - actually, we can't because from a transaction commit we
> +	 * cannot start a new transaction or we could deadlock.
> +	 */


Any thoughts how to handle the unmapped page under case 1)? Right now I
see it fails. Your comments here saying that we still have the issue
that "can't start a new transaction while commiting", but likely, with
delayed allocation, starting a new transaction could to happen a lot to
do defered block allocation. 

I really hope this new mode could be easy to add delayed allocation
support.  Any thoughts that we could workaround the locking in the JBD2
layer?


> +	if (!wbc->skip_unmapped) {
> +		handle = ext3_journal_start(inode,
> +				ext3_writepage_trans_blocks(inode));
> +		if (IS_ERR(handle)) {
> +			ret = PTR_ERR(handle);
> +			goto out_fail;
> +		}
>  	}
> +	else if (!PageMappedToDisk(page))
> +		goto out_fail;
> 

Thanks & Regards,

Mingming